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Shear wave crustal velocity model of the Western Bohemian Massif from Love wave phase velocity dispersion
Abstract and Figures
We propose a new quantitative determination of shear wave velocities for distinct geological units in the Bohemian Massif,
Czech Republic (Central Europe). The phase velocities of fundamental Love wave modes are measured along two long profiles
(~200km) crossing three major geological units and one rift-like structure of the studied region. We have developed a modified
version of the classical multiple filtering technique for the frequency-time analysis and we apply it to two-station phase
velocity estimation. Tests of both the analysis and inversion are provided. Seismograms of three Aegean Sea earthquakes are
analyzed. One of the two profiles is further divided into four shorter sub-profiles. The long profiles yield smooth dispersion
curves; while the curves of the sub-profiles have complicated shapes. Dispersion curve undulations are interpreted as period-dependent
apparent velocity anomalies caused both by different backazimuths of surface wave propagation and by surface wave mode coupling.
An appropriate backazimuth of propagation is found for each period, and the dispersion curves are corrected for this true
propagation direction. Both the curves for the long and short profiles are inverted for a 1D shear wave velocity model of
the crust. Subsurface shear wave velocities are found to be around 2.9km/s for all four studied sub-profiles. Two of the
profiles crossing the older Moldanubian and Teplá-Barrandian units are characterized by higher velocities of 3.8km/s in the
upper crust while for the Saxothuringian unit we find the velocity slightly lower, around 3.6km/s at the same depths. We
obtain an indication of a shear wave low velocity zone above Moho in the Moldanubian and Teplá-Barrandian units. The area
of the Eger Rift (Teplá-Barrandian–Saxothuringian unit contact) is significantly different from all other three units. Low
upper crust velocities suggest sedimentary and volcanic filling of the rift as well as fluid activity causing the earthquake
swarms. Higher velocities in the lower crust together with weak or even missing Moho implies the upper mantle updoming.
KeywordsLove waves-Phase velocity dispersion-Frequency-time analysis-Structure inversion-Bohemian Massif
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... By analyzing teleseismic records with the receiver function technique (e.g., the receiver functions at stations MOX, WET, BRG, and NKC shown in Figure 1), Wilde-Piórko et al. (2005) revealed the existence of an S-wave LVZ in the middle crust (depth range of 10-15 km) of the northwestern Bohemian Massif. With the help of Love wave phase velocity dispersion curves, Kolínský et al. (2011) discovered an S-wave LVZ in the middle and lower crust beneath the TB zone and a gradually increasing S-wave velocity structure in the crust of the ST zone. By applying ambient seismic noise interferometric surface wave tomography (ASNT) to the recordings of broadband seismic stations, Růžek et al. (2016) inverted group and phase dispersion curves to obtain crustal velocity models for the Bohemian Massif that monotonically increase with depth; they concluded that the differences among different tectonic units are small and that the most homogeneous part among them generally being the middle crust. ...
... Nevertheless, the crustal models derived from previous studies for the Bohemian Massif present different, even contradictory results regarding the existence of the LVZ in the middle crust (e.g., Enderle et al., 1998;Wilde-Piórko et al., 2005;Grad et al., 2008;Kolínský et al., 2011;Růžek et al., 2016;Kästle et al., 2018;Lu et al., 2018;Kvapil et al., 2021). To address this inconsistency, in this study, we apply our newly developed multimodal ambient noise dispersion curve tomography method, denoted the frequency-Bessel transform (F-J) method , to investigate the crustal structure beneath the study area by using available ambient seismic noise datasets. ...
... The models obtained via the joint inversion of multimodal dispersion curves based on the F-J method reveal obvious lowvelocity characteristics in the crust and uppermost mantle beneath the study area. Other studies similarly provided evidence for the existence of an LVZ in this region (e.g., Wilde-Piórko et al., 2005;Kolínský et al., 2011;Kvapil et al., 2021). Based on our inversion results, we construct a simple S-wave velocity profile of the crust and uppermost mantle beneath the western side of the study area ( Figure 14B). ...
The northwestern Bohemian Massif and adjacent areas are a tectonically active region associated with complex geodynamic activities, that manifest as Quaternary volcanism, earthquake swarms in the upper and middle crust, degassing of CO2, and crustal fluid migration. The intricate tectonic evolution and activities of this region reflect the complexity of the crustal structure therein. However, the crustal models derived from previous studies in this area offer different, even contradictory information regarding the existence of a mid-crustal low-velocity zone (LVZ). In this study, we apply the frequency-Bessel transform (F-J) method to extract the fundamental-mode and up to five higher-mode Rayleigh wave dispersion curves from ambient seismic noise data recorded in the study area and perform multimodal ambient noise dispersion curves inversion. The addition of higher-mode dispersion curves enhances the vertical resolution of the velocity structure inversion results. Our models support the view that the general S-wave velocity level of the crust is high within the study area. We detect two S-wave LVZs beneath the study area that are distributed mainly in the middle crust rather than the lower crust, and these LVZs are separated by a high-velocity zone. Considering the results of previous studies in the area, we infer that these S-wave LVZs may be the consequence of crustal fluids, plastic deformation and even partial melting of the felsic middle crust at relatively high crustal temperatures. Furthermore, these S-wave LVZs could be responsible for the origin and foci depth distribution of earthquake swarms. S-wave low-velocity anomalies are also observed in the uppermost mantle beneath the study area. These S-wave models based on the joint inversion of multimodal dispersion curves can provide new references for understanding the tectonic activity and geodynamic evolution of the northwestern Bohemian Massif and adjacent areas.
... Most of the events are located in the Pacific seismic zone stretching from Chile to Japan, which results in reduced sampling from the south-east (Fig. 2a). Following Kolínský et al. (2011), a two-station method is used to determine phase velocity of the Rayleigh waves recorded on the Z and R components. We start by multiple filtering of each record. ...
... The number of layers was also tested to be sufficient for imaging the structural variations. Higher number of layers do not bring any qualitative change of the resulting models (Kolínský et al., 2011), it only smears the velocity gradients over more thinner layers. The ak135 model is used to set the density and vP/vS ratio. ...
... The resulting best model is then used as a starting model for the inversion at all grid nodes. While the inversion does not depend on the starting model, using a model close to the final one saves computational time (Kolínský et al., 2011(Kolínský et al., , 2014. Unrealistic velocity oscillations are avoided by constraining the velocity difference between neighboring layers. ...
The interaction between the Adriatic microplate (Adria) and Eurasia is the main driving factor in the central Mediterranean tectonics. Their interplay has shaped the geodynamics of the whole region and formed several mountain belts including Alps, Dinarides and Apennines. Among these, Dinarides are the least investigated and little is known about the underlying geodynamic processes. There are numerous open questions about the current state of interaction between Adria and Eurasia under the Dinaric domain. One of the most interesting is the nature of lithospheric underthrusting of Adriatic plate, e.g. length of the slab or varying slab disposition along the orogen. Previous investigations have found a low-velocity zone in the uppermost mantle under the northern-central Dinarides which was interpreted as a slab gap. Conversely, several newer studies have indicated the presence of the continuous slab under the Dinarides with no trace of the low velocity zone. Thus, to investigate the Dinaric mantle structure further, we use regional-to-teleseismic surface-wave records from 98 seismic stations in the wider Dinarides region to create a 3D shear-wave velocity model. More precisely, a two-station method is used to extract Rayleigh-wave phase velocity while tomography and 1D inversion of the phase velocity are employed to map the depth dependent shear-wave velocity. Resulting velocity model reveals a robust high-velocity anomaly present under the whole Dinarides, reaching the depths of 160 km in the north to more than 200 km under southern Dinarides. These results do not agree with 1 most of the previous investigations and show continuous underthrusting of the Adriatic lithosphere under Europe along the whole Dinaric region. The geometry of the down-going slab varies from the deeper slab in the north and south to the shallower underthrusting in the center. On-top of both north and south slabs there is a low-velocity wedge indicating lithospheric delamination which could explain the 200 km deep high-velocity body existing under the southern Dinarides.
... The assumption about great-circle-parallel propagation biases surface wave tomography (Lay & Kanamori 1985). For that reason many studies have corrected the two-station measurements of phase velocities for the true arrival angles (Baumont et al. 2002;Pedersen 2006;Kolínský et al. 2011;Foster et al. 2014b;Chen et al. 2018). Some (e.g. ...
... It takes advantage of two traditional approaches. The first one is the classical two-station method of measuring phase velocities between pairs of stations in the time domain as described by Kolínský et al. (2011). The second approach is array beamforming for determining the phase velocity and the true arrival angles of propagation for a given array. ...
... The measurement of time differences between quasimonochromatic signals is carried out according to the method described by Kolínský et al. (2011Kolínský et al. ( , 2014. Filtered and tapered quasi-monochromatic signals are shifted in time sample by sample and their correlation is computed for each time shift. ...
The dense AlpArray network allows studying seismic wave propagation with high spatial resolution. Here we introduce an array approach to measure arrival angles of teleseismic Rayleigh waves. The approach combines the advantages of phase correlation as in the two-station method with array beamforming to obtain the phase-velocity vector. 20 earthquakes from the first two years of the AlpArray project are selected, and spatial patterns of arrival-angle deviations across the AlpArray are shown in maps, depending on period and earthquake location. The cause of these intriguing spatial patterns is discussed. A simple wave-propagation modelling example using an isolated anomaly and a Gaussian beam solution suggests that much of the complexity can be explained as a result of wave interference after passing a structural anomaly along the wave paths. This indicates that arrival-angle information constitutes useful additional information on the Earth structure, beyond what is currently used in inversions.
... These dispersion curves are averaged and then the representative dispersion curve is inverted to obtain a 1D S-wave velocity profile representing the monitoring network. We apply the methodology for the surface wave inversion developed and tested by Kolínský et al. (2011) and recently used by Belinić et al. (2021) with layer boundaries derived from the P-wave model. This allows us to constrain S-wave velocities in geological layers and compare resulting Vp/Vs ratio with known geology. ...
... To reconstruct the S-wave velocity profile, we invert the Rayleigh wave group velocity dispersion using the method developed and tested by Kolínský et al. (2011). The inversion starts with an initial velocity profile which is randomly modified in each iteration. ...
... For the purposes of the current paper, we reprocessed the data mainly to obtain the measurements for more periods. To do so, we followed the procedure of phase velocity measurement described by Kolínský et al. (2011) and the array processing developed by Kolínský et al. (2014). The overall procedure is the same as used by Kolínský and Bokelmann (2019). ...
... Group velocity was estimated using the method described by Kolínský and Brokešová (2007) based on multiple filtering. This procedure is anyway part of the phase velocity calculation, as mentioned in Kolínský et al. (2011). Figure The comparison of the phase-wavefront propagation is shown in Figure C1, lower panel. ...
Stripe‐like patterns of surface wave arrival angle deviations have been observed by several seismological studies around the world, but this phenomenon has not been explained so far. Here we test the hypothesis that systematic arrival angle deviations observed at the AlpArray broadband seismic network in Europe are interference patterns caused by diffraction of surface waves at single small‐scaled velocity anomalies. We use the observed pattern of Rayleigh waves from two earthquakes under the Southern Atlantic Ocean, and we fit this pattern with theoretical arrival angles derived by a simple modeling approach describing the interaction of a seismic wavefield with small anomalies. A grid search inversion scheme is implemented, which indicates that the anomaly is located in Central Africa, with its head under Cameroon. Moreover, the inversion enables the characterization of the anomaly: The anomaly is inferred to be between 320 and 420 km wide, matching in length the 2,500 km long upper mantle low‐velocity region under the volcano‐capped swells of the Cameroon volcanic line. We show that this approach can be generally used for studying the upper mantle anomalies worldwide.
... The result of the inversion is independent of the starting model. However, by setting a proper starting model which is close to the resulting one, we can save the computational time (Kolínský et al., 2011). ...
... Such a group velocity dispersion curve is considered as reliable and filter-independent. For details of this approach see Kolínský (2004); Kolínský and Brokešová (2007) and Kolínský et al. (2011). ...
The study area of Western Carpathians belongs to one of the most seismically active regions in Slovakia, where the available velocity models are not precise enough for accurate location and determination of focal mechanism. To improve the uppermost part of the currently used models, seismic surface wave data from two blasts in the quarry Rohožník, recorded at the MKnet local seismic network, were analyzed. Group velocity dispersion curves of Rayleigh and Love waves were determined by the frequency-time analysis. A mean group velocity dispersion curve for Love and Rayleigh waves for the central part of the MKnet network was settled and consequently joint inversion, for the period range 0.9 – 2.7 s, was performed to obtain a 1-D velocity model using the isometric method. The 11-layered model extends to depth of 2.5 km and indicate increase of the velocities from the surface down to the depths of 0.9 km. Then, velocities slightly decrease. The minimum of the low-velocity zone is at the depth of 1.8 km after which the velocities moderately increase to the velocity of the halfspace with vP = 5.35 km/s and vS = 2.96 km/s.
... Using the modeling approach by Nolet and Dahlen (2000), an array measurement of surface wave phase velocities based on the two-station method (Kolínský et al. 2011;Legendre et al. 2014) and array beamforming technique (Kolínský et al. 2014) can be computed. The predicted wavefield of the surface-wave fundamental mode Rayleigh waves over a selected region can be computed (Kolínský et al. 2019). ...
Data quality is of utmost importance in assessing seismic hazards and risk mitigation related to geohazards. Erroneous data incorporation may lead to inaccurate observations and judgments. Lack of data or clipped data can be dealt with easily, but errors in the data (e.g., timing, amplitudes) or in the metadata (e.g. location, gain) can induce some strong bias in the estimation of geohazards. As a result, strict and standard quality controls are required to guarantee the accuracy of the assessments. This chapter will discuss some of the common issues with seismological data and offer tips on identifying inaccuracies. Despite recent efforts by the seismological community, there is no uniform framework to identify the errors in seismological data nor to perform some established quality controls. Therefore, we will examine the current state of the art and best practices in seismological quality controls. Some recommendations for new implementations, including some cutting-edge technologies like “Dynamic Time Warping” will be suggested. Missing data (i.e., gaps) are permanent, but all erroneous data do not need to be discarded. In some cases, identifying the problem in the data or metadata allows for reprocessing. The problems with data availability will be examined first, then the challenges with data quality. Finally, a discussion on the metadata errors will be provided.KeywordsData qualityData metricsSeismic network performanceSeismologySeismic hazard
... The region has become the subject of many studies , and many references found in that review). Since the 1990s, many seismic structural measurements of a local or regional extent have been performed in the region (Novotný et al., 2013), namely the deep seismic sounding (Novotný, 1997), vertical incidence reflection (Tomek et al., 1997), surface wave dispersion (Kolínský & Brokešová, 2007;Kolínský et al., 2011), or the receiver function method (Wilde-Piórko et al., 2005;Heuer et al., 2006). The extended refraction and wide-angle reflection measurements GRANU'95 (Enderle et al., 1998), CELEBRATION 2000(Hrubcová et al., 2005, and SUDETES 2003 (Grad et al., 2003) enabled an investigation of the crust of the West Bohemia/Vogtland region on the regional scale up to Moho depths. ...
The intraplate West Bohemia region is characterized by relatively frequent earthquake swarms mostly concentrated in the vicinity of the Nový Kostel village. Records of selected microearthquakes of the 2008 seismic swarm from a small circular area around Nový Kostel with a radius of 11 km are processed with focus on availability of precise onset readings from all the selected events at all the selected stations. These data were used in the joint hypocenter-velocity inversion, employing the so-called isometric method which is especially efficient for this task. The results are two new one-dimensional velocity models, the constrained (smoother) and the unconstrained one, of the upper crust down to the depth of 11 km, and absolute locations of the selected earthquakes in the new constrained model. The foci delineate two segments of a steeply dipping fault zone. Time residuals at each of the stations display a remarkable azimuthal dependence, for both P and S waves, caused by anisotropy. No systematic dependence on epicentral distance was found even for stations outside the target area, which means that the new 1D models seem to be reasonable approximations of the real structure in the whole, much larger, West Bohemia region. No time dependence indicating possible changes of \(v_P\), \(v_S\) and their ratio during the swarm was observed. A set of P- and S-wave station corrections that should be used to correct onset times in order to reach precise absolute locations was derived as an integral part of the new models.
In this study, we show results from ambient noise tomography around the KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland), a continental deep drilling site located at the western edge of the Bohemian Massif, within the Variscan belt of Europe. At the KTB site, crustal rocks have been drilled down to 9 km depth. Before the drilling activity started, several active seismic surveys had been performed to explore its surroundings during the ’80s and early ’90s, in the frame of an extensive exploration of the area aimed at unraveling the characteristics of the continental lower crust that is exposed at surface in this location. Despite the exploration campaigns held at and around the KTB drilling site, there are important targets that are worth further investigation; these are related in particular to the obduction of lower crustal units to the surface, and to the mechanism of orogenic processes in general. Here we present a new 3D shear-wave velocity model of the area from cross-correlations of ambient seismic noise. The model is obtained by a unique data-set composed of two years of continuous data recorded at nine 3-component temporary stations (installed from July 2012 to July 2014) located on top and around the drilling site, and together with the data from 19 permanent stations throughout the region. This paper is focusing on the upper crustal layers, and we show velocity variations at short scales that correlate well with known geological structures in the region of the KTB site, at the surface and at depth. These are used to discuss features that are less well-resolved at present.
To calculate phase-velocity dispersion curves, we introduce a method which reflects both structural and dynamic effects of wave propagation and interference. Rayleigh-wave fundamental-mode surface waves from the South Atlantic Ocean earthquake of 19. August, 2016, M = 7.4, observed at the AlpArray network in Europe are strongly influenced by the upper-mantle low-velocity zone under the Cameroon Volcanic Line in Central Africa. Predicting phase-delay times affected by diffraction from this heterogeneity for each station gives phase velocities as they would be determined using the classical two-station method as well as the advanced array-beamforming method. Synthetics from these two methods are thus compared with measurements. We show how the dynamic phase velocity differs from the structural phase velocity, how these differences evolve in space, and how two-station and array measurements are affected. In principle, arrays are affected with the same uncertainty as the two-station measurements. The dynamic effects can be several times larger than the error caused by the unknown arrival angle in case of the two-station method. The non-planarity of the waves and its relation to the arrival angle and dynamic phase-velocity deviations is discussed. Our study is complemented by extensive review of literature related to the surface-wave phase-velocity measurement of the last 120 years.
We apply a taditional method of surpace wave tomographys as a new approach to investigate the uppermost crust velocities in the Western Bohemia region (Czeh Republic). It enables us to look for velocity distribution in a small scale of tens of kilometers. We measure Rayleigh wave group velocity dispersion curves in a period range 0.25-2.0s alon paths crossing the region of interest. We use modified multiple-filtering method for frequency-time analysis. We compute 2-D tomography maps of group velocity distrinution in the region for eight selected periods using the standard methods and programs described in literature. We discuss the velocity distribution with respect to results of former study by Nehybka and Skácelová (1997). We present a set of local dispersion curves which may be further inverted to obtain a 3-D shear wave velocity image of the area.
Measurements of group velocities by means of band-pass filtering lead to systematic errors when group in velocity changes rapidly with frequency. A new method hereafter referred to as “residual dispersion measurement” avoids this difficulty by transforming the signal prior to the filtering. An observed seismogram is cross-correlated with a theoretical seismogram in which the dispersion approximates the observed dispersion within a few per cent. Dispersion of the resulting pulse can be measured with a much higher precision, since du/dω has been reduced by at least an order of magnitude. In addition, the method can be employed iteratively to obtain a greater precision of measurement.
The dispersion of mantle Rayleigh waves is measured from a sum of 48 auto-correlograms of the recordings of the Alaskan earthquake of March 28, 1964. Group and phase velocities are obtained for order numbers between 0S9 and 0S47. It is estimated that the absolute error of the group-velocity measurement does not exceed 0.015 km/sec.
The analysis of a sum of 13 auto-correlograms of horizontal component seismograms with predominantly transverse motion reveals that the fundamental-mode Love-wave data may be contaminated by higher torsional modes. Comparison of several sets of average Love-wave phase velocities shows a discrepancy of the order of 0.4 per cent in the period range from 350 to 170 sec. The corresponding figure for Rayleigh waves is less by at least an order of magnitude.
Version 3.1 of the Generic Mapping Tools (GMT) has been released. More than 6000 scientists worldwide are currently using this free, public domain collection of UNIX tools that contains programs serving a variety of research functions. GMT allows users to manipulate (x,y) and (x,y,z) data, and generate PostScript illustrations, including simple x-y diagrams, contour maps, color images, and artificially illuminated, perspective, and/or shaded-relief plots using a variety of map projections (see Wessel and Smith [1991] and Wessel and Smith [1995], for details.). GMT has been installed under UNIX on most types of workstations and both IBM-compatible and Macintosh personal computers.
A summary of seismic surface waves and time-frequency analysis theory is presented here. The main goal is to introduce the multiple-filter technique, which is to be used for processing of records. The SVAL program has been built and tested. The SVAL computes the spectrogram of a given signal, a filtered spectrogram, a filtered seismogram and group velocity dispersion curves of Rayleigh and Love fundamental modes. Filtered seismograms reveal also other wavegroups contained in a record of each component. The dispersion of overtones and Rayleigh waves at a transverse component and Love waves at vertical and radial component is also studied. The function of the program is demonstrated on Asian earthquakes recorded at the Praha seismic station.
Phase velocity as a function of period has been determined for Rayleigh waves in the period range 100 to 400 seconds. The results were derived from a study of seismograms from the southeastern Alaska earthquake of July 10, 1958, and from published data on the Assam earthquake of August 15, 1950. The method depends on measurement of the travel time of wave crests along an arc of known length, with proper correction for change of period with distance. For observations of a single Rayleigh wave train at a single pair of observing stations, crest identification is uncertain, and so too is the resulting curve of phase velocity versus period. A set of phase velocity curves may be computed, each one corresponding to a different choice of crest identification. Only one of these is consistent with the data from several earthquakes and several pairs of observing stations. In the present work, high precision in phase velocity measurement is achieved by using the observations of the Rayleigh waves R3 and R5 at Pasadena of the Assam earthquake. Data from the southeastern Alaska earthquake are used to resolve the ambiguity resulting from uncertainty in crest identification. The final phase velocity curve is estimated to be accurate to better than one per cent in the range of periods 100 to 400 seconds.
Fundamental mode Rayleigh and Love wave phase velocities in the period range 5 to 80 secends have been measured in the central U. S. Using the concept of constant partial derivatives of phase velocity with respect to layer parameters in a least-squares inversion scheme a structural model for the area was determined. Several parameters were constrained on the basis of other available data. It was not possible to fit satisfactorily both Rayleigh and Love data with a simple isotropic model. Conclusions require either anisotropy in the upper mantle or systematic errors in the phase velocity measurements.
We investigate the upper mantle shear-wave speed structure beneath the south Indian shield by measuring and modeling fundamental mode Rayleigh wave phase-velocity dispersion. Observed phase velocities for the south Indian shield closely match those observed for the Canadian shield. We constrain the south Indian crust using published receiver function results and invert the dispersion data for upper mantle shear-wave structure. The 155-km-thick seismic lithosphere of the south Indian shield is composed of a 35 km-thick, two-layer crust and a 120-km-thick, high-velocity upper mantle lid. Beneath the Moho the average S n wave speed is 4.7 km sec 1. Both S n travel times data and the dispersion data suggest a positive sub-Moho shear-wave speed gradient. Beneath the seismic lithosphere there is a low-velocity layer where the shear-wave speed drops to 4.4 km sec 1 .
Two station fundamental mode Rayleigh wave phase velocity dispersion measurements for the south Indian Shield have been made between 20 and 200 s period. The dispersion curve for southern India is compared with those from other major shield regions. The south Indian Shield phase velocity dispersion matches closely the dispersion curve of the Canadian Shield (CANSD) while the Siberian Shield dispersion curve is much slower than the south Indian curve. The south African Shield phase velocity curve lies close to the south Indian curve at periods below 40 s, but is slower at longer periods. The differences in these phase velocity dispersion data from the different shield regions largely results from differences in the crustal structure between the various shields. The inversion of the south Indian dispersion data yields a 155±10~km thick high velocity upper mantle lid with Sn-velocity of 4.68~km/s, and an 80~km thick LVZ beneath below the high velocity lid.