No full-text available
To read the full-text of this research,
you can request a copy directly from the author.
On the nature of regional seismic phase-I. Phase representations for Pn, Pg, Sn, Lg
Abstract
An operator development of the seismic wavefield is used to generate descriptions of the propagation processes contributing to the main regional seismic phases Pn, Pg, Sn, Lg. These operator forms are valid for laterally heterogeneous crust and mantle models and include the major processes of interconversion between wavetypes. These representations of the regional phases are used to examine the theoretical basis for discriminants between earthquakes and underground explosions based on the relative amplitudes of P and S phases. The ratio of Sn to Pn amplitude looks promising as a high frequency discriminant. However, the ratio of Lg to Pn amplitudes is not as useful because of the complex nature of the propagation characteristics of Lg.
... upper mantle effectively scatter Pn energy, i.e. head waves and diving waves propagating mainly along and just below the Moho (Kennett 1989(Kennett , 1994. up into the crust. A prominent feature is the coda excitation. ...
... Observationally, the Pn amplitudes may vary considerably and explanations here range from geological compositions, via tectonic reworkings of an area, to complex multilayered I-D velocity models (Gajewski et al. 1990;Kennett 1989Kennett , 1994. In our synthetic modelling it was sufficient to introduce sub-hloho velocity perturbations in order to ensure sufficiently strong Pn-and Sn-phase amplitudes. ...
... Coda velocities are roughly similar for the and 25. This is clearly seen in the NORESS event (Fig. 25) two arrays, but in exceptional cases ARCESS exhibits while for the ARCESS event (Fig. 24) character of these arrivals, as has been noted by others (Kennett 1989(Kennett , 1994. Likewise, the often presumed identifications of phases like pPn, PnPn, etc., from 'within' the Pn-Pg wave train is seldom justified unless based on objective wave-propagation criteria; even the simple models which we have used are complex in this respect. ...
A description is given of the numerical FD scheme used to solve the elastic wave equation, including a few remarks on the source functions used. The FD method has been used for computing synthetic seismograms for 2-D crust/upper mantle models of size 150 × 400 km2, with options for free-surface topography. The strategy was to introduce successively more complex lithosphere models for generating the synthetics; the reference model was laterally homogeneous lithosphere. The essence of the 2-D FD synthetic seisogram experiments is that a simple lithosphere model, being moderately heterogeneous, gives rise to complex seismograms which are grossly similar to the observation recordings. In contrast, complex models derived from profiling surveys (but lacking the fine-scale random variations) give simple, "ray tracing' like synthetics, not necessarily similar to the observed records. -from Authors
... The crustal guided phases like Pg and Lg originate due to the presence of thick low seismic velocity structure in the crust (Kennett and Furumura, 2001). It has been observed that Pg converts into S waves in shield region e.g. in eastern United States, which causes it to decay faster (Kennett, 1989). Variations in attenuation properties of Pg are sensitive to the changes in crustal structure (Bao et al., 2011c), though very few attempts have been made to understand its propagation. ...
... Inefficient Pg wave propagation has been reported at few places such as Zagros fold and thrust belt of Iran (Sandvol et al., 2001) and Teisseyre-Tornquist area of Eastern Europe (Schweitzer, 1995). Some studies have demonstrated that the amplitude of Pg can be affected by crustal scattering (Kennett, 1989;Kennett and Furumura, 2001) while Bao et al. (2011c) and Ranasinghe (2016) have observed that the intrinsic attenuation is the major cause of high Pg attenuation in northern middle East and northeast China respectively. ...
We investigate the spatial variations of Pg attenuation structure in Nepal Himalaya. We implement the “Two-Station method” on 2,325 waveforms obtained from 435 events recorded at 151 stations
deployed across Nepal Himalaya to measure the Pg-wave quality factor (Q). These station-station measurements are inverted further for investigating the spatial variations of Pg attenuation using a back projection algorithm. The model contains variations across the region, with much of the region being dominated by high attenuation. It is observed that the areas around the existing faults and lineaments exhibit very low Q values. A few exceptional high Q pockets are observed to be limited predominantly in the north western side of our study area. We hypothesize that intrinsic attenuation plays a major role in causing the high apparent Pg attenuation in the crust of Nepal Himalaya. This may be mainly caused by highly pressurized fluids trapped within a thin low velocity layer (LVL) at shallow depths. Moreover it is interesting to observe that all the past earthquakes of
since 1934, fall into the regions of low Q values but not the lowest. It may be anticipated that the very low Q areas are too weak to sustain the accumulation of high energy release during a large magnitude earthquake. Therefore the observed low Q structures across the study area may imply a weak crust and subsequently a favorable zone for future rupture.
... For example, it has been demonstrated that laterally heterogeneous velocity structure and changes in crustal discontinuities and crustal thickness affect regional phases (e.g., Kennett, 1986Kennett, , 1989. Blockage of Lg phases by segments of oceanic crust as short as 100 to 200 km is one classic example (e.g., Press and Ewing, 1952;Zhang and Lay, 1995). ...
... To a large extent, the low-frequency trends reflect the partial and complete blockage of Lg phases traversing the margin of the Tibetan Plateau, which is also partly responsible for the trends with topography (note the strong correlations between Figs. 1 and 2). This is consistent with modeling efforts that used ray diagrams and topography in central Asia, including Tibet, to demonstrate that Pg and Lg propagation are affected by changes in the crustal thickness (Kennett, 1989;Bostock and Kennett, 1990). The correlations appear to be stronger at lower frequencies because low-frequency Lg has more predictable effects than the strongly scattered high-frequency energy (e.g., Shih et al., 1994). ...
Reducing scatter in measurements of regional-phase amplitude ratios is desirable for seismic discrimination applications and also provides insight into crustal structure controls on energy partitioning of regional seismic waves. Our previous analysis (Fan and Lay, 1998) of the regional seismic discriminant, Pg/Lg, indicated that variations in crustal structure cause path-specific fluctuations of those amplitude ratios for earthquakes recorded at broadband station WMQ in western China. In this study, we extend our multivariate regression analysis to Pn/Sn, Pn/Lg, and Pg/Sn amplitude ratios recorded at WMQ in frequency bands of 0.75 to 1.5 Hz, 1.5 to 3.0 Hz, and 3.0 to 6.0 Hz for the suite of path-specific parameters: path length, mean path elevation, variance of topography along the path, rms topographic slope varia-tions, mean crustal thickness, and mean sediment thickness. Optimal three-and four-parameter models all achieve reductions in variance of the measurements relative to conventional distance corrections. At low frequency, the improvements for ratios involving Pg can be more than a factor of 2. For all short-period amplitude ratios, mean path elevation seems to play an important role. While crustal thickness and sediment thickness affect ratios involving Pg, topographic variance and surface slope variations have more influence on ratios involving Pn. Strong crustal variations as-sociated with the structure of the Tibetan Plateau are responsible for much of the amplitude variations.
... For example, it has been demonstrated that laterally heterogeneous velocity structure and changes in crustal discontinuities and crustal thickness affect regional phases (e.g., Kennett, 1986Kennett, , 1989. Blockage of Lg phases by segments of oceanic crust as short as 100 to 200 km is one classic example (e.g., Press and Ewing, 1952;Zhang and Lay, 1995). ...
... To a large extent, the low-frequency trends reflect the partial and complete blockage of Lg phases traversing the margin of the Tibetan Plateau, which is also partly responsible for the trends with topography (note the strong correlations between Figs. 1 and 2). This is consistent with modeling efforts that used ray diagrams and topography in central Asia, including Tibet, to demonstrate that Pg and Lg propagation are affected by changes in the crustal thickness (Kennett, 1989;Bostock and Kennett, 1990). The correlations appear to be stronger at lower frequencies because low-frequency Lg has more predictable effects than the strongly scattered high-frequency energy (e.g., Shih et al., 1994). ...
Regional seismic signals play an important role in identifying low-magnitude explosion and earthquake sources for monitoring the Comprehensive Test Ban Treaty (CTBT). To enhance the performance of seismic discriminants, regional phases need to be corrected for propagation effects of large-and intermediate-scale crustal wave-guide heterogeneity. Multivariate regression analysis using parametric path characteristics has shown promise for reducing the propagation-induced scatter in short-period regional phase amplitude ratios. While path length is an important parameter for most seismic phase amplitudes, other parametric features of crustal wave-guide structure are often significantly correlated with regional amplitude ratios of P-wave energy (Pn, Pg) to S-wave energy (Sn, Lg) as demonstrated by our prior work on station WMQ. A question remains as to whether these effects can be re-gionalized so that they apply to all observations in a given region. We explore the viability of regionalization of wave-guide effects on regional seismic discriminants for frequencies of 0.75 to 6 Hz in central Asia and southwest China by analyzing single-and multi-station data. Linear regressions for single-station data indicate that the significance of specific path parameters extracted from available crustal models varies from one station to another and from one subregion to another. In general, corrections for optimal combinations of wave-guide parameters obtained from mul-tivariate regression reduce the data variance more than conventional corrections for path length alone for both single-station and multi-station data. However, in central Asia, several stations show path dependence on certain variables with opposite signs, so regionalization with multi-station data does not perform well. This is due to highly path-specific effects. In southwest China, there is greater commonality in behavior among stations, and fairly good regionalization can be achieved. However, the re-sulting variance reduction is still less than that for individual stations because some path effects appear to be direction dependent. Our results indicate that individual station calibration is the preferred procedure, but this should include corrections for path effects beyond simple distance correction.
... For example, it has been demonstrated that laterally heterogeneous velocity structure and changes in crustal discontinuities and crustal thickness affect regional phases (e.g., Kennett, 1986Kennett, , 1989. Blockage of Lg phases by segments of oceanic crust as short as 100 to 200 km is one classic example (e.g., Press and Ewing, 1952;Zhang and Lay, 1995). ...
... To a large extent, the low-frequency trends reflect the partial and complete blockage of Lg phases traversing the margin of the Tibetan Plateau, which is also partly responsible for the trends with topography (note the strong correlations between Figs. 1 and 2). This is consistent with modeling efforts that used ray diagrams and topography in central Asia, including Tibet, to demonstrate that Pg and Lg propagation are affected by changes in the crustal thickness (Kennett, 1989;Bostock and Kennett, 1990). The correlations appear to be stronger at lower frequencies because low-frequency Lg has more predictable effects than the strongly scattered high-frequency energy (e.g., Shih et al., 1994). ...
Short-period regional phases play an important role in identifying low-magnitude seismic events in the context of monitoring the Comprehensive Test Ban Treaty. Amplitude ratios of regional phases comprised mainly of P-wave energy (Pn, Pg) to those comprised mainly of S-wave energy (Sn, Lg) effectively discriminate between explosions and earthquakes in many regions, particularly at frequencies higher than 3 Hz. At lower frequencies, discrimination is usually poor due to large scatter that causes overlapping of event populations. Scatter in regional discriminant measures such as Pg/Lg ratios is caused by both source and propagation effects, and reducing the scatter imparted by the latter is essential to improving the discriminant performance when events do not share identical paths. Regional phases experience distance-dependent amplitude variations due to effects such as critical angle ampli-fication, geometric spreading, and attenuation. Discriminant measures are usually corrected for empirically determined distance trends for a given region, but large scatter persists after such corrections. This study seeks to develop more sophisticated empirical corrections for path properties in order to further reduce the scatter in regional discriminant measures caused by propagation effects. Broadband seismic waveforms recorded at station WMQ, in western China, demonstrate that regional Pg/Lg ratios show significant distance dependence for frequencies less than 6 Hz. However, variations in crustal structure cause additional path-specific amplitude fluc-tuations that are not accounted for by regionally averaged distance corrections. Blockage of Lg phases on paths traversing the margins of the Tibetan Plateau is one such effect. Regression analysis demonstrates that Pg/Lg ratios measured at WMQ display significant correlations with path-specific properties such as mean elevation, topographic roughness, basement depth, and crustal thickness. Multiple regressions using optimal combinations of parameters yield corrections that reduce variance in Pg/Lg measurements for frequencies less than 3 Hz by a factor of 2 or more relative to standard distance corrections. This should systematically enhance the performance of the Pg/Lg discriminant at low frequencies. The method presented here can be used for all regions and all short-period regional discriminants. It is likely that the extraor-dinary crustal heterogeneity in western China represents an extreme case of path-dependent effects.
... Almost the entire eastern Anatolia, separated by the north Anatolian fault zone and east Anatolian fault zone, is characterized by low , while central and western Anatolia exhibit unevenly distributed low values. Strong attenuation was observed in the Aegean Sea block, where the crust thickness sharply decreases to ∼25 km ( Figure S1 in Supporting Information S1), which affects Lg propagation and its amplitude decay (Kennett, 1989;Zhang & Lay, 1995). Some researchers reported inefficient Lg wave propagation in the Anatolian Plateau and even Lg-wave blockage for ray paths across the southern Black Sea and the Bitlis-Zagros thrust (Al-Damegh et al., 2004;Zor et al., 2007). ...
Plain Language Summary
Different parts of the Anatolian Plateau are in different evolution stages between oceanic subduction and continental collision and currently undergoing plateau uplift and tectonic escape. The regional seismic velocity and attenuation can be used to characterize crustal partial melting and lateral heterogeneity, which can further identify the underlying subduction process. In this study, we construct a high‐resolution broadband Lg‐wave attenuation model for the Anatolian Plateau. Strong Lg attenuation in Anatolia correlates well with late Cenozoic magmatism distributions and can be an indicator of high temperature or partial melting in the crust. Combined with previous studies, we suggest that the mantle upwelling induced by the delamination of the Bitlis slab is likely reworking the crust in eastern Anatolia and is the cause of widespread thermal anomalies there. The lithospheric dripping process in central Anatolia may facilitate the mantle flows through the window between the Cyprus and Aegean slabs, and results in a piecemeal low QLg ${Q}_{\mathit{Lg}}$ anomaly pattern in central Anatolia.
... This is critical, as P-to-S converted phases have a higher 375 incidence angle at the bottom of the crust than both the S and the P phase (e.g. 376 Kennett, 1989) and are more likely to pass into the mantle, where the energy would 377 be lost to attenuation. This would prevent the Pg phase from developing. ...
The seismometer deployed on the surface of Mars as part of the InSight mission (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) has recorded several hundreds of marsquakes in the first 478 sols after landing. The majority of these are classified as high‐frequency (HF) events in the frequency range from approximately 1 to 10 Hz on Mars' surface. All the HF events excite a resonance around 2.4 Hz and show two distinct but broad arrivals of seismic energy that are separated by up to 450 s. Based on the frequency content and vertical‐to‐horizontal energy ratio, the HF event family has been subdivided into three event types, two of which we show to be identical and only appear separated due to the signal‐to‐noise ratio. We show here that the envelope shape of the HF events is explained by guided Pg and Sg phases in the Martian crust using simple layered models with scattering. Furthermore, the relative travel times between these two arrivals can be related to the epicentral distance, which shows distinct clustering. The rate at which HF events are observed varies by an order of magnitude over the course of one year and cannot be explained by changes of the background noise only. The HF content and the absence of additional seismic phases constrain crustal attenuation and layering, and the coda shape constrains the diffusivity in the uppermost shallow layers of Mars.
... We now face the issues that the effects of heterogeneity in different parts of the model do not linearly superpose, but instead can have complex and interfering interactions on the seismic wavefield. Kennett [1989] provided a description of the components of the seismic wavefield for regional to far-regional distances in terms of an operator representation of the reflection and transmission properties of different parts of the structure. The operators allow for the redistribution of energy between horizontal wave numbers introduced by the presence of lateral variations in structure. ...
... We now face the issues that the effects of heterogeneity in different parts of the model do not linearly superpose, but instead can have complex and interfering interactions on the seismic wavefield. Kennett [1989] provided a description of the components of the seismic wavefield for regional to far-regional distances in terms of an operator representation of the reflection and transmission properties of different parts of the structure. The operators allow for the redistribution of energy between horizontal wave numbers introduced by the presence of lateral variations in structure. ...
We examine the nature of seismic heterogeneity in the continental lithosphere, with particular reference to Australia. With the inclusion of deterministic large-scale structure and realistic medium-scale features there is not a need for strong fine-scale variations. The resulting multi-scale heterogeneity model gives a good representation of the character of observed seismograms and their geographic variation, and is also in good agreement with recent direct results on P-wave reflectivity in the lithosphere. Fine-scale heterogeneity is pervasive, but strongest in the crust. There is a weak quasi-laminar component above the lithosphere-asthenosphere transition with horizontal correlation length of 10 km and vertical correlation length of 0.5 km. Within the transition the aspect ratio of heterogeneity changes and can be well represented with a horizontal correlation length of 5 km and vertical correlation length of 1 km. For the Australian cratons this transition zone needs low intrinsic attenuation (high Q) to sustain the long high-frequency coda of both P and S waves. The interaction of the different aspects of the heterogeneity is complex and produces a diversity of behaviour depending on the relative thickness of the different lithospheric zones. The multi-scale model reconciles many of the divergent concepts of the character of heterogeneity based on interpretations of particular aspects of the seismic wavefield. The varying nature of the heterogeneity also ties well with the variations in tectonic character across the Australian continent. This article is protected by copyright. All rights reserved.
... На региональных расстояниях Lg-волны от землетрясений и взрывов зачастую являются доминирующими фазами [1]. Механизм образования Lg-волн, распространяющихся с групповыми скоростями между 3,5 км/с и 2,8 км/с и с периодами от 0,3 с до нескольких секунд, может быть интерпретирован различными способами [2,3]. Динамические характеристики Lg-волн, распространяющихся в земной коре, часто используются при оценке мощности взрывов на региональных расстояниях [4]. ...
В работе в рамках лучевого приближения моделировалось волновое поле объемных волн, генерируемое сейсмическими источниками взрывного типа, на региональных расстояниях в земной коре со слабой латеральной неоднородностью. Показано, что наличие крупномасштабных латеральных неоднородностей может приводить к изменению на десятки процентов амплитуд различных объемных региональных сейсмических фаз, в том числе и Lg-волн. Как показали расчеты, величина наклона в соотношении “магнитуда-логарифм мощности взрыва” практически не зависит от геометрии границ раздела и хорошо согласуется с экспериментальными данными.
... The theoretical and numerical exploration of wave propagation in various crustal wave guides has been extensively studied during the past several decades. Kennett (1989) developed a representation theory of crustal phases based on the operator method of Kennett (1984). This operator description of propagation processes systematically examines the complex nature of regional phases in a laterally heterogeneous crust. ...
A major impetus to study the topographic scattering effects is because most of our seismic observations are either at or very near to the earth's surface. The sensitivity of regional phases to topographic roughness in the crust has been widely observed. Comparisons of several approximation solutions to rough surface scattering are conducted in this study for an analytical description of the close relation of topographic statistics and regional phase attenuation. These approximations include Kirchhoff approximation theory, Taylor expansion-based perturbation theory, two-scale model, Rytov phase approximation, and Born series method, with each valid for a range of roughness scales. Kirchhoff approximation ignores multiple scatterings between any two surface points. In general, it has been considered valid for the large-scale roughness components. Perturbation theory based on Taylor series expansion is valid for the small-scale roughness components where the surface heights deviate from a planar at z = 0 by less than a wavelength. Rytov phase approximation to large-scale topographic roughness is not subject to the stringent restrictions that apply to the Kirchhoff approximation. Tests with the Gaussian and semicircular convex topographies show that the Rytov approximation improves the Kirchhoff approximation in both amplitude and phase. For a two-scale topography that consists of two extreme roughness components (large and small scales), some assumptions are valid to combine the Kirchhoff and perturbation theories for rough surface scattering. The realistic methods for the multiscale surfaces come with the Born series approximation that accounts for multiple scattering between surface points. For instance, the second-order Born approximation might be sufficient to guarantee the accuracy for general rough surfaces without infinite gradients and extremely large surface heights. It must be stressed that the approximation solutions described in this article miss the conversion of energy between SH and P-SV waves that is one of the main features of the crustal wave guide in real situations. Extension to the elastic case must be conducted in the future.
... Many seismologists agree that Pg, as a prominent second regional arrival after Pn, has a group velocity at approximately 6:1 km=s (Langston, 1982;Hearn and Clayton, 1986;Steck et al., 2009) and may have a similar excitation function to Lg (Stump et al., 2003). Studies using synthetic seismograms have suggested that Pg amplitude is sensitive to scattering and conversions (Kennett, 1989;Kennett and Furumura, 2001) and can be blocked by significant varia-tions in crustal wave guide (Furumura and Kennett, 1997). The attenuation of Pg should be investigated for calibrating the widely used Lg=Pg discriminant, in which Lg amplitude variations are assumed to be the majority compared with relatively stable Pg amplitude variations (Sandvol et al., 2001). ...
A seismic attenuation map of regional phase Pg is constructed for the northern Middle East using two-station frequency-dependent Q measurements and seismic attenuation tomography. Pg is widely used in source discrimination, but its attenuation behavior is not well understood. We investigate the functional form of the geometrical spreading of Pg for both uniform and more complex seismic velocity structures. We find that there is significant trade-off between attenuation and geometrical spreading of Pg, and we quantify this trade-off to interpret the observed Pg attenuation in the Middle East. The northern Middle East attenuation tomographic images show significant lateral Q(Pg) variations, which may be caused by lateral variations of crustal rheology if intrinsic attenuation dominates the seismic attenuation mechanism of this region. We observe very large variations in the Q(Pg) model, such as low Q(Pg) within the Anatolian plate and high Q(Pg) within much of the stable Arabian plate. The low Q(Pg) anomalies generally coincide with active fault systems or Quaternary volcanisms. In order to perform a systematic comparison with Lg propagation, we have also updated the Q(Lg) map of Zor et al. (2007). These two new maps provide estimates for crustal Q(alpha) (Pg) and Q(beta)(Lg) for the northern Middle East.
... Mode coupling can have the effect of shortening the primary L 8 wave train and reducing the amplitude of later S,, (Clouser and Langston, 1993). However, scattering and wave conversions can broaden overall Lg and S,, waveforms (Kennett, 1989), so the effects on S,/Lg ratios are not easily predicted. To deepen our understanding of these empirical observations , we plan further studies of both theoretical and empirical aspects. ...
Propagation of regional seismic phases is highly dependent on path effects, but we have a limited understanding of these effects and no general procedure for accounting for path variation influence on regional waveforms. Thus, there is strong regional variability in the effectiveness of regional wave discriminants used to identify small earthquakes and nuclear explosions. Moti- vated by many observations of correlation between surface geology and regional phase behavior, we empirically explore the relationship between short-period regional and upper mantle distance signal energy and statistics of topography along different travel paths using data for underground nuclear explosions at Semipalatinsk, Kazakhstan. We find strong linear correlations of the logarithmic rms amplitude ratio Sn/Lg (and to a lesser extent, P/Lg) with mean altitude, rms roughness, rms slope, and skewness of topography along the paths to receivers in Eurasia. This indicates that energy partitioning in the regional wave field is controlled by wave-guide structure and attenuation variations that are manifested in surface topography. This suggests that it is feasible in many cases to directly calibrate regional discriminants for path effects in terms of observable surface topography, as a surrogate for overall path properties. The relationships also help to understand the nature of regional phase propagation.
... Confined explosive sources couple into the surrounding rock primarily through compressional waves. Shear waves may be generated by multiple scattering in the source region, P to S wave conversion, tectonic stress release, spall and slapdown, and three dimensional heterogeneity along the propagation path (Kennett. 1989). Of these factors, only P to S wave conversion is easily modeled by ray methods and by Snell's law, S wave conversions at near horizontal boundary surfaces are too steep to account for the Lg reverberations. It is important to note that even at short distance from an explosive source, Lg energy is equalized on all components of motion, ...
The Lg wave is simulated three dimensionally by ray methods, in homogeneous models of the crust with smooth variations in thickness. Variations typical of mountainous regions are utilized, and the effect of these variations is examined in detail. Ray modeling shows that Lg is strongly attenuated by propagation across the thickened crustal root found beneath mountain ranges. While variations in the width of the mountain root are found to be unimportant, variations in the width of transition between thick and thin crust strongly influence attenuation. Propagation paths across transition regions 100 km wide result in weak Lg while 200 km transitions cause very little Lg attenuation. Lg is found to propagate very efficiently within a mountain root, with amplitude increased slightly by lateral reflections. Propagation across mountain belts at angles from 90 to 20 degrees (relative to strike) is shown to cause strong attenuation at all angles, with no identifiable optimal path. The technique is applied to a rough thickness model of the Central Asian crust and although attenuation is predicted, close comparison of the synthetics with earthquake recordings reveals discrepancies that can not be entirely explained by ray methods in a homogeneous crust. This suggests that a more accurate model of the crust is required. Including variations in crustal Q, surface sedimentary layers in the basin regions and the large scale 3-D heterogeneities associated with the uplifted regions.
... And our model shows a very similar pattern of lateral variation Q Lg0 with the result of Phillips et al. [2000]. [8] A complex velocity structure may lead to scattering attenuation and regional phase mode conversion when Lg propagates in a strongly heterogeneous crust [e.g., Kennett, 1989]. But a large number of studies imply that the Q Lg is a good estimation for crustal shear wave Q b , and the scattering only plays a minor role and is probably negligible at Here SS represents the site response of each station relative to the average of site responses within the whole area. ...
The Q of regional seismic phases Lg and Pg within the crust is assumed as a proxy for crustal Qβ and Qα, which is used as a constraint of crustal rheology. We measure regional-phase Q of the eastern Tibetan Plateau and adjacent areas. This method eliminates contributions from source and site responses and is an improvement on the Two-Station Method (TSM). We have generated tomographic images of crustal attenuation anomalies with resolution as high as 1° In general we observe low Q in the northernmost portions of the Tibetan Plateau and high Q in the more tectonically stable regions such as the interior of the Qaidam basin. The calculated site responses appear to correlate with topography or sediment thickness. Furthermore the relationship between earthquake magnitudes and calculated source terms suggest that the RTM method effectively removes the source response and may be used as an alternative to source magnitude.
... On these recordings at distances of about 950 km, Pg is not well developed, in agreement with an earlier study of regioi;al phases from Soviet nuclear explosions (Gupta et al., 1980). A possible reason is that in a shield region, Pg energy leaks rapidly into S with each surface reflection (Kennett, 1989). ...
The spectral characteristics of regional phases from East Kazakh, USSR underground nuclear explosions are studied for their dependence on parameters such as m sub b (generally related to shot depth) and spatial location (Shagan versus Degelen). The observed results are compared with those of the Nevada Test Site (NTS), where the near-source conditions are better known. Pn and Lg from 25 Soviet nuclear shots recorded at the Chinese Digital Seismic Network (CDSN) station WMQ are analyzed by obtaining spectral and time domain measurements on each phase. The average amplitude ratio Pn/Lg is found to be stable with mb but to vary strongly with frequency. For both Shagan and Yucca Flat explosions of similar yield, the reduction in amplitude with frequency is considerably larger for Lg than for Pn. At higher frequencies (3-7 Hz), the amplitude ratios Pn/Lg for explosions from Shagan, Degelen, Pahute Mesa, and Yucca Flat test sites show significant differences that appear to be due to variations in their source medium velocities. Over the frequency range of about 0.5 to 5.0 Hz, Pn/Lg increases by almost two orders of magnitude for the USSR shots and considerably less for the NTS shots. A possible explanation for the observed Lg spectra varying systematically with shot medium velocity is that Lg from USSR explosions is dominated by S* whereas that from NTS shots includes contributions from both pS and S.
... First is simply the extended coherent nature of Pg. There is no one distinct Pg arrival but several equal amplitude arrivals, some of which may be a direct result of the aforementioned source zone reverberations, that have traveled slightly different crustal paths from source to receiver (Kennett, 1989). The second similarity between the ANISO and observed array data is that subtle feature in waveforms observed at adjacent sensors exhibit a high degree of coherence, while those observed at sensors separated by distances on the order of a wavelength exhibit an unexpected reduction in correlation, and those observed at sensors separated by several wavelengths are seemingly unrelated. ...
Preliminary results for numerical simulations of regional propagation in realistic earth models are presented. The modeling is motivated in part by three-component array observations which show that the P and S codas, and Lg wavetrains for regional sources are comprised of substantial amounts of forward scattered/multipathed energy, and in part by results from numerical simulations of teleseismic wave propagation which suggest a correlation between the coda level and rate of decay and the aspect ratio of lithospheric heterogeneities. Numerical simulations are conducted using a two-dimensional, fourth order accurate, P-SV finite difference routine. The models consist of a heterogeneous layer over a homogeneous half-space. Perturbations are in velocity and parameterized using a gaussian correlation function. The wavefields generated using models containing high aspect ratio heterogeneities (ANISO) exhibit several characteristic consistent with regional array observations. First, the Pg wavetrain and coda consists of forward scattered, coherent plane wave energy trapped in the crustal waveguide. Models containing isotropic heterogeneities (ISO) inhibit the extent to which energy is trapped in the crustal waveguide and produce codas dominated by randomly scattered energy. Waveforms generated using models containing ANISO heterogeneities exhibit another feature commonly observed in regional array data. That is, that detailed features in waveforms observed at adjacent sensors exhibit a high degree of coherence, while those observed at sensors separated by distances on the order of a wavelength exhibit a somewhat surprising loss of coherence. Waveforms generated using models containing ISO heterogeneities exhibit a degree of incoherence inconsistent with regional array observations.
... In their study of discrimination between NTS explosions and earthquakes, Walter et al. (1995) noted improved performance at higher frequencies. Kennett (1989) provided theoretical arguments, supported by observations, that the Pn/Sn ratio would perform better as a source discriminant at higher frequencies. Another example is the regional discrimination study of explosions and earthquakes in the eastern United States by Kim et al. (1993) who observed significantly improved discrimination capability of the amplitude ratio P/Lg in the 5-25 Hz band than in the lower frequency bands. ...
Most source discriminants make use of the prominent regional phase Lg, so that a complete understanding of the generation and spectral characteristics of Lg from explosions is essential for improved and reliable monitoring of the Comprehensive Test Ban Treaty (CTBT). Our research provides valuable new information regarding the origin of both low and high frequency Lg. The low-frequency (up to about 2 Hz) part of the Lg spectra (including the most prominent peaks and nulls) appears to be due to the near-source scattering of explosion-generated Rg into S. Excellent agreement between observations and theory leaves no doubt regarding the contribution of Rg-to-S scattering to the low-frequency Lg from explosions. The high-frequency Lg appears to originate from explosion-generated cracks around the source region. Evidence for these results comes mostly from analysis of broadband regional data from over 40 Nevada Test Site (NTS) and several Kazakh Test Site (KTS) explosions. The results obtained in this study not only improve our understanding of broadband Lg from explosions, but also provide potentially useful source discriminants. Knowledge of the physical basis of discriminants is important to CTBT monitoring because a physical understanding of how and why they work allows for the prediction of discrimination performance in different geologic settings where adequate seismic data may not be available.
... More examples were shown in Fan and Lay (2002). The 0.2 to 1.0-Hz passband is actually that for the classical definition of Lg (e.g., Press and Ewing, 1952;Ewing et al., 1957), so there is no question that this energy is the Rayleigh-wave overtone energy recognized to constitute Lg over all frequencies (e.g., Kennett, 1989;Xie and Lay, 1994;Zhang and Lay, 1995). ...
The widespread existence of strong Lg attenuation in the Tibetan Plateau is further demonstrated by analysis of Lg spectra on many paths within the plateau and quantitative estimates of the Lg attenuation from low-frequency Lg signals for new, localized path geometries. Strong path-length-dependent shifts of the Lg spectra to lower corner frequencies with increasing distance are observed across the plateau, consistent with a low regional
average 1-Hz Lg attenuation value, Q0, of about 125. There are clearly lateral variations within the plateau found in this and other recent studies, with localized
areas having Q0 values of 60–90, low enough to eliminate high-frequency Lg energy over path lengths of just several hundred kilometers, while some localized areas may have higher Q0 values of up to 147 or higher. A Q0 value of 103 is found in south-central Tibet, compatible with recent work by others for higher frequencies on very localized
scales, and values from 83 to 147 are found in eastern Tibet. The lowest Q0 estimates found in Tibet tend to be in areas for which there is evidence of volcanism and/or partial melting within the crust;
however, the strong regional attenuation may have a contribution from scattering by small-scale crustal heterogeneity. The
strong Lg attenuation in Tibet gives a new constraint for understanding the tectonic development of the plateau and presents challenges
to seismic monitoring of the region for possible clandestine nuclear tests.
... Unlike Figure 10, For seismic waves generated by shallow sources, regional seismic wave propagation at ranges up to several hundred km is complex. The complexities arise because of strong scattering due to the heterogeneity of the crust and at frequencies above a few Hz, to the heterogeneity of the topography as well, and to the interaction of the waves with the crustal waveguide bounded by the free surface and the Moho crustmantle boundary (e.g., Kennett, 1989). The identification of constituent phases in iceberg tremor is further complicated by the quasi-continuous nature of the source. ...
Satellite imagery reveals that a series of large icebergs (B15B in April 2001, C19 in June 2003, and B15A in October 2005) broke up or fractured while exiting the Ross Sea in a narrowly defined area off Cape Adare, Antarctica. Examination of recent swath-mapped bathymetric observations revealed that the principle agent of these breakups is a previously unknown 9 km long ridge with minimum depths of 215 m that we call Davey Shoal. Satellite imagery shows that the icebergs are driven into the shoal by coastal currents that converge over the narrow continental shelf. One of the icebergs, the 120 km by 30 km B15A, was instrumented with a seismograph, GPS, and fluxgate compass. This instrumentation provided a unique opportunity to establish the details of the iceberg kinematics that were not revealed by satellite imagery alone and to correlate seismic events observed both on the iceberg and in the far field during breakup. B15A fractured from multiple strikes against Davey Shoal and the adjacent Possession Islands; these strikes were driven by the combination of tidal currents and the coastal mean flow. The periods of iceberg-sourced seismic radiation were correlated with the strikes. The iceberg- and land-based seismic signals showed that the iceberg fracture, its sliding across the shoals, and the ice-on-ice stick-slip contacts among the postbreakup iceberg fragments generated the strong chaotic and harmonic tremor episodes that were observed at distances as far as the South Pole, where these signals propagated as seismically coupled hydroacoustic T phases.
... Thus, various P/Lg amplitude measures have been developed for source identification applications, with good success for frequencies above 2 – 3 Hz [e.g., Walter et al., 1995; Taylor, 1996; Hartse et al., 1997]. Lg propagation is affected by laterally heterogeneous crustal thickness and velocity structure [e.g., Kennett, 1989; Bostock and Kennett, 1990; Zhang et al., 1994] and Lg does not propagate efficiently in thin oceanic crust [e.g., Press and Ewing, 1952; Zhang and Lay, 1995], but it is otherwise extensively observed. In Eurasia, Lg is the most stable phase observed at regional distances [Sereno et al., 1990; Baumgardt, 1990; Rapine et al., 1997], with efficient broadband Lg transmission over large distances in much of the continental crust. ...
1] Lg, a regional seismic wave comprised of multiple shear wave reverberations trapped in the crustal waveguide, is important for magnitude estimation and source discrimination for monitoring nuclear testing treaties. In stable continental regions, Lg propagates with a group velocity of about 3.5 km/s and can often be observed at distances up to 4000 km. To better understand the absence of high-frequency Lg arrivals for paths traversing the northern boundary of the Tibetan Plateau, we investigate spatial variations of broadband (0.15–5.0 Hz) energy in the Lg group velocity window (3.6–3.0 km/s) using regional waveforms recorded at the Chinese Digital Seismic Network station WMQ. Vertical component seismograms are analyzed for 90 events with magnitudes of 4.4 m b 6.4 that occurred between 1987 and 1999 in the Tibetan Plateau and around its margins. The Lg amplitude spectra for events located near the northern margin of the plateau have apparent corner frequencies of 1–2 Hz, nearly identical to those for comparable size events at similar distances outside the plateau. High-frequency (>1 Hz) Lg energy recorded at WMQ decreases rapidly as a function of source distance into the plateau. A path length of 300–400 km within the northern Plateau suffices to eliminate 2–5 Hz Lg energy. For events in southern Tibet with paths crossing the central portion of the Tibetan Plateau, almost total Lg extinction occurs, even for energy in the low-frequency band of 0.2–1 Hz. Corresponding apparent corner frequencies of the ''Lg'' amplitude spectra range between 0.2 and 0.4 Hz. The corner frequency shift is found to vary systematically with path length across the plateau. Linear regressions demonstrate that the shift in apparent corner frequency of Lg amplitude spectra is negatively correlated with features of the Tibetan Plateau, such as mean elevation along the paths or travel distance within the plateau above specified elevation thresholds. The systematic variations in the amplitude and frequency content of energy in the Lg window as a function of path length within the plateau indicate that strong crustal attenuation plays an important role in Lg extinction for paths traversing central and northern Tibet, superimposed on any structural blockage effects associated with abrupt thinning of the crust near the northern boundary of the plateau. Spectral ratios of many event pairs along great circle paths give estimates of frequency-dependent Lg attenuation for paths crossing western, central, and eastern sectors of Tibet. The region of strong Sn attenuation in northern central Tibet also has strong Lg attenuation with Q Lg for 1 Hz on the order of Q o = 80–90, while in southern central Tibet, Q o increases to about 316, and in eastern and western Tibet, Q o is on the order of 120–200 for paths traversing the entire plateau. The strong Lg attenuation in northern central Tibet is responsible for the so-called Lg blockage and may be associated with partial melting in the crustal low-velocity layer in northern Tibet. Our Q Lg values for Tibet are significantly lower than most earlier estimates, primarily as a result of not excluding blocked observations along with allowing for lateral variations within Tibet.
... The L~ phase can be viewed as a guided wave in the crust, very dependent upon the lateral continuity of the wave guide. Given that disruptions of the wave guide are expected near continental margins, and even within tectonically active continental regions, there are many concerns about the reliability of Lg in discrimination measurements (e.g., Kennett, 1989), independent of its remarkable relative stability on a given path for relative yield estimation (e.g., Ringdal et al., 1992; Hansen et al., 1990). Crustal thinning apparently can reduce Lg al., 1990), labeled N-S in (a), across the North Kara plate and south Kara syncline. ...
Short-period seismic recordings at regional and upper mantle distances from underground explosions at Novaya Zemlya demonstrate that propagation across the continental shelf under the Barents and Kara Seas appears to modify the partitioning of energy between Lg and Sn phases relative to purely continental paths in the Eurasian crust. While the underwater segments of the paths are relatively short, variations in bathymetric characteristics from path to path influence the regional wave field, with systematic behavior that can be used to establish empirical amplitude corrections for regional phases. We analyze a large set of Eurasian recordings to explore the relationship between regional phase energy partitioning and bathymetric characteristics. Maximum water depth along the path is the most influential factor for the Novaya Zemlya data. It has strong linear correlations with the logarithmic rms amplitude of Lg and the ratios Sn/Lg and P/Lg. The maximum water depth probably reflects the extent of necking
... Since the global studies by Molnar and Oliver (1969), several regional studies following the same philosophy were conducted for the Indian shield and Tibetan Plateau (Ni and Barazangi, 1983 ), for the Nazca plate and across the Altiplano in South America (Chinn et al., 1980), for the Banda Arc (Barazangi et al., 1977), for southern Africa (Nyblade et al., 1996), for the Turkish and Iranian Plateaus (Kadinsky-Cade et al., 1981) and for China (Rapine et al., 1997), for the Middle East (Rodgers et al., 1997), and for the Africa-Iberia plate boundary (Calvert et al., 2000). The efficiency of S n and P n propagation is also of importance in the Comprehensive Test Ban Treaty (CTBT) for discrimination of earthquakes and explosions (e.g., Kennett, 1989). The blocking of S n propagation at mid-ocean ridges has two likely explanations. ...
We infer the fine structure of a sub-Moho boundary layer (SMBL) at the top of the lithospheric mantle from high-resolution seismic observations of Peaceful Nuclear Explosions (PNE) on superlong-range profiles in Russia. Densely recorded seismograms permit recognition of previously unknown features of teleseismic propagation of the well known Pn and Sn phases, such as a band of incoherent, scattered, high-frequency seismic energy, developing consistently from station to station, apparent velocities of sub-Moho material, and high-frequency energy to distances of more than 3000 km with a coda band, incoherent at 10 km spacing and yet consistently observed to the end of the profiles.
Estimates of the other key elements of the SMBL were obtained by finite difference calculations of wave propagation in elastic 2D models from a systematic grid search through parameter space. The SMBL consists of randomly distributed, mild velocity fluctuations of 2% or schlieren of high aspect ratios (≥40) with long horizontal extent (~20 km) and therefore as thin as 0.5 km only; SMBL thickness is 60-100 km.
It is suggested that the SMBL is of global significance as the physical base of the platewide observed high-frequency phases Pn and Sn. It is shown that wave propagation in the SMBL waveguide is insensitive to the background velocity distribution on which its schlieren are superimposed. This explains why the Pn and Sn phases traverse geological provinces of various age, heat flow, crustal thickness, and tectonic regimes. Their propagation appears to be independent of age, temperature, pressure, and stress.
Dynamic stretching of mantle material during subduction or flow, possibly combined with chemical differentiation have to be considered as scale-forming processes in the upper mantle. However, it is difficult to distinguish with the present sets of Pn/Sn array data whether (and also where) the boundary layer is a frozen-in feature of paleo-processes or whether it is a response to an on-going processes; nevertheless, the derived quantitative estimates of the SMBL properties provide important constraints for any hypothesis on scale-forming processes. Models to be tested by future numerical and field experiments are, for example, repeated subduction-convection stretching of oceanic lithosphere (marble-cake model) and schlieren formation at mid-ocean ridges. It is also proposed that the modeling of the observed blocking of Sn and Pn propagation at active plate margins offers a new tool to study the depth range of tectonics below the crust-mantle boundary.
Finally, the deduced schlieren structure of the SMBL closes an important scale gap of three to four orders of magnitude between structural dimensions studied in petrological analysis of mantle samples (xenoliths or outcrop of oceanic lithosphere) and those imaged in classical seismological studies of the lithosphere.
The deformation mechanism in southeastern Tibet since the continental collision between the Indian and Eurasian plates could be explained by several models, including two major classic end-member models, the rigid-block extrusion model, and the crustal flow model. Crustal channel flow is likely an important tectonic regime for properly explaining a large number of geological and geophysical observations but remains in competition with the block extrusion model. Consequently, detecting ductile flow connectivity would play a key role in understanding the tectonic evolution of the southeastern Tibetan Plateau. Here, we established a high-resolution broadband QPg model for the crust in SE Tibet by using a joint inversion tomography method based on both single- and two-station Pg data. We verified the stability of the QPg tomography by comparing the QPg values at 1 Hz between the joint inversion and the two-station method. Two low-QPg zones were observed, isolated by the high-QPg Emeishan large igneous province (ELIP). Strong Pg attenuation beneath the Songpan-Ganzi Block and Western Sichuan Block may indicate the presence of crustal material flow due to relatively weak rheological strength. Cooled basaltic magma remnants in the inner zone of the ELIP likely block the southeastward migration of crustal materials driven by the gravity and lateral pressure gradient, and restrict the flow to the Western Sichuan Block, resulting in surface uplift and crustal thickening. Strong Pg attenuation near the Xiaojiang Fault and the Red River Fault may result from mantle upwelling in this region. Our QPg model, combined with previous results, suggests that the tectonic deformation in the southeastern Tibetan Plateau has been mainly controlled by the effects of crustal channel flow and asthenospheric upwelling since the Late Miocene.
The lateral variations in Pg attenuation characteristics in western Tibet have been investigated. Data from 48 broadband seismic stations operated across western Tibet and western Kunlun are used to generate the two-station method (TSM) and reverse two-station method (RTSM) PgQ at 1 Hz (Q0). Subsequently, 735 TSM and 561 RTSM pairs are used to obtain the Pg attenuation tomography model at 1 Hz by using LSQR algorithm. Both TSM and RTSM models exhibit spatial variations across the diverse tectonics with a predominance of low Q structure. Our observations complement the reported geophysical studies, which enlighten the high intrinsic attenuation caused by high temperature, partial melting or channel flow as the dominant factor for the low PgQ structure. The northern part, such as Kunlun suture and Tianshuihai terrane belongs to high attenuation zones consistent with the reported low LgQ0 values. This might explain the active tectonics caused by several geological units which are directly linked to the earthquake genesis in this area. The Lhasa terrane is mostly dominated by moderate PgQ0 values, with very few pockets of high Q structures evident in both TSM and RTSM images, which is relatively dissimilar from the reported LgQ values. This could be attributed to the presence of strong crustal heterogeneity as a result of the underthrusting of the Indian lithosphere beneath western Tibet.
Cambridge Core - Fluid Dynamics and Solid Mechanics - The Seismic Wavefield - by B. L. N. Kennett
I present results from elastic finite-difference simulations of regional wave propagation conducted in an effort to characterize, in a statistical sense, the nature of lithospheric heterogeneities required to generate scattered wave fields with characteristics consistent with those observed in regional array data. In particular, regional P, S, and Lg wave trains that are comprised not of the occasional coherent deterministic phase emersed in randomly scattered coda, but of a continuous succession of coherent forward-scattered arrivals. My modeling suggests that lithospheric heterogeneities should be parameterized using spatially anisotropic correlation functions. Models containing spatially isotropic heterogeneities inhibit the extent to which energy is forward scattered and trapped in the crustal wave guide and, consequently, produce regional wave fields whose characteristics are inconsistent with array observations. Models containing spatially anisotropic heterogeneities - which preferentially forward scatter energy that is subsequently trapped in the crustal wave guide - produce wave fields whose characteristics are consistent with regional array observations and provide intuitively appealing representations of subsurface structure.
The earth's topography is generally rough at various scales. Numerical simulation techniques are used in this study to investigate
the energy attenuation of regional phases across a randomly rough topography. We demonstrate a clear statistical correlation
of the distance-dependent energy distribution with path topographic properties parameterized by the surface correlation length
a and the surface root-mean-square height σ. Numerical experiments show that interference of randomly scattered waves by topography can cause regional wave amplitude
undergoing strong variations. The topographic-scattering-driven energy distribution over a long distance is usually characteristic
of an attenuation trend on the long distance scale, accompanied by amplitude fluctuations on the smaller distance scale. Total
energy attenuation can be divided into large-scale and small-scale components that are correlated, respectively, in quite
different manners, with along-path topographic statistics. On the one hand, the small-scale energy component is strongly related
to the near-receiver topographic geometry. It has a striking similarity to the corresponding topographic curve. The spatial
fluctuation of the small-scale component depends on a, whereas its amplitude amplification/deamplification is mainly related to σ, wavelength, and local incident angles. On the other hand, the large-scale component of energy curve, described by a scattering
Q, demonstrates a scale-dependent relation with topographic statistics. A two-step analysis method is presented to evaluate
the mantle leakage loss due to topographic scattering. The resultant topographic scattering Q is comparable with some observations with Q measured as a mean value in the crust. In summary, the study suggests the concept that topographic scattering might be a
powerful mechanism to attenuate regional waves.
Crustal wave guides are usually heterogeneous on many scales, some of which can be investigated for regional phases by one-way approximation methods. The advantage of one-way propagation methods is the greater saving of computing time and memory, often up to several orders, than the full-waveform numerical methods. By slicing a half-space crustal wave guide into a number of slabs perpendicular to the propagation direction, Wu et al. (2000) formulate a wide-angle pseudoscreen method for the SH half-space problem, which leads to a generalized screen propagator (GSP) for simulating Lg propagation. In this article we introduce a broadband constant-coefficient propagator for the SH half-space problem, which accounts for wide angles in large-contrast media while allowing implementation using Fourier transforms alone. Particular attention is paid to the first-order separation-of-variables screen propagator (SVSP1) that significantly improves the split-step Fourier (SSF) method for large lateral variations at the cost of one more Fourier transform in each slab. Advancing wave fields by SVSP1 is actually a linear interpolation in the wave-number domain between two split-step terms. We benchmark the SSF, GSP, and SVSP1 synthetic seismograms against the full-waveform boundary-element synthetics for flat, belling, and necking crustal wave guides, which shows that the SVSP1 method can model the Lg phase and the mantle wave (head wave) quite well in both travel time, energy, and waveform for most common mantle velocity perturbations. These numerical comparisons also demonstrate some limitations (especially in waveform) of the one-way propagation methods to model the Lg code attributed by forward scatterings as a result of lateral irregularities of the Moho.
We present results from a study conducted to characterize regional wave propagation in southern California and Nevada. The data are from events whose paths sample the southern Coast Ranges, the southern Sierra Nevada Mountains and the southern Basin and Range. These regional earthquake data were recorded by a small-aperture three-component seismic array located at the Pition Flat Observatory, southern California. The data are analyzed using a statistical array processing algorithm that provides estimates of the propagation direction, frequency, wavenumber, and particle motion polarization characteristics of the coherent arrivals observed at the array. Perhaps the most striking feature shared by these data is the extent to which the P, $ and Lg wave trains are composed of coherent, forward scattered/multipathed energy. The nature othe P, $ and Lg wave trains suggests that spatially anisotropic scattering plays an important role in regional wave propagation in southern California and Nevada, and that scattering sources are located not on elliptical shells whose foci are the source and the receiver but instead in a limited volume subparallel to the path of the direct arrivals. The high-amplitude, coherent nature of numerous of the Pg and Lg coda waves suggests that strong scattering in the source zone plays an important role in shaping the Pg and Lg wave trains. While the P, $ and Lg wave trains for these regional events share some general characteristics they also exhibit fairly significant differences. These differences illustrate the important role regional and source zone structure play in shaping the wave field, and the need to account for both complex scattering and three-dimensional propagation effects in regional waveform modeling. The coda for these regional data do not begin to exhibit a character consistent with the random scattering model for coda generation until the arrival of the surface waves. The fact that much of the surface wave coda cannot be effectively modeled as plane waves suggests that it is not the product of scattering from distant heterogeneities.
Location relative to a reference event is often more useful and precise than absolute event location. Such location requires relative times for pairs of events observed at a common station, which can be obtained with high precision for similar events by waveform cross-correlation. The precision can be further boosted by applying the cross-spectral analysis method, which can provide relative delay times with resolution up to an order of magnitude better than the seismogram sampling interval. Software for cross-spectral analysis based on use of both single cosine taper and the significantly more sophisticated multitaper was developed and tested on real data. Analysis of data from six closely-located Yucca Flat explosions provided relative locations with mean location error of only about 1 km; an impressive result if one considers the large epicentral distances and the complex geology of the Nevada Test Site. Mean velocities along the four source-receiver paths were found to be stable, and significantly different, suggesting considerable anisotropy. The cross- spectrum method was also applied to regional data from two pairs of closely located NTS explosions recorded at common stations. Our results indicate that the analysis can detect small differences (of the o 0.1%) in the propagation velocities of regional phases generated by the two shots.... Multitaper, Relative location, Cross-spectral analysis, Inter-source coherence, Regional phases, Colocated explosions.
A number of proposed discriminants for distinguishing the character of seismic sources use the amplitude of the phase Pn as a reference. The merit of Pn is that it constitutes the onset of regional seismograms, but the behaviour of the arrivals at an individual station can be quite complex. Results from long-range refraction experiments in Eurasia suggest that the complexity arises from the superimposition of a number of sub-phases returned from fine- scale structure in the uppermost mantle. The behaviour is consistent with fine scale horizontal variations superimposed on a gentle increase in seismic velocity with depth, so that equivalent one-dimensional models will show multiple low velocity zones. Long range refraction data for Sn is much less common but similar trends can be discerned. As frequency increases such complexity is likely to become more important. Sn is often observed at distance beyond 300 km but emerges from Sg (Lg) much less clearly in general than Pn emerges from Pg. However some refraction profiles in the Finnish Shield areas show very clear Sn arrivals at short distances. The differences in character can be associated with differing velocity gradients for S in the uppermost mantle. The variability in Sn behaviour means that it may prove difficult to generate discriminants based on amplitudes which can be readily transportable between different regions.
The earth's topography is generally rough at various scales. Numerical simulation techniques are used in this study to investigate the energy attenuation of regional phases across a randomly rough topography. We demonstrate a clear statis-tical correlation of the distance-dependent energy distribution with path topographic properties parameterized by the surface correlation length a and the surface root-mean-square height r. Numerical experiments show that interference of randomly scattered waves by topography can cause regional wave amplitude undergoing strong variations. The topographic-scattering-driven energy distribution over a long distance is usually characteristic of an attenuation trend on the long distance scale, accom-panied by amplitude fluctuations on the smaller distance scale. Total energy atten-uation can be divided into large-scale and small-scale components that are correlated, respectively, in quite different manners, with along-path topographic statistics. On the one hand, the small-scale energy component is strongly related to the near-receiver topographic geometry. It has a striking similarity to the corresponding to-pographic curve. The spatial fluctuation of the small-scale component depends on a, whereas its amplitude amplification/deamplification is mainly related to r, wave-length, and local incident angles. On the other hand, the large-scale component of energy curve, described by a scattering Q, demonstrates a scale-dependent relation with topographic statistics. A two-step analysis method is presented to evaluate the mantle leakage loss due to topographic scattering. The resultant topographic scatter-ing Q is comparable with some observations with Q measured as a mean value in the crust. In summary, the study suggests the concept that topographic scattering might be a powerful mechanism to attenuate regional waves.
We used over 1000 regional waveforms recorded by 60 seismic stations located in northwest Africa and Iberia to map the efficiency of L g and Sn wave propagation beneath the Gulf of Cadiz, Alboran Sea and bounding Betic, Rif and Atlas mountain belts. Crustal attenuation is inferred from the tomographic inversion of L g/Pg amplitude ratios. Upper mantle attenuation is inferred from maps of Sn propagation efficiency derived by inversion of well-defined qualitative efficiency assignments based on waveform characteristics. Regions of L g attenuation correlate well with areas of thinned continental or oceanic crust, significant sedimentary basins, and lateral crustal variations. Comparison of the Sn efficiency results with velocities obtained from an anisotropic Pn traveltime inversion shows a fairly good correlation between regions of poor Sn efficiency and low Pn velocity. A low Pn velocity (7.6–7.8 km s−1 ) and significant Sn attenuation in the uppermost mantle is imaged beneath the Betics in southern Spain, in sharp contrast to the relatively normal Pn velocity (8.0–8.1 km s−1 ) and efficient Sn imaged beneath the Alboran Sea. Slow Pn velocity anomalies are also imaged beneath the Rif and Middle Atlas in Morocco. We do not identify any conclusive evidence of lithospheric-scale upper mantle attenuation beneath the Rif, although the crust in the Gibraltar region appears highly attenuating, making observations at stations in this region ambiguous. Paths crossing the Gulf of Cadiz, eastern Atlantic and the Moroccan and Iberian mesetas show very efficient Sn propagation and are imaged with high Pn velocities (8.1–8.2 km s−1 ).
The spatial distribution of attenuation and velocity anomalies lead us to conclude that some recovery of the mantle lid beneath the Alboran Sea must have occurred since the early Miocene episode of extension and volcanism. We interpret the low-velocity and attenuating regions beneath the Betics and possibly the Rif as indicating the presence of partial melt in the uppermost mantle which may be underlain by faster less attenuating mantle. In the light of observations from other geophysical and geological studies, the presence of melt at the base of the Betic crust may be an indication that delamination of continental lithosphere has played a role in the Neogene evolution of the Alboran Sea region.
The most prominent feature of the regional seismic wavefield from about 150 to over 1000 km is usually the Lg phase. This arrival represents trapped S-wave propagation within the crust as a superposition of multiple reflections, and its amplitude is quite sensitive to the lateral variation in the crust along a propagation path. In an environment where the events occur in a subduction zone, such as the western coast of Mexico, quite complex influences on the character of the regional wavefield arise from the presence of the subduction zone.
The great 1985 Michoacan earthquake (MW=8.1), which occurred in the Mexican subduction zone, was one of the most destructive earthquakes in modern history and its notable character was that at Mexico City, located over 350 km from the epicentre, there was strong ground shaking almost comparable to that in the epicentral region that lasted for several minutes. Considerable effort has been expended to explain the origin of the unusual observed waves that caused the severe damage in the capital city during the destructive earthquake.
The nature of the propagation process in this region can be understood in part by using the detailed strong-motion records from the 1995 Copala, Guerrero (MW=7.4) earthquake near the coast to the south of Mexico City, which also had an enhanced amplitude in the Valley of Mexico.
Numerical modelling of both P and S seismic waves in 2-D and 3-D heterogeneous crustal models for western Mexico using the pseudospectral method provides direct insight into the nature of the propagation processes through the use of sequences of snapshots of the wavefield and synthetic seismograms at the surface. A comparison of different models allows the influences of different aspects of the structure to be isolated.
2-D and 3-D modelling of the 1985 Michoacan and 1995 Copala earthquakes clearly demonstrates that the origin of the long duration of strong ground shaking comes from the Sn and Lg wave trains. These S-wave arrivals are produced efficiently from shallow subduction earthquakes and are strongly enhanced during their propagation within the laterally heterogeneous waveguide produced by the subduction of the Cocos Plate beneath the Mexican mainland. The amplitude and duration of the Lg coda is also strongly reinforced by transmission through the Mexican Volcanic Belt from the amplification of S waves in the low-velocity surficial layer associated withS-to-P conversions in the volcanic zone.
The further amplification of the large and long Lg wave train impinging on the shallow structure in the basin of Mexico City, with very soft soil underlain by nearly rigid bedrock with a strong impedance contrast, gives rise to the destructive strong ground shaking from the Mexican subduction earthquakes.
-- The seismic records from significant earthquakes are profoundly affected by 3-D variations in crustal structure both in the source zone itself and in propagation to some distance. Even in structurally complex zones such as Japan and Mexico relatively coherent arrivals are found associated with different classes of propagation paths. The presence of strong lateral variations can disrupt the arrivals, and impose significant variations in propagation characteristics for different directions from the source as illustrated by observations for the 1995 Kobe and 2000 Tottori-ken Seibu earthquakes in western Japan. Such effects can be modelled in 3 dimensions using a hybrid scheme with a pseudospectral representation for horizontal coordinates and finite differences in depth. This arrangement improves parallel implementation by minimising communication costs. For a realistic 3-D model for the structure in western Japan the 3-D simulations to frequencies close to 1 Hz provide a good representation of the observations from subduction zones events such as the 1946 Nankai earthquake and the 2000 Tottori-ken Seibu earthquake. The model can therefore be used to investigate the pattern of ground motion expected for future events e.g., in current seismic gaps.
A simple approach for the estimation of onset times of regional arrivals was tested. The method consists of the computation of the cumulative sum (CUSUM) of a test statistic, in this case, the absolute value of the trace amplitude, addition of a linear trend to the result and finding minima of the resulting function by various methods. The minima are located at the times where sudden increases in the trace amplitudes occur, i.e., at the phase onsets. The method requires frequency prefiltering in order to enhance the amplitude changes at the phase onsets. It was found that this algorithm performed comparably to a human analyst in finding onset times for Pn arrivals with varying S/N ratios. Moreover, when applied to the rest of the seismogram, it was effective in finding the onset times of the rest of the regional arrivals as long as these were clearly visible. In the rest of the cases, where there are many poorly defined arrivals, an analyst would also have difficulties in uniquely defining onset times.
It is unlikely that regional data recorded at a newly installed array of stations could be used immediately to reliably discriminate events because of uncertainties about the local characteristics of wave propagation. However, the stations would immediately begin to record P sub nl data from small local events. This raises the possibility that the earthquake data base could be used to calibrate the array for discrimination purposes. The relationships is studied between long-period P/sub nl/ data and high-frequency regional phases in the western U.S. This report examines the phase P/sub g/. The high-frequency p/sub g/ phase for small, shallow events typically appears as a complex arrival with a duration of several tens of seconds. This is much longer than the possible duration of the effective source time function even with depth phases taken into account. This phenomenon can be explained by synthesizing P/sub g/ using generalized rays which is a practical computational procedure for a simple layer over a half-space crustal model. If model responses are computed by progressively adding in rays with one reverberation in the crust, then two and so on, it is observed that a sequence of high-frequency arrivals develops spanning the typical duration of P/sub g/. Each successive resonance phase is associated with a higher-order reverberation in the crust. If each arrival contributes a typical amount of high-frequency scattered energy, the long duration of P/sub g/ is simple to understand.
The propagation of Lg waves in complex media can be described either by means of a modal superposition scheme with numerical integration through the heterogeneity or by using ray diagrams. Rays are set off at equal horizontal intervals in a stratified zone adjacent to the heterogeneity, with phase velocities appropriate to particular modes. The constructive interference pattern in the stratified medium is modified in the horizontally varying region to give a graphic illustration of the propagation effects. The ray method agrees well with the modal calculations but may be conveniently applied in more general circumstances, e.g., to include surface topography or permanent changes in crustal structure. Structural boundaries which involve sudden thinning of the crustal wave guide are particularly disruptive to the Lg train, as at a pinch in crustal thickness or the continent-ocean transition. The effects of localized thickening are more subtle. The relatively sharp cutoff for Lg waves in some structures, e.g. in the Tibetan Plateau, can be explained by the source being no longer able to couple into the crustal wave guide at the receiver.
The reflectivity method for the computation of synthetic seismograms, as devised by Fuchs, is extended to include the elastic transmission losses and time shifts due to a stack of layers on top of the reflecting medium. Numerical details of this method are described, and a comparison with the ray-theoretical method, as devised by Müller, is given. The results of both methods agree well if the models are not too complicated. The field of application of these methods is the comparison with observed seismograms obtained from refraction studies of the Earth's crust and upper mantle. The reflectivity method is applied to the interpretation of observations along a profile in Central Europe. The compressional velocity of the lower crust beneath this profile increases gradually without showing a pronounced structure. At the Mohorov010Di0107 discontinuity whose depth is about 27 km the velocity increases from 6.8 to 8.0 km s-1. The zone immediately below the Moho is homogeneous, followed by an increase in velo
An extension of the Aki (1969) coda model takes into account the instrument and Earth Q-filter effects on the dispersion of the coda. The coda dispersion and coda shapes are easily derived and plotted as a function of t* = t/Q. Observed data can be overlaid on master curves to directly estimate shear-wave Q and to normalize for earthquake moment to estimate the attenuation coefficient of γ. The method works well when the earthquakes studied have corner frequencies greater than the combined peak response of the instrument and Earth Q-filters. Comparing results from central California and the southeastern United States, there is a slight indication that the excitation of the coda is inversely proportional to Q.
The amplitude ratio of the maximum motion before Sn (Pmax) to the maximum after Sn (Lg) is a good discriminant between earthquakes and explosions at regional distances. Experimental data shows that the ratio is not seriously affected by site or source geology or by event depth. However propagation effects can be severe, especially from the USSR to the South, and can lead to the requirement for regionalization. The amplitude-distance relation for Pmax and Lg is A ~ r−3 in the Western United States (WUS), and r−2.5 and r−2.0 for Pmax and Lg respectively in the EUS and within the USSR. The radial to transverse ratio for Lg does not appear to be a discriminant, being controlled by the local geology. Evidence is presented that there are no useful spectral discriminants in the 1 to 10 Hz band. It appears that spectra of Lg are contaminated by compressional wave coda which can lead to an overestimate of the high frequency energy in the direct Lg wave.
Surface shear waves (Lg) with initial period about 1/2 to 6 sec. with sharp commencements and
amplitudes larger than any conventional phase have been recorded for continental paths at distances
up to 6,000 km. These waves have a group velocity of 3.51±.07 km/sec. and for distances
greater than 20° they have reverse dispersion. For distances less than about 10° the periods shorten
and Lg merges into the recognized near-earthquake phase Sg.
An additional large amplitude phase in which the orbital motion of the particle is retrograde
elliptical and the velocity is 3.05±.07 km/sec. has also been observed for continental paths.
It is believed that these phases are propagated through a wave guide formed by a superficial
sialic layer. The problem of explaining the propagation of these surface waves is that of finding a
crustal structure which is consistent with the other data of geology and geophysics and which will
provide a suitable wave guide for the new phases. A possible nature of the wave guide is described.
We present a theoretical study of the generation and geometrical attenuation of regional crustal phases. We do this through the computation of seismograms in the epicentral distance range from 60 to 500 km. The geometrical attenuation of Lg waves with epicentral distance is of the form r−0.83. Pg wave amplitudes display a much stronger decay of the form r−1.5. The spectral density of the crustal transfer function for Pg waves is relatively flat for frequencies between 0.1 and 5 Hz while Lg wave spectra strongly fall off beyond 2 to 3 Hz.
The excitation of Pg wave is insensitive to the depth of the source within the crust while the Lg amplitude is about 50 per cent higher for a source in the upper and middle crust than in the lower crust. The amplitudes of these two phases drastically decrease when the source is below the Moho. These results illustrate the important role of wave guide played by the crust for the propagation of Lg and Pg.
We find that the geometrical attenuation of Pg and Lg waves is independent of source mechanisms. In the case of an explosion, the excitation of Pg is insensitive to the source depth. The Lg wave amplitude is small in comparison to Pg and Rayleigh waves and depends on the closeness of the source to an interface or to the free surface.
This paper summarizes the use of regional (Δ ≦ 30°) seismic data in a test ban context for detecting, locating, identifying, and determining the yield of underground nuclear explosions. In many areas of the world (Eastern North America, Africa, Eastern USSR), Lg is the largest amplitude wave recorded on standard seismograph systems and thus is the most appropriate phase for monitoring small magnitude events. Excellent location capability for near-regional events has been demonstrated at the Norwegian small aperture array (NORESS) using Lg and P waves. Lg and other regional phases may contain information on source depth, but such information has not been exploited to date. Fifteen classes of regional discriminants have been identified including
Each of these proposed discriminants has, with differing degrees of success, separated some explosions from some earthquakes. However, most have been tested only on limited data, usually from one geographic region and only one or two recording stations. No systematic analyses have been done to determine the best individual discriminant or combination of them.
Preliminary evaluation of the use of Lg for yield determination suggests that regional waves hold promise in this application. Theoretical studies have contributed significantly to the understanding of propagation characteristics of regional waves but further studies are required emphasizing modeling for realistic anisotropic sources.
The major conclusion of this study is that a systematic and comparative evaluation of all the proposed regional discriminants is now required, utilizing a common data base derived from all present-day test sites. This evaluation would suggest the optimal discrimination procedure using regional waves, and would also define areas of needed research. Without such an integrated evaluation, it is still possible to speculate, using existing results, on the most promising regional discriminants.
Explosions are more efficient than earthquakes in exciting P waves, but earthquakes are more efficient than explosions in exciting Rayleigh waves. If an earthquake and an explosion have the same mb, then Ms for the earthquake should be about 0.5 larger than for the explosion. These results are derived by taking into account the moment tensor of a particular source and the partition of potential energy into compression and shear.
Lg propagates efficiently with predominant periods of about 1 to 1 1/2 s across the stable regions of Asia: the Indian shield, the Eurasian platform, the Tarim basin, and (less efficiently) eastern China. This suggests that the structure is relatively homogeneous and Q must be high in the crust in these regions. For paths across the Tien Shan, where the crust is abnormally thick, Lg is observed with somewhat less sharp arrivals, but nevertheless, frequencies are high, and amplitudes large. For paths along the TienShan, signals ae weaker, and arrivals are still less sharp. Lg was not observed for paths crossing the Tibetan Plateau. We infer that the change in crustal structure on the margins of the Tibetan Plateau and at the Tien Shan may disrupt the wave guide for Lg and may scatter energy out of the relatively homogeneous wave guide of the more stable crust. Alternatively or in addition, Lg may not propagate across the Tibetan Plateau becuase of an unusual velocity structure or because of high attenuation in the crust.
Teleseismic P n waves are modeled as a sum of whispering galley waves, which propagate in a waveguide composed of a simple high velocity mantle lid underlain by a low velocity zone. This model is able to account for those P n wave propagation properties that are not dominated by scattering, i.e., their apparent velocity and lack of long period energy. P n apparent velocity data constrain the P wave velocity gradient in the upper 100 km of mantle to be low; dv p /dz<0.001 s ⁻¹ . The whispering gallery rays are shown to have significant amplitude (when compared to the direct P wave) and to have spectra that rapidly fall off at long periods. Yet this model cannot account for certain details of the observed P n amplitude spectra. Most important among these is the spectral ratio of P n to P, which the model underestimates by a factor of 10. Nevertheless, the model presents a useful framework for understanding some characteristics of P n wave propagation and provides an estimate of the distribution of energy with depth in the P n waves.
The interaction of a seismic wavefield with a heterogeneous medium can be described by using a plane-wave decomposition and allowing for coupling between different horizontal wavenumber vectors. Such a procedure is possible for a generally anisotropic medium but is simplified when the seismic parameters can be represented via a horizontally stratified reference structure with superimposed lateral heterogeneity. This form of the seismic velocity distribution is well suited to describing the conditions in the crust and mantle. With this parameterization the reflection and transmission properties of a region can be represented as a combination of a term associated with the stratification and a second which represents the wavenumber mixing introduced by the presence of the heterogeneity.
The response of the heterogeneous medium to excitation by a source can be expressed in terms of reflection and transmission operators which can be derived from the plane-wave representation by inverse Fourier transformation. These operators provide a physically based treatment which encompasses the matrix methods used for stratified media. The operators can be split into a part associated with the reference medium and a part representing the heterogeneity terms. This allows the terms corresponding to the stratified reference model to be extracted from the full response in a way which clearly shows the modifications wrought by the presence of lateral heterogeneity. Further, by employing partitioned operators it is possible to examine the interaction between wavetypes, especially in the case of multiple interactions. An explicit form for the partitioned operator inverse enables a convenient approximate development to allow a simple expression for coupling between wavetypes which can be used to assess the extent to which scalar wave propagation schemes can be employed in the presence of heterogeneity.
In an effort to study the effect of strong crustal lateral variations on the propagation characteristics of Lg waves, we perform a numerical modelling of these waves at 1Hz in a graben structure. We use a coupled mode method and a simplified 2-D model of the North Sea Central Graben.
We show that at 1 Hz the energy reflected by the structure is negligible, and that on the average only 20 per cent of the Lg incoming energy is converted to waves propagating into the mantle. The energy remaining in the Lg wavetrain is subject to a strong redistribution with depth, which has an effect at the surface of the model which is very dependent on source type and depth, multiplying the surface amplitude by a factor of 0.6 for shallow sources and 0.9–1.4 for mid-crustal sources, on the average. The phase of the wavetrain is shown to be stable enough with period to avoid a destructive interference of the signal. Crossing the Graben perpendicularly or at an angle leads to equivalent results.
Lg wave propagation appears to be surprisingly robust in crustal structures with strong lateral variations. The large-scale geometric features of the Graben prove insufficient to explain the strong attenuation of the Lg waves which has been observed in the North Sea area.
Recordings of regional earthquakes near Denmark and the North Sea have been collected in an attempt to find the Lg-wave propagation properties in the area. Propagation paths of lengths between 300 and 1100 km are investigated. The amplitudes of the Lg-waves and the other seismic phases in the seismograms are compared. The propagation paths are classified according to whether the Lg-wave propagation compared to P- and S-wave propagation is good, intermediate or poor. It is noted that there is a distinct regional pattern in the Lg-wave attenuation. The Lg-wave propagation is poor across the North Sea. The North Sea is surrounded by areas of good, continental Lg-wave propagation except towards the south-east in the Netherlands and western Germany. An area in Skagerrak is characterized by good, but slow propagation of a surface wave similar to Lg. Since this is exactly the area of a cluster of earthquakes in Skagerrak, it is suggested that the two phenomena are caused by the same structural inhomogeneities in the area, just off the coast of north-western Denmark. The Lg-wave propagation is as good in the northern part of Denmark as it is in the Fennoscandidn shield.
The extent to which the propagation is poor across the North Sea is dependent on the frequency of the Lg-waves observed with different seismographs. The Lg-waves are more attenuated, relative to the other seismic phases, the higher the frequency is between 0.25 and 2 Hz.
As the structural differences between the Fennoscandian bedrock area and the Norwegian—Danish basin with 8–10 km of sediments has no effect on the Lg-wave propagation, it is suggested that the structural anomalies causing the poor Lg-wave propagation across the North Sea are deep crustal anomalies, possibly significant variations in crustal thickness. It is likely that such deep crustal differences are located along the most proniinent upper crustal differences, in the sedimentary grabens in the middle of the North Sea. The poor propagation across the North Sea is similar to that observed across a sedimentary basin in eastern Greenland, which also developed mainly before seafloor spreading separated Greenland from Europe. The frequency-dependent propagation of the Lg-waves is explainable in terms of surface wave propagation across structural boundaries as well as in terms of anelastic attenuation.
The short-period seismic phases known as and Lg are often recorded at distances of 200–1000 km on long-range refraction profiles and are usually the largest-amplitude features on record sections for this distance range. and Lg propagate as multiply reflected compressional and shear waves in a crustal waveguide whose principal boundaries are the Moho and the free surface. Equivalently, they can be interpreted as the interference pattern produced by a superposition of higher-mode P, SV and SH waves propagating in a leaky waveguide. For compressional waves, the waveguide efficiency is a strong function of frequency and depends on the presence or absence of low-velocity layers within a few kilometers of the surface, such as deep sedimentary sections commonly found in active tectonic areas. Such low-velocity surface layers create constructive interference effects for upcoming P waves incident at near grazing angles at the free surface and lead to efficient propagation. Several good examples of strong phases can be found on long-range refraction profiles for the tectonically active western United States; the 550 km profile eastward from SHOAL to Delta, UT is analyzed here. We have used a modified reflectivity-method computer program to model crustal phases for the SHOAL-Delta profile. The reflectivity technique accounts for all body and surface waves contributing to the short-period seismograms. It is found that the synthetic waveforms realistically model the observed characteristics. In this case, the decay of amplitudes with distance appears to be dominated by surface-reflection leakage from the waveguide rather than by anelastic attenuation due to Q of crustal rocks.
Reflection and transmission operator methods are particularly effective in problems where multiple reflections are important. When two regions are joined together, the overall reflection operators for the composite region can be built up from the reflection and transmission operators for the individual regions. The two regions are linked by a sequence of successive reflections between the zones which may be represented via a reverberation operator. For source problems, the model can be broken along a surface passing through the source and the overall displacement can be found by following the physical sequence of propagation processes. This leads to a representation in terms of reflection and transmission operators for the regions above and below the source. The two zones are linked by the reverberation operator for the structure, which determines the modal characteristics in the model.
This study demonstrates the existence and determines the pattern of lateral variations of attenuation in the upper%most mantle on a worlwide scale. The evidence comes mostly from a comparison of the gross characteristics of the seismic phase Sn for over 1500 paths, which taken together cross as many regions of the earth as possible with the current configuration of the World-Wide Standardized Seismograph Network. Sn is a seismic shear wave that propagates in the uppermost mantle and that does not penetrate the low-velocity channel.
Das Beben von 8. Okt. 1909, Jahrb, meterol
- A Mohorovieic
MohoroviEiC, A., 1909. Das Beben von 8. Okt. 1909, Jahrb, meterol. Obs. Zagreb (Agram), 9, Teil IV
Lg wave propagation and crustal structure differences near Denmark and the North Sea A mechanism for the generation of short-period transverse motion from explosions, Bull. seism Q estimates using the coda of local earthquakes
- J R Soc Astr
- S Gregersen
- I N Gupta
- R R Blandford
J. R. astr. SOC., 33, 487-488. Gregersen, S., 1984. Lg wave propagation and crustal structure differences near Denmark and the North Sea, Geophys. J. R. astr. SOC., 79, 217-234. Gupta, I. N. & Blandford, R. R., 1983. A mechanism for the generation of short-period transverse motion from explosions, Bull. seism. SOC. Am., 73, 571-591. Herrmann, R. B., 1980. Q estimates using the coda of local earthquakes, Bull. seism. SOC. Am., 70, 447-468
Seismic discrimination problems at regional distances, in Ident@cation of Seismic sources -Earthquake or Underground Explosion
- R R Blandford
Blandford, R. R., 1980. Seismic discrimination problems at regional
distances, in Ident@cation of Seismic sources -Earthquake or
Underground Explosion, eds Husebye, E. S. & Mykkeltveit,
S., pp. 695-740, Reidel, Dordrecht.
Das Beben von 8. Okt. 1909
- Mohorovičić
The discrimination potential of crustal resonance phasesAFGL Technical Report
- L. J. Burdick
- D. V. Helmberger