Fig 2 - uploaded by Mamoru Nakamura
Content may be subject to copyright.
Example of travel-time diagrams for an earthquake. Reduction velocity is 7.5 km/s. Solid circles show travel time data.
Source publication
First P-wave arrival times recorded by the seismic network in the Ryukyu arc were analyzed in order to image the lateral variation in crustal thickness beneath the Ryukyu arc. The results indicate a low P
n velocity (7.5 km/s) in the Ryukyu arc and a relatively high P
n velocity (7.9 km/s) in the Okinawa Trough. The crustal thickness changes betwee...
Context in source publication
Context 1
... Ryukyu arc is an approximately 1200-km-long island arc that runs along the Ryukyu Trench between Kyushu Island and Taiwan. The Philippine Sea plate subducts beneath the Eurasian plate northwestward. The Okinawa Trough, which is at the beginning of the rifting stage and extends toward the NW-SE (Sibuet et al. , 1995), is located northwest of the Ryukyu arc. The northern-and-central and southern parts of both the Ryukyu arc and Okinawa Trough have different characteristics. The Ryukyu arc and the Okinawa Trough are divided geologically into three blocks (northern part, central part, and southern part) that are bounded by the Tokara Strait and the Kerama Gap (Konishi, 1965) ( Fig. 1). The southern Okinawa Trough (SOT) and the northern-and-central Okinawa Trough (NCOT) have different features. The NCOT is a gentle depression with a maximum bathymetric depth of 1000 m. In contrast, the SOT consists of a broad and flat basin with a bathymetric depth of 2000 m. In the northern-and-central Ryukyu arc, the volcanic front is located to the east of the axis position of the Okinawa Trough (Fig. 1); in the southern Ryukyu arc, the volcanic front is located at the axis of the Okinawa Trough. Surveys conducted using an ocean bottom seismometer (OBS) have revealed a distinct difference in crustal structure between the northern-and-central Okinawa Trough and the SOT. The Moho in the NCOT is flat, with a crustal thickness of 26 km; in contrast, the crustal thickness is 18 km in the SOT. Several OBS surveys have been carried out in the Ryukyu subduction zones in order to image the crustal structures and OBS velocity models have been con- structed to model crustal thickness and P n velocity for the various parts of the Ryukyu arc and Okinawa Trough. Ac- cording to the respective models, the crustal thickness and P n velocity for the northern Ryukyu are 26 km and 7.6 km/s, respectively (Iwasaki et al. , 1990); for the central Ryukyu arc, 26 km and 7.7–7.8 km/s, respectively (Nakahigashi et al. , 2001); for the SOT, 15 km and 8.2 km/s, respectively (Lee et al. , 1980); for the SOT, 18 km and 7.8 km/s, respectively (Hirata et al. , 1990). The results of a gravity anomaly analysis aimed at deter- mining the crustal structure provides support for the results of the seismic survey. The crust model estimated from the gravity anomaly also reveals differences in crustal thickness between the central Okinawa Trough and the SOT (Sibuet et al. , 1995). Although crustal thickness is generally regu- lar in the NCOT, that in the SOT is thinner beneath the axis and thickens in increasing distance from the axis. Therefore, according to the model predictions, crustal thickness is different between the NCOT and the SOT, and this difference may affect the crustal structure in the Ryukyu arc. The change in crustal thickness along the Ryukyu arc may yield clues to the tectonic process of the extension of the Okinawa Trough. With the aim of inves- tigating the crustal structure along the Ryukyu arc, we imaged variations in crustal thickness and averaged P n velocity. We selected 96 events with depths 20 km and magni- tudes > 3.5 from the Japan Meteorological Agency (JMA) catalog. The P n travel time data from the 19 stations of the JMA network and one station from the National Research Institute for Earth Science and Disaster Prevention (NIED) network were used to investigate the variations in crustal thickness. Almost all of these stations are distributed along the Ryukyu arc. Data from a station located in the Philippine Sea plate (MINAM2) were also included in the study. A total of 744 first P -wave arrival time data were selected from all of the events that occurred from January 1997 to August 2003. The epicentral distances are in the range of 150–550 km from the stations (Fig. 2). Ray paths for the data set are shown in Fig. 3. Most of these P n arrivals passed along the Ryukyu arc. The travel time residuals were inverted for station delays and event delays using the modified time-term equation (Hearn et al. , 1994): where a i is the static delay for station i , b j is the static delay for event j , d i jk is the distance traveled by ray i j in mantle cell k , and S k is the slowness (inverse of P n velocity) of cell k . The P n velocity in the cell has been changed in three areas: the Okinawa Trough (OT) area, Ryukyu arc (RA) area, and the Philippine Sea plate (PHS) area (Fig. 3). The boundary between the Philippine Sea plate and the Ryukyu arc is set to the position of the Ryukyu Trench, while that between the Ryukyu arc and the Okinawa Trough is set to the position of the western margin of the Ryukyu arc, where the Ryukyu arc is separated from the Okinawa Trough by faults. The delay between the stations at NAKANO and KUCHIE, which are located north of the Ryukyu arc, has been fixed to 2.6 s using the velocity structure (Iwasaki et al. , 1990). Figure 4 shows the variation in root mean square (RMS) travel time residuals as a function of P n velocity in each area. It can be seen from this figure that the minimum RMS travel time residuals of P n velocity in the Philippine Sea plate cannot be constrained well. We then imaged the RMS, fixing the P n velocity of the Philippine Sea plate at 8.0 km based on the refraction survey in the Philippine Sea plate (Murauchi et al. , 1968; Nishizawa et al. , 1983; Iwasaki et al. , 1990; Kodaira et al. , 1996). The minimum RMS is 0.57 s when the P n velocity beneath the Okinawa Trough and the Ryukyu arc are 7.9 and 7.5 km/s, respectively. Errors in the inversion were examined using the bootstrap method (Hearn and Ni, 1994). The averaged error in the station delays is 0.2 s. The maximum error in the station delays is 0.5 s and the maximum error in the P n velocity estimation is < 0.1 km/s. The largest errors in station delays occur at the Minami-Daito Island (MINAM2), which is located in the Philippine Sea plate. The station delays between the northern-and-central Ryukyu arc (range 2.4–3.8 s) and the southern Ryukyu arc (range 2.9–4.6 s) are an indication of differences in features between these two areas. The station delay at TAMAG2 station reaches 3.8 s, that at MINAM2 (PHS) reaches 1.7 s, and those near the Okinawa Trough range from 2.6 to 2.9 s. The errors of the station delays are less than 0.5 s. The station delays were converted to crustal thickness assuming a mean crustal velocity and mean mantle velocity of 5.9 and 7.5 km/s, respectively (Table 1). The station delay at MINAM2 is converted to crustal thickness assuming a mean crustal velocity and mean mantle velocity of 6.0 and 8.0 km/s, respectively (Murauchi et al. , 1968). The estimated crustal thickness is shown in Fig. 5. For the southern Ruykyu arc, it ranges from 29 to 44 km; for the northern-and-central Ryukyu arc, 23–37 km; for the Moho, 27–29 km near the Okinawa Trough. Crustal thickness at MINAM2 is estimated to be 16 km. The errors in the crustal thicknesses are < 5 km. The P n velocities in the Ryukyu arc and Okinawa Trough are 7.5 and 7.9 km/s, respectively. To check the results, we added the random noise of 20 km for the horizontal vector and 10 km for the vertical vector to the hypocenters and cal- culated the inversion. The maximum change in the station delays is 0.3 s, corresponding to a change in crustal thickness of 3 km. The changes in P velocities are 0.1 km/s. Based on our study results, the P n velocity in the Okinawa Trough is 7.9 km/s; as such, it is similar to that obtained from refraction experiments in the Okinawa Trough (Lee et al. , 1980; Hirata et al. , 1990; Nakahigashi et al. , 2001). The time-term inversion of P n in the Kyushu area in- dicated that the P n velocity beneath the Kyushu is 7.7 km/s. A refraction experiment in the south Kyushu area showed the P n velocity to be 7.8 km/s (Ono et al. , 1978). It would therefore appear that the P n velocity beneath the Okinawa Trough is similar to that beneath Kyushu Island. In contrast, the P n velocity beneath the Ryukyu arc is 7.5 km/s. The results of an earlier study in the northern Ryukyu arc suggested that the P n velocity is ≤ 7.6 km (Iwasaki et al. , 1990). Consequently, the P n velocity beneath the Ryukyu arc is close to that beneath the Kyushu Island. The crustal thickness ranges from 23 to 37 km in the northern-and-central Ryukyu arc. The crustal thickness at the center of the NCOT, based on data from refraction seismic surveys and gravity anomaly, has been estimated to be 26 km (Sibuet et al. , 1995). Thus, the crustal thickness in the NCOT and northern-and-central Ryukyu arc changes gradually from 23 km to 37 km, showing a flat Moho. This suggests that the extension area of crustal thinning is wide in the NCOT. The crustal thickness ranges from 29 to 44 km in the southern Ryukyu arc. ...
Similar publications
For a horizontal and homogeneous transversely isotropic reflector with a vertical symmetry axis (VTI), we derive the generalized moveout approximations for P-SV converted-wave in group (t-x) domain and phase (τ-p) domain. They are called generalized approximation due to following the concept proposed by Fomel and Stovas. Both generalized moveout ap...
Citations
... Based on previously published studies on intraplate earthquakes (Chen et al., 2022;Pollitz et al., 2017) and tomography studies of this region (Lallemand, 2016;M. Nakamura & Umedu, 2009;Salah & Zhao, 2004;Yoshioka & Ito, 2001), we have constructed a 3D viscoelastic FEM (Figure 1b, Figure S1 and Text S1 in Supporting Information S1) to study the processes responsible for the postseismic deformation of the 2016 Kumamoto earthquake sequence, that is, the viscoelastic relaxation and afterslip. The coseismic slip model utili ...
Plain Language Summary
We have derived the first 5‐year postseismic deformation at more than 200 Global Positioning System (GPS) stations following the 2016 Kumamoto earthquake to constrain the rheological properties beneath the Kyushu Island, as well as the evolution of afterslip. Test models have determined the viscosity of the lower crust and upper mantle to be ∼2 × 10²⁰ and ∼2 × 10¹⁹ Pa s, respectively, while the viscosity of the lower crust beneath the volcanic arc is ∼2 × 10¹⁹ Pa s, notably lower than that of the other area. The lower crust viscosity in the Beppu‐Shimabara Graben, except the volcano dominant area, is estimated to be no smaller than 10²⁰ Pa s. The complex near‐field postseismic deformation cannot be fully explained by one single deformation process. The viscoelastic relaxation and afterslip must both be at play to control the postseismic deformation. In addition to the rheological heterogeneity, the afterslip may also be heterogeneous. We propose additional afterslip possibly induced by the Aso volcano to better fit the near‐field GPS observations. Our models provide insights into the lithospheric structure of Kyushu Island and the intricate behavior of fault slips.
... In the overriding EU, the crustal thickness is flat along the northern and central Ryukyu Arc and OT and changes abruptly along the southern Ryukyu Arc and OT (Nakamura and Umedu 2009). The OT shows an obvious variation in seafloor topography: the northern OT is a gentle depression with shallow bathymetry (< 1000 m); water depths are within 1000-2000 m in the central OT, while the southern OT consists of a relatively narrow basin with a maximum depth greater than 2000 m ( Fig. 1). ...
The Ryukyu trench-arc-back arc system is part of the subduction margins of the Philippine Sea plate. Previous studies have indicated that several geophysical and geological characteristics reveal significant variations (including convergent rate, topography, subducting slab angle etc.) along this subduction system. In addition, the strength of plate coupling and the potential of large earthquake occurrence in the Ryukyu subduction zone have been major subjects of debate for decades. To gain new insights into the spatial variations in the crustal structure and strength of plate coupling along the Ryukyu subduction zone, in the present study, based on three P-wave seismic velocity profiles, we construct density models for 2-D gravity modeling. Then, we estimate the mantle lithosphere buoyancy (H m ) using these three density models to determine the strength of plate coupling between the subducting Philippine Sea plate and the overriding Eurasian plate, which could provide information for evaluating large earthquakes potential. 2-D gravity modeling results reveal that oceanic plateaus and/or submarine ridges with obviously less dense and thick oceanic crust are subducting in the northern and central parts of the Ryukyu Trench, which could increase the slab buoyancy in these regions. The H m results indicate that the strength of plate coupling is almost weak in the north and is relatively strong in the central Ryukyu subduction zone.
... K. Wang et al., 2004;K. McIntosh et al., 2005;Nishizawa et al., 2019), regional body wave and surface wave tomography (Chou et al., 2009;Friederich, 2003;Gorbatov & Kennett, 2003;Huang et al., 2014;Nakamura & Umedu, 2009;Nakamura et al., 2003;Tao et al., 2018;Wang et al., 2008Wang et al., , 2017, magnetic (J. Y. Gao et al., 2008;Liang et al., 2001), and gravity data analysis (J. ...
The Ryukyu arc (RA)‐trough system, which is uniquely characterized by the Okinawa trough (OT) experiencing an early stage of continental disintegration, provides an ideal location to study continental rifting processes. However, the crustal structure beneath this region is not well resolved due to limited seismic surveys and station coverage. Here, by taking advantage of the well‐distributed intermediate‐depth earthquakes beneath the RA, we invert for the crustal structures beneath the RA‐trough system from the depth phases pP and sP and their precursors, underside reflected from the Moho and intracrustal interface. Along the RA, the Moho depth varies from 22 to 27 km indicating a thinner continental crust. Beneath the OT, the crustal thickness gradually decreases from 26 km to 15 km toward the south, which correlates with the trough rifting rates. At the south continental shelf, a thick crust with Moho depth of 29 km presents. In the transition region from the middle OT to the continental shelf, we observe multipathed precursors, which can be modeled with a 10 km Moho offset. The offset occurs in a gentle slope with angle of 27° dipping to the northwest, which supports a slow spreading process across the boundary between the middle OT and the continental shelf.
... The Mohoroviciç discontinuity and upper mantle structures in and around the Japanese territories has been well investigated with active and passive seismic studies (e.g. Iwasaki et al., 1990;Yoshii, 1994;Iwasaki et al., 2002;Nakamura et al., 2003;Yoshimoto et al., 2004;Hasegawa et al., 2005;Shiomi et al., 2006;Nakamura and Umedu, 2009;Chou et al., 2009 andUchida et al., 2010). Analogous data for the broader study also exist in the Only earthquakes with magnitudes M3 are included in the analysis; this is a compromise adopted so as to ensure completeness, sufficiently large earthquake populations to guarantee statistical rigour and exclusion of distant small events that are ostensibly uncorrelated and may randomize the statistics (for more details see Sections 4 and 5). ...
We inquire the statistical nature and dynamics of shallow and deep seismogenesis along major plate margins of the NW Circum-Pacific Belt, by examining whether earthquakes are generated by Poisson processes and are independent (uncorrelated), or by Complex processes and are dependent (correlated). Analysis is based of Non Extensive Statistical Physics and the complete and homogeneous catalogue of the Japan Meteorological Agency for the period 2002-2016.5. Emphasis is given to background seismicity recovered by removing aftershocks with stochastic declustering. Long-term correlation and long-range interaction is mostly weak to moderate in shallow background seismicity. Conversely, deep seismicity is mostly uncorrelated (quasi-Poissonian), particularly in Wadati-Benioff zones. As function of time, shallow background seismicity exhibits persistent weak to moderate correlation; sub-crustal background seismicity is generally quasi-Poissonian but may dynamically transition to moderate-significant in association with temporal clusters of large earthquakes. A universal such effect was observed ahead of the 2011.19 Tohoku mega-earthquake. These results contrast observations along the Pacific-North American transformational plate boundaries of California and Alaska which are generally correlated, suggesting that the geodynamic setting is essential in the development of self-organization and Complexity. Moreover, observations appear consistent with simulations of small-world fault networks in which free boundary conditions at the surface allow for Complexity to develop, while fixed boundary conditions at depth do not.
... The lower depth limit of the crustal earthquakes is 15 km in the northern region (Y = 60 km) and 25 km in the southern region (Y = 10 km). A previous study estimated the depth of the land Moho, based on an analysis of arrival times of the Pn-phase, to be 25 to 35 km beneath Iriomote Island (Nakamura and Umedu 2009), which is similar to the depth of the land Moho estimated from the receiver function analysis. ...
Seismic tomography and receiver function analysis were carried out to investigate the relation between the slab structure in the southern Ryukyu arc region and the occurrence of slow-slip events that repeat biannually. For calculation of the receiver function, 212 teleseismic earthquakes with magnitudes larger than 6.0 were selected, and teleseismic waveforms were observed using two short-period seismometers and one broadband seismometer. Assuming that each later phase in a receiver function was a wave converted from P to S at depth, the time-domain receiver function was transformed into depth domain along each ray path using a reference velocity model. P- and S-wave arrival times, selected manually by the Japan Meteorological Agency (JMA), were used for the analysis of seismic tomography. In all, 6,750 earthquakes from January 2002 to March 2014 were used. The results showed that a cluster of slab earthquakes is distributed about 10 km below the plate interface. This suggests that the slab earthquakes occur in the lower part of the oceanic crust near the oceanic Moho within the slab. Moreover, the fault of the slow-slip events corresponds with the plate interface. The results also showed that a low Vp and high Vp/Vs area is distributed within the subducted oceanic crust underlying the fault associated with the slow-slip events. These structures are similar to those in the Tokai district, which suggests that the thermal condition of the southern Ryukyu arc is similar to the case of a hot-slab area where slow-slip events occur.
... Values of 650 and 430 kg m −3 were taken for the crust-mantle density contrast in land and sea areas, according to Sjöberg and Bagherbandi (2011). Mean Moho depths of 37 km and 23 km were obtained from the spatial averaging of the 2 @BULLET × 2 @BULLET CRUST2.0 (Bassin et al., 2000 ) Moho depths for the corresponding areas, respectively (see, for example, the discussion concerning the seismic method and its accuracy in Nakamura and Umedu, 2009). Our goal is to determine 2 @BULLET × 2 @BULLET Moho models with the same resolution as that of CRUST2.0, from 1 @BULLET × 1 @BULLET GOCE and terrestrial gravimetric data. ...
One of the GOCE satellite mission goals is to study the Earth's interior
structure including its crustal thickness. A gravimetric-isostatic Moho
model, based on the Vening Meinesz-Moritz (VMM) theory and GOCE
gradiometric data, is determined beneath Iran's continental shelf and
surrounding seas. The terrestrial gravimetric data of Iran are also used
in a nonlinear inversion for a recovering-Moho model applying the VMM
model. The newly-computed Moho models are compared with the Moho data
taken from CRUST2.0. The root-mean-square (RMS) of differences between
the CRUST2.0 Moho model and the recovered model from GOCE and that from
the terrestrial gravimetric data are 3.8 km and 4.6 km, respectively.
During the differentiation of arc magmas, fractionating liquids follow a series of cotectics, where the co‐crystallization of multiple minerals control the melt compositional trajectories, commonly referred to as liquid lines of descent (LLD). These cotectics are sensitive to intensive properties, including fractionation pressure and melt H2O concentration, and changes in these variables produce systematic differences in the LLDs of arc lavas. Based on a compilation of experimental studies, we develop two major element proxies that exploit differences in LLDs to constrain the fractionation conditions of arc magmas. Near‐primary fractionating magmas evolve along the olivine‐clinopyroxene cotectic, which is pressure‐sensitive. We use this sensitivity to develop a proxy for early fractionation pressure based on the normative mineral compositions of melts with 8 ± 1 wt.% MgO. Fractionation in more evolved magmas is controlled by the clinopyroxene‐plagioclase cotectic, which is strongly sensitive to magmatic H2O contents. We use this relationship to develop an H2O proxy that is calibrated to the normative mineral components of melts with 2–4 wt.% MgO. These two proxies provide new tools for estimating the variations in pressure and temperature between magmatic systems. We applied these proxies to compiled major element data and phenocryst assemblages from modern volcanic arcs and show that in island arcs early fractionation is relatively shallow and magmas are dominantly H2O‐poor, while continental arcs are characterized by more hydrous and deeper early fractionation. These differences likely reflect variations in the relative contributions of decompression and flux melting in combination with distinct upper plate controls on arc melt generation.
The Okinawa Trough is an active back-arc basin of the Ryukyu Island arc-trench system caused by the subduction of the Philippine Sea plate. The different controlling effects of plate subduction result in the unique characteristic of the crustal structure, fault development and magmatic activities. The research on the crustal structure and properties is of great significance to the evolution mechanism of back-arc basins. To overcome the non-uniqueness of gravity interface inversion and spatial limitation of seismic profiles, 19 published seismic results are selected as the soft constrained strategy, and the partitioned interface inversion with the exponential density contrast model is utilized to obtain more detailed undulations of the Moho interface and crustal thickness. The crustal thickness and the stretching factors of the Okinawa Trough are obtained as well. The results are reliable after the evaluation of the fitting degree of observed data, and the evaluation of recovered data and selected constraint points respectively. The results show that the overall trend of the crustal thickness variation in the study area gradually decreases from the west to east. The crustal thickness of the Okinawa Trough is larger than that of the East China Sea shelf basin, ranging from 12 to 22 km. The Okinawa Trough is a transitional crust formed during the transformation from continental crust to oceanic crust, with the overall characteristics of extensional and thinning continental crust. However, a new oceanic crust may have accreted in the central graben of the southern sections of the trough.
Antarctica has traditionally been considered continental inside the coastline of ice and bedrock since Press and
Dewart (1959). Sixty years later, we reconsider the conventional extent of this sixth continent. Geochemical
observations show that subduction was active along the whole western coast of West Antarctica until the mid-
Cretaceous after which it gradually ceased towards the tip of the Antarctic Peninsula. We propose that the entire
West Antarctica formed as a back-arc basin system flanked by a volcanic arc, similar to e.g. the Japan Sea,
instead of a continental rift system as conventionally interpreted. Globally, the fundamental difference between
oceanic and continental lithosphere is reflected in hypsometry, largely controlled by lithosphere buoyancy. The
equivalent hypsometry in West Antarctica (−580 ± 335 m on average, extending down to −1.6 km) is much
deeper than in any continent, but corresponds to back-arc basins and oceans proper. This first order observation
questions the conventional interpretation of West Antarctica as continental, since even continental shelves do
not extend deeper than −200 m in equivalent hypsometry.
We present a suite of geophysical observations that supports our geodynamic interpretation: a linear belt of
seismicity sub-parallel to the volcanic arc along the Pacific margin of West Antarctica; a pattern of free air
gravity anomalies typical of subduction systems; and extremely thin crystalline crust typical of back-arc basins.
We calculate residual mantle gravity anomalies and demonstrate that they require the presence of (1) a thick
sedimentary sequence of up to ca. 50% of the total crustal thickness or (2) extremely low density mantle below
the deep basins of West Antarctica and, possibly, the Wilkes Basin in East Antarctica. Case (2) requires the
presence of anomalously hot mantle below the entire West Antarctica with a size much larger than around
continental rifts. We propose, by analogy with back-arc basins in the Western Pacific, the existence of rotated
back-arc basins caused by differential slab roll-back during subduction of the Phoenix plate under the West
Antarctica margin. Our finding reduces the continental lithosphere in Antarctica to 2/3 of its traditional area. It
has significant implications for global models of lithosphere-mantle dynamics and models of the ice sheet
evolution.
