Three-dimensional Seismic Velocity Structure Research Articles

High-frequency S and S-coda waves at ocean-bottom seismometers

To clarify the characteristics of high-frequency (> 1 Hz) S and S-coda waves at ocean-bottom seismometers (OBSs), we analyzed seismograms observed at permanent OBSs and inland broadband seismometers around the Kii Peninsula in southwest Japan along the Nankai Trough. The coda amplitudes (both horizontal and vertical) at the OBSs were much larger than those at the inland rock-site stations. Because coda amplitudes relative to those at inland rock-site stations have been used as site-amplification factors, large site amplifications for both components can be expected due to the presence of thick oceanic sediments just below the OBSs; however, the observed maximum S-wave amplitudes in the vertical component exhibited similar attenuation trends against epicentral distances at both OBS and inland stations. To clarify the causes of this discrepancy, we conducted numerical simulations of seismic wave propagation using various three-dimensional seismic velocity structure models. The results demonstrated that coda waves at OBSs mostly comprise multiple scattered waves within a thick (> 2 km) sedimentary layer; consequently, coda amplitudes at OBSs become much larger than those at inland rock-site stations. Our numerical simulations also confirmed the generation of large coda amplitudes at regions with seawater depths ≥ 4 km, where no OBS was deployed. However, the thick sedimentary layer and seawater have limited effects on maximum S-wave amplitudes at the OBSs. Given that the effects of a thick sedimentary layer and seawater on S and S-coda waves differ, we concluded that the coda-normalization technique for site-amplification correction against a rock-site station could not be applied if stations are located within regions above the thick sedimentary layer or deeper sea depths. The site amplifications at the OBSs were corrected according to the horizontal-to-vertical ratios at each OBS; we adjusted the simulated horizontal envelopes at the OBSs using these ratios of the observed S-coda waves. As well as inland seismometers, the site-corrected simulation results practically reproduced the observed high-frequency envelopes at OBSs.Graphical

A Possible Roll-Over Slab Geometry Under the Caroline Plate Imaged by Monte Carlo Finite-Frequency Traveltime Inversion of Teleseismic SS Phases

The shape of a subducting slab varies as a function of trench motion. Two end-members of subduction modes are geodynamically possible: roll-back mode underneath neighboring plates and roll-over mode underneath the plate itself. Whereas most of major slabs seem to roll back while the Pacific plate shows a slab piling behavior down to ∼1,000 km depth under the Mariana trench, no clear evidence of slab roll-over in nature has been reported so far. Here we show a possible roll-over slab beneath the Caroline microplate, revealed from its three-dimensional seismic velocity structure derived by analyzing teleseismic reverberating SS phases. We suggest that slab roll-over is driven by at least two factors: 1) the overall buoyancy and fragility of the Caroline microplate at the surface, induced by a thin hot mantle plume that rises from depths ≥800 km; and 2) the pushing force of the Pacific plate acting on the trailing edge of the Caroline plate.

阿多カルデラ周辺の地殻の 3次元地震波速度構造, 及び加久藤カルデラ下の速度構造との比較

阿多カルデラ周辺の地殻の 3次元地震波速度構造, 及び加久藤カルデラ下の速度構造との比較

Characteristics of relocated hypocenters of the 2018 M6.7 Hokkaido Eastern Iburi earthquake and its aftershocks with a three-dimensional seismic velocity structure

I relocated the hypocenters of the 2018 M6.7 Hokkaido Eastern Iburi earthquake and its surrounding area, using a three-dimensional seismic structure, the double-difference relocation method, and the JMA earthquake catalog. After relocation, the focal depth of the mainshock became 35.4 km. As previous studies show, in south-central Hokkaido, the Hidaka collision zone is formed, and anomalous deep and thickened forearc crust material is subducting at depths of less than 70 km. The mainshock and its aftershocks are located at depths of approximately 10 to 40 km within the lower crust of the anomalous deep and thickened curst near the uppermost mantle material intrusions in the northwestern edge of this Hidaka collision zone. Like the two previous large events, the aftershocks of this event incline steeply eastward and appear to be distributed in the deeper extension of the Ishikari-teichi-toen fault zone. The highly inclined fault in the present study is consistent with a fault model by a geodetic analysis with InSAR. The aftershocks at depths of 10 to 20 km are located at the western edge of the high-attenuation (low-Qp) zone. These kinds of relationships between hypocenters and materials are the same as the 1970 and 1982 events in the Hidaka collision zone. The anomalous large focal depths of these large events compared with the average depth limit of inland earthquakes in Japan could be caused by the locally lower temperature in south-central Hokkaido. This event is one of the approximately M7 large inland earthquakes that occurred repeatedly at a recurrence interval of approximately 40 years and is important in the collision process in the Hidaka collision zone.

Modeling of Long-Period Ground Motions in the Nankai Subduction Zone: Model Simulation Using the Accretionary Prism Derived from Oceanfloor Local S-Wave Velocity Structures

The accretionary prism in the subduction zone, which consists of thick low-velocity oceanic sediments, significantly affects the propagation of seismic waves for shallow, offshore earthquakes, including large interplate earthquakes. In order to simulate long-period (> 5 s) ground motions in the Nankai subduction zone, we constructed a three-dimensional (3D) seismic velocity structure model of the accretionary prism by interpolation/extrapolation of local S-wave velocity structures beneath 46 oceanfloor seismic stations (DONET), which are deployed just above the accretionary prism off the southern Kii and eastern Shikoku regions. We modeled local S-wave velocity structures using a simple two-parameter depth-varying velocity function. To investigate the effects of the accretionary prism on ground and seafloor motions, we conducted numerical simulations of seismic wave propagation for three local earthquakes that occurred in southwestern Japan. The simulations reasonably reproduced the observed seismograms, not only for the period ranges of the moment tensor inversion (~ 50 s), but also for the strong, long-period ground motions in the sedimentary basins (~ 5 s), especially in the region where DONET stations are densely deployed. Since depth-varying, local S-wave structures significantly improve the reproducibility of long-period ground motions, our modeling procedure is useful for modeling long-period ground motions of local and regional offshore subduction zone earthquakes.

Three-Dimensional Seismic Velocity Structure of the Aegean Region of Turkey from Local Earthquake Tomography

This study brings new insights to elucidate the 3-D seismic velocity structure of the Aegean region of Turkey by using Local Earthquake Tomography (LET). The study area has remarkable potential for geothermal resources. To provide the subsurface geological structure of seismogenic layers and geothermal areas, we develop new high-resolution depth-cross sections through Buyuk Menderes, Kucuk Menderes and Gediz grabens. Travel times of almost 265.000 readings (14.963 P-phase and 11.969 S-phase picks) from 2.085 well-located events recorded between 2007 and 2016 by a permanent seismic network of 75 broad-band seismometers were used to precisely interpret the 3-D Vp (lithological) and Vp/Vs (petrological) models of the study area. We detected four main layers down to 35-40 km depths with different Vp velocities ranging from 3.5 to 8.5 km/s. Mid-crustal interface (Conrad discontinuity) is discovered at 15 km depth in nearly all depth-cross-sections. Our results suggest an average of 25 km Moho depth in the Aegean region of Turkey. The depths range from around 18 km beneath SE of Aydin to 29 km beneath Aliaga, and approximate values of 19, 25 and 31 km beneath the Doganbey, Kutahya and Selendi-Demirci regions, respectively. The geothermal areas of the studied region are imaged by Vp and Vp/Vs anomalies. We propose the Aliaga, Denizli, Doganbey and Kosk areas as low Vp, low Vp/Vs anomalies which are an indicator of steam, CO2 or a mixture of both. Low Vp, high Vp/Vs models, suggesting geothermal fluids, are clearly visible near the Buharkent, Gumuskol, Guzelhisar, Kosk, Kuyucak, Saraykoy and Suzbeyli regions. We also report that the Bademler, Candarli, Kalekoy, Karahalli, Merdivenli, Ortakoy, Saruhanli, Yelki and Yuntdagikoseler regions might be good candidates for new potential geothermal resources.

Joint 3-D tomographic imaging of Vp, Vs and Vp/Vs and hypocenter relocation at Sinabung volcano, Indonesia from November to December 2013

We conducted travel time tomography using P- and S-wave arrival times of volcanic-tectonic (VT) events that occurred between November and December 2013 to determine the three-dimensional (3D) seismic velocity structure (Vp, Vs, and Vp/Vs) beneath Sinabung volcano, Indonesia in order to delineate geological subsurface structure and to enhance our understanding of the volcanism itself. This was a time period when phreatic explosions became phreatomagmatic and then magma migrated to the surface forming a summit lava dome. We used 4846 VT events with 16,138 P- and 16,138 S-wave arrival time phases recorded by 6 stations for the tomographic inversion. The relocated VTs collapse into three clusters at depths from the surface to sea level, from 2 to 4km below sea level, and from 5 to 8.5km below sea level. The tomographic inversion results show three prominent regions of high Vp/Vs (~1.8) beneath Sinabung volcano at depths consistent with the relocated earthquake clusters. We interpret these anomalies as intrusives associated with previous eruptions and possibly surrounding the magma conduit, which we cannot resolve with this study. One anomalous region might contain partial melt, at sea level and below the eventual eruption site at the summit. Our results are important for the interpretation of a conceptual model of the “plumbing system” of this hazardous volcano.

Three-Dimensional Seismic Velocity Structure Beneath Japanese Islands and Surroundings Based on NIED Seismic Networks Using both Inland and Offshore Events

Tomographic analysis of the seismic velocity structure beneath oceans has always been difficult because offshore events determined by onshore seismic networks have large uncertainties in depth. In order to use reliable event locations for our computations, we have developed a method to use the hypocentral depths determined by the NIED F-net with moment tensor solutions using long-period (20-50 s) waves from offshore events away from onshore seismic networks. We applied seismic tomographic method to events occurring between the years 2000 and 2015 to generate a tomographic image of the Japanese Islands and the surrounding using travel time data picked by the NIED Hi-net, hypocenteral information for onshore earthquakes from the Hi-net, and hypocenter information for offshore events from the F-net. The seismic velocity structure at depths of 30-50 km beneath the Pacific Ocean off the east coast of northeastern Japan and onshore Japan was clearly imaged using both onshore and offshore event date. The boundary between high and low P-wave velocities (Vp) is clearly seen at the Median Tectonic Line beneath southwestern Japan at depths of 10 and 20 km. We discuss how the high-Vp lower crust and low-Vp upper crust beneath central Japan and towards the Sea of Japan are responsible for the failed rift structures formed during the opening of the Sea of Japan. Due to consequent shortening, the crustal deformation has been concentrated along the failed rift zone. Resolution of shallow structures beneath the ocean is investigated using S-net data, confirming the possibility of imaging depths of 5-20 km. In future studies, application of S-net data will be useful in evaluating whether the failed rift structure, formed during the late Cretaceous to early Tertiary, continues towards the shallow regions beneath the Pacific Ocean.

Seismic imaging of the crust and uppermost mantle beneath the Qilian Orogenic Belt and its geodynamic implications

Three-dimensional (3-D) seismic velocity structure of the crust and uppermost mantle under the Qilian Orogenic Belt (QLOB) is important for better understanding the Cenozoic deformation mechanism of the QLOB and the deep tectonic relationship between the QLOB and the Alashan Block. We determined a detailed 3-D P and S wave velocity (Vp and Vs) model of the crust and uppermost mantle beneath the QLOB and its adjacent regions. The data set used in this study consists of 75,999 P wave and 72,178 S wave arrival times of 7485 local earthquakes recorded by 72 seismic stations in the study area during 2009 to 2015. Our tomographic results show obvious low-velocity (low-V) anomalies in the lower crust beneath the QLOB, and the low-V zones extend to the uppermost mantle beneath a few local areas. These features are interpreted as mechanically weakened zones caused by fluids and melt related to the northward extension of the Tibetan Plateau and local hot upwelling in the upper mantle. The weakened zones in the lower crust are capable of ductile flow on a geological timescale, and the intermittent distribution of the low-V zones indicates that the scale of the ductile flow is not very large. The obtained Vp and Vs images indicate southward subduction of the Alashan Block beneath the QLOB, which may result from the Indo-Eurasia collision. Large crustal earthquakes in the study region generally occurred along large fault zones where Vp and Vs change drastically over a short distance. Beneath source areas of the large earthquakes, low-V anomalies caused by fluids and melt exist widely in the lower crust and uppermost mantle. When the fluids migrate upward to the active faults in the upper crust, the fault zone friction is reduced and so large crustal earthquakes can be triggered.

Three-dimensional Seismic Velocity Structure beneath Unzen Volcano, Kyushu, Japan Inferred by Tomography from Experimental Explosion Data

Seismic travel time inversion is widely used to estimate the velocity structure within a target area. For a given velocity structure a suitable ray tracer is needed to calculate the ray path and travel time between points. The velocity structure of volcanic areas varies considerably. Any seismic ray tracer used in volcanic areas should therefore be robust to velocity variation. A seismic ray tracer developed by the author to be effective with strongly varying velocity fields is used to calculate the travel time in an inversion scheme, and is shown to work well in volcanic regions (Nishi, 2001). The present paper gives the results of this travel time inversion method applied to data obtained from experimental explosions in Unzen volcano, Kyushu Japan. Seismic velocity in the center of the volcanic edifice is in general higher than peripheral area, as was already known from an analysis of the travel time by the refraction method (Explosion Seismic Research Group of Unzen Volcano and Shimizu, 1997). In detail, however seismic velocity structure in this region is laterally heterogeneous. Among the high velocity zone, some low velocity zones and also still higher velocity zone are found. Low velocity zone beneath the Fugendake peak (near to the lava dome generated in the 1991-1995 activity) is seemed to be related to the pressure source beneath the Fugendake peak estimated from leveling survey.

A2-12 霧島火山周辺の上部地殻内3次元地震波速度構造(2)(地震・地殻変動,口頭発表)

A2-12 霧島火山周辺の上部地殻内3次元地震波速度構造(2)(地震・地殻変動,口頭発表)

B38 稠密地震観測による富士山の3次元地震波速度構造(火山の地震, 日本火山学会2005年秋季大会)

B38 稠密地震観測による富士山の3次元地震波速度構造(火山の地震, 日本火山学会2005年秋季大会)

Waveform inversion for localized three-dimensional seismic velocity structure in the lowermost mantle beneath the Western Pacific

We infer 3-D localized shear velocity structure in the lowermost 400 km of the mantle at the western edge of the Pacific large low shear velocity province (LLSVP) by applying waveform inversion to transverse component body-wave waveforms from the F-net seismic array in Japan. Our data set consists of relatively long period (12.5–200 s) broad-band seismic waveforms of Tonga-Fiji deep focus and intermediate deep earthquakes. We conduct several tests to confirm the robustness of the inversion results. We find two low-velocity zones at the bottom of the target region, with a high-velocity zone in the middle, and a low-velocity zone above the high-velocity zone and contiguous with the two deeper low-velocity zones at a depth of 200–300 km above the core-mantle boundary (CMB). This supports the idea that the Pacific LLSVP may be an aggregation of small upwelling plumes rather than a single large thermochemical pile.

Three-dimensional seismic velocity structure and earthquake relocations at Katmai, Alaska

We invert arrival time data from local earthquakes occurring between September 2004 and May 2009 to determine the three-dimensional (3D) upper crustal seismic structure in the Katmai volcanic region. Waveforms for the study come from the Alaska Volcano Observatory's permanent network of 20 seismic stations in the area (predominantly single-component, short period instruments) plus a densely spaced temporary array of 11 broadband, 3-component stations. The absolute and relative arrival times are used in a double-difference seismic tomography inversion to solve for 3D P- and S-wave velocity models for an area encompassing the main volcanic centers.The relocated hypocenters provide insight into the geometry of seismogenic structures in the area, revealing clustering of events into four distinct zones associated with Martin, Mageik, Trident-Novarupta, and Mount Katmai. The seismic activity extends from about sea level to 2km depth (all depths referenced to mean sea level) beneath Martin, is concentrated near 2km depth beneath Mageik, and lies mainly between 2 and 4km depth below Katmai and Trident-Novarupta. Many new features are apparent within these earthquake clusters. In particular, linear features are visible within all clusters, some associated with swarm activity, including an observation of earthquake migration near Trident in 2008. The final velocity model reveals a possible zone of magma storage beneath Mageik, but there is no clear evidence for magma beneath the Katmai-Novarupta area where the 1912 eruptive activity occurred, suggesting that the storage zone for that eruption may have largely been evacuated, or remnant magma has solidified.

Three-Dimensional Seismic Velocity Structure and Precise Earthquake Relocations in the Salton Trough, Southern California

Research Article| October 01, 2013 Three‐Dimensional Seismic Velocity Structure and Precise Earthquake Relocations in the Salton Trough, Southern California Guoqing Lin Guoqing Lin Division of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149 Search for other works by this author on: GSW Google Scholar Author and Article Information Guoqing Lin Division of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149 Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1943-3573 Print ISSN: 0037-1106 Bulletin of the Seismological Society of America (2013) 103 (5): 2694–2708. https://doi.org/10.1785/0120120286 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Guoqing Lin; Three‐Dimensional Seismic Velocity Structure and Precise Earthquake Relocations in the Salton Trough, Southern California. Bulletin of the Seismological Society of America 2013;; 103 (5): 2694–2708. doi: https://doi.org/10.1785/0120120286 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract I present a new 3D seismic velocity model and high‐precision earthquake relocations between 1981 and 2010 near the Salton Trough and the San Jacinto fault zone. The simul2000 tomography algorithm is applied to derive the 3D VP and VP/VS models using first arrivals of a composite event data set. The final VP model correlates with geological features at shallow depths and is more consistent with the refraction studies than the starting 3D regional‐scale model. The VP/VS model shows strong variations in the upper crust and is dominated by relatively low values (below 1.7) in the middle crust except for the Salton Trough and the Imperial Valley. The newly resolved model is used to relocate all the seismicity in the study area. I then apply the similar‐event cluster analysis and differential‐time relocation approach based on waveform cross‐correlation data to the 3D relocated events. A dramatic sharpening of seismicity patterns is obtained after using these methods. The new locations are more tightly clustered than the previous catalogs mainly due to the different absolute locations. The depth distribution along a basal surface may suggest the existence of a detachment fault from the north end of the Salton Trough to United States–Mexico border. The fine‐scale velocity structure and precise earthquake relocations will be helpful for investigating the fault geometry, seismicity, stress interaction, and other seismological studies for southern California.Online Material: Figures of model resolution tests, cross‐sectional comparisons of different P‐velocity models, and earthquake depth distributions. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Rayleigh-wave phase-velocity maps and three-dimensional shear velocity structure of the western US from local non-plane surface wave tomography

SUMMARY We utilize two-and-three-quarter years of vertical-component recordings made by the Transportable Array (TA) component of Earthscope to constrain three-dimensional (3-D) seismic shear wave velocity structure in the upper 200 km of the western United States. Single-taper spectral estimation is used to compile measurements of complex spectral amplitudes from 44 317 seismograms generated by 123 teleseismic events. In the first step employed to determine the Rayleigh-wave phase-velocity structure, we implement a new tomographic method, which is simpler and more robust than scattering-based methods (e.g. multi-plane surface wave tomography). The TA is effectively implemented as a large number of local arrays by defining a horizontal Gaussian smoothing distance that weights observations near a given target point. The complex spectral-amplitude measurements are interpreted with the spherical Helmholtz equation using local observations about a succession of target points, resulting in Rayleigh-wave phase-velocity maps at periods over the range of 18–125 s. The derived maps depend on the form of local fits to the Helmholtz equation, which generally involve the non-plane-wave solutions of Friederich et al. In a second step, the phase-velocity maps are used to derive 3-D shear velocity structure. The 3-D velocity images confirm details witnessed in prior body-wave and surface-wave studies and reveal new structures, including a deep (>100 km deep) high-velocity lineament, of width ∼200 km, stretching from the southern Great Valley to northern Utah that may be a relic of plate subduction or, alternatively, either a remnant of the Mojave Precambrian Province or a mantle downwelling. Mantle seismic velocity is highly correlated with heat flow, Holocene volcanism, elastic plate thickness and seismicity. This suggests that shallow mantle structure provides the heat source for associated magmatism, as well as thinning of the thermal lithosphere, leading to relatively high stress concentration. Our images also confirm the presence of high-velocity mantle at ≳100 km depth beneath areas of suspected mantle delamination (southern Sierra Nevada; Grande Ronde uplift), low velocity mantle underlying active rift zones, and high velocity mantle associated with the subducting Juan de Fuca plate. Structure established during the Proterozoic appears to exert a lasting influence on subsequent volcanism and tectonism up to the Present.

Three-dimensional seismic velocity structure across the 2008 Wenchuan Ms 8.0 earthquake, Sichuan, China

We present three-dimensional (3D) seismic compressional wave velocity ( V p), shear wave velocity ( V s) and V p/ V s models around the Longmen Shan fault, Sichuan, China region, using aftershocks associated with the 2008 Wenchuan Ms 8.0 earthquake. The velocity and ratio models are obtained using a new version of the double-difference seismic tomography code tomoDDPS (Zhang, 2003) to simultaneously solve for V p, V s, V p/ V s and event locations. The data used in inversion include 73,013 P arrival times, 62,287 S arrival times and 61,823 S–P travel times recorded by 63 stations from both permanent and temporary networks in a region 400 km northeast-southwest by 200 km northwest-southeast. The velocity model shows structure heterogeneity both along and across the fault zone. Generally, the velocity model is consistent with the local geology, with older rocks having high velocity and younger rocks having low velocity. The Longmen Shan fault zone is a clear boundary in V p, V s and V p/ V s ratio. Down to the depth around 15 km, higher V p and V s and lower V p/ V s ratio exist to the west of the fault, corresponding to the Songpan–Ganze Fold System, while lower V p and V s and higher V p/ V s ratio exist to the east, corresponding to the Sichuan Basin. Along the fault zone, the velocity structure is generally consistent with various rupture slip models, with two high velocity bodies corresponding well to the two large slip patches. This shows the structure control on the slip distribution along the fault plane.

Application of teleseismic tomography to the study of shallow structure beneath Shizigou in the western Qaidam basin

Teleseismic body wave traveltime tomography is used to inverse the three-dimensional seismic velocity structure beneath Shizigou in the western Qaidam basin. The travel time are picked from the continuous observation data on a small seismic array of stations deployed during 2004–2007. The tomographic results obtained indicate that a NW-trending low velocity anomaly just beneath the target region insert northeastwards with a high dip angle. In the north, northeast and east of the low velocity anomaly, some high-velocity anomalies distribute with the same strike and coverage as those of Shizigou anticline.

Three-dimensional seismic velocity structure of the crust beneath the central Tien Shan, Kyrgyzstan: Implications for large- and small-scale mountain building

The central Tien Shan, Kyrgyzstan is located approximately 2000 km from the main collisional suture between India and Eurasia and is sandwiched between the Kazakh Shield to the north and the Tarim Basin to the south. A three-dimensional (3-D) seismic velocity structure of the crust beneath the central Tien Shan was estimated to a depth of 50 km by means of seismic tomography using arrival-time data of P- and S-waves from local earthquakes recorded by a temporary seismic network of the Tien Shan during 1998–2000. Velocity structure images show a good correlation with the subsurface geology at shallow depths. A low-velocity zone is distributed beneath the basins probably due to the Cenozoic deposition, whereas a high-velocity zone is visible beneath the mountain ranges likely due to Paleozoic crystalline basement rocks at the surface layer. Results obtained at greater depths revealed the following features: (1) A low-velocity zone is clearly observed in the middle to lower crust, and is probably caused by melt and/or fluids conveyed through the flow of mantle upwelling; (2) Both north-dipping and south-dipping low-velocity zones were imaged at locations where the Kazakh Shield and the Tarim Basin underthrust beneath the Tien Shan, respectively, and are probably caused by mantle materials somehow being intruded into the crust. (3) The lateral variation of high- and low-velocity zones seen in the middle to lower crust beneath basins and surrounding mountain ranges, respectively, seems to be caused by melt and/or fluids vertically penetrated and/or laterally spread. All these findings suggest that in-process northward shortening and crustal thickening owing to collision, and mantle convective motion, i.e. upwelling and, as a consequence, partial melting are “work-horses” in the mountain building beneath the Tien Shan.

Seismic velocity structure around the Hyuganada region, Southwest Japan, derived from seismic tomography using land and OBS data and its implications for interplate coupling and vertical crustal uplift

The Hyuganada region, a forearc region of Southwest Japan, is characterized by several interesting geological and geophysical features, i.e., significant aseismic crustal uplift of ∼120 m during the past ∼120 thousand years at the Miyazaki Plain, negative free-air gravity anomalies with the maximum magnitude of −130 mgal, and relatively less cohesive interplate coupling compared with that for off the Shikoku and Kii Peninsula. In order to examine the causes of these observations, we determined a detailed three-dimensional seismic velocity structure based on the seismic data observed by ocean bottom seismometers (OBS) and land stations. P- and S-wave tomographic velocity structures clearly indicate the subducting slab and also the zones of high Poisson's ratio at 25–35 km depth along the coastline of the northeastern part of the Hyuganada. The region with high Poisson's ratio may correspond to the serpentinized mantle wedge as suggested for other mantle wedges, and appears to be coincident with the zone for observed aseismic slips such as the slow-slip and after-slip events. Also, the detection may be related to a relatively weak interplate coupling in the Hyuganada region. The tomographic structures also indicate low velocity zones with a horizontal scale comparable to the Kyushu-Palau Ridge in and around the subducting slab. If we assume that the low velocity zones correspond to the subducted Kyushu-Palau Ridge, then the predicted gravity anomaly due to the density contrast between the low velocity zones and the surrounding region can explain about 60% of the gravity anomaly in the Hyuganada region. The buoyancy is probably an important factor for the crustal uplift observed in the Miyazaki Plain, the steep bending of the subducting slab and the normal fault-type earthquakes around the Hyuganada region.