T13E-1621
Variation of Rayleigh and Love Wave Fundamental Mode Group Velocity Dispersion Across India and Surrounding Regions
We present group velocity dispersion results from a study of regional fundamental mode Rayleigh and Love waves propagating across India and surrounding regions. Data used in this study comes from broadband stations operated in India by us in addition to data from seismograms in the region whose data is archived at the IRIS Data Management Centre. The large amount of new and available data allows an improved path coverage and accordingly increased lateral resolution than in previous similar global and regional studies. 1D path- averaged dispersion measurements have been made using multiple filter analyis for source-receiver paths and are combined to produce tomographic group velocity maps for periods between 10 and 60 s. Preliminary Rayleigh wave group velocity maps have been produced using ~2500 paths and checkerboard tests indicate an average resolution of 5 degrees with substantially higher resolution achieved over the more densely sampled Himalayan regions. Short period velocity maps correlate well with surface geology resolving low velocity regions (2.0-2.4 km/s) corresponding to the Ganges and Brahmaputra river deltas, the Indo-Gangetic plains, the Katawaz Basin in Pakhistan, the Tarim Basin in China and the Turan Depression. The Tibetan Plateau is well defined as a high velocity region (2.9-3.2 km/s) at 10 s period, but for periods greater than 20 s it becomes a low velocity region which remains a distinct feature at 60 s and is consistent with the increased crustal thickness. The southern Indian shield is characterized by high crustal group velocities (3.0-3.4 km/s) and at short periods of 10 and 15 s it is possible to make some distinction between the Singhbhum, Dharwar and Aravali cratons. Initial Love wave group velocity maps from 500 dispersion measurements show similarly low velocities at short periods across regions with high sedimentation but higher velocities compared to Rayleigh waves across the Indian shield.
T13E-1622
Constraints on the evolution of East Asia's mantle from P-wave travel time tomography
High resolution tomographic images of the mantle structure beneath East Asia have been obtained through inversion of travel time data from global and regional seismograph stations and regional (temporary) arrays. These data resolve three-dimensional (3-D) upper mantle heterogeneity in unprecedented detail. In the west, high velocity anomalies are dominant beneath the Himalayas and the western portion of the Tibetan plateau to 300 km depth, which we interpret as the image of the northeastward subducting Indian lithospheric mantle. In contrast, P-wave tomography does not provide evidence for underthrusting of the Indian lithosphere beneath much of central and eastern Tibet. Beneath East China, slabs subducting from the Japan and Izu-Bonin trenches are deflected in the mantle transition zone. These stagnant slabs likely influence upper mantle convection beneath East Asia and might be related to volcanism in Korea and northeast China (such as the Changbai volcanic area). Low wavespeed structures in the shallow mantle beneath the Red River fault region connect to deep, slow anomalies beneath the South China Foldbelt. Tomographic imaging also reveals high wavespeed continental roots of the Precambrian Ordos block and Sichuan Basin (to 250-300 km depth) and strong heterogeneity between the latter and the Burma ranges further to the west. Together these structures may mark a transition in tectonic regime from the continental collision control in the west to control by subduction of Pacific, Philippine Sea and Indonesia plates to the east and the southeast.
T13E-1623
Crustal and Upper-mantle Structures Beneath the Chinese Continent From Joint Inversion of Rayleigh-wave Group Velocities and Waveforms
Traditional fundamental-mode surface wave dispersion tomography is the most common method used to study continental-scale lithosphere structures. But due to its limited resolution in deep upper mantle, most of previous models based on the fundamental-mode tomography can hardly offer information deeper than about 200 km. However, dispersion processing does not require event focal mechanisms, so it can be applied to smaller earthquakes and consequently has good path coverage. Surface waveform tomography is another useful way to study upper-mantle structures. Because of higher-mode content of the surface waveforms, surface waveform tomography gives better resolution in deep upper mantle. However, as waveform processing requires event focal mechanisms, waveform tomography can only be applied to strong earthquakes and consequently has worse path coverage. To strengthen the advantage and suppress the disadvantage of these two types of tomography, the present study derived a new method combining surface wave dispersion and waveform constraints in a 3D inversion. The final jointly inverted 3D S-velocity model is improved both in lateral resolution and resolution in depth which can be seen in checkerboard test results. The final S-velocity model can simultaneously fit both dispersion observations and waveform observations. The Chinese continent is an attractive region for geoscientists because of its diversity and complexity of geological evolutions. Upper mantle structure features revealed in our new model are well consistent with known geotectonic units.
T13E-1624
The Uppermost Mantle Structure in China from Pseudowave Tomography
We have obtained pseudowave velocity images of the uppermost mantle beneath China by performing tomographic inversion of travel time differences between Sn and Pn. The arrival pairs were selected from the Annual Bulletin of Chinese Earthquakes from 1984 to 2005. The dataset includes 50,136 arrival pairs from 11,470 earthquakes recorded by 116 stations. The average pseudowave velocities are 10.6 km/s. The preliminary tomographic results show that 1) the pseudowave velocity is high in the Tarim and Junggar basins, the Ordos craton, the southern region of the Sichuan basin, and the Taiwan Strait; and 2) the pseudowave velocity is low in North China, the North-South Seismic Zone, and the central and western Tibetan Plateau. The pseudowave velocity pattern obtained is very similar to the distribution of Pn and Sn velocities in the study area.
T13E-1625
Crustal and Uppermost Mantle Structure of the Yanshan Belt and Adjacent regions at the northeastern boundary of the North China Craton from Rayleigh Wave Dispersion Analysis and Inversion
We present a high resolution shear wave velocity imaging profile of the crust and uppermost mantle structure beneath the northeastern boundary regions of the North China Craton (NCC) by Rayleigh wave dispersion analysis and inversion. The data came from 46 broadband seismometers densely deployed along N-S direction in the region under a sub-project of the NCISP. Rayleigh wave phase velocities were measured at periods from 10 to 48 s, and were utilized in subsequent inversions to solve for the shear wave velocity structure from 15 km up to 120 km depth. The inverted lower crust and uppermost mantle velocities, about 3.75 km/s and 4.3 km/s on average, are apparently low compared with the global average. The Moho was constrained within the depth range of 30-40 km with an average of about 35 km, indicating a generally normal-in-thickness crust in the region. A thin lithosphere of no more than 100 km, however, was imaged under a large part of the profile with the thinnest of only ~60 km under the Inner Mongolian Axis (IMA) where an abnormally slow anomaly was observed below 60-km depth. The overall structural features resemble those of typical continental rift zones and are likely associated with the lithospheric reactivation and tectonic extension widespread in the eastern NCC during the Mesozoic-Cenozoic time. Distinctly, high velocities were found in both the crust and uppermost mantle immediately to the south of the IMA beneath the northern Yanshan Belt (YSB). In particular, the mantle anomaly reaches a deep extent of > 100 km and shows velocities as high as ~4.6 km/s, presumably representing the cratonic lithospheric lid of the region that may have not been affected by the Mesozoic-Cenozoic deformation process as strongly as other regions in the eastern NCC. Based on our shear wave velocity structural image in together with other geophysical observations, we proposed a possible lithosphere-asthenosphere interaction scenario at the northeastern boundary of the NCC. We speculate that significant undulations of the base of the lithosphere, which might have resulted from the uneven Mesozoic-Cenozoic lithospheric thinning, may induce mantle flows concentrating beneath the weak IMA and probably also under the southern edge of the YSB where it borders the Bohai Bay Basin to the south. The relatively thick lithospheric lid in the northern YSB likely serves as a tectonic barrier separating the on-craton and off-craton regions into different upper mantle convection systems at the present time.
T13E-1626
Observations of Frequency-Dependent Sn Blockage in Northern Tibet
We present new observations of the frequency-dependent propagation efficiency of the seismic phase Sn over the Tibetan plateau. Our measurements are the ratio of the Sn amplitude to the Pcoda amplitude on Tibetan regional seismograms. We map the lateral variation in the maximum and mean values of this ratio across the plateau. Good path density and azimuthal coverage allow the area of Sn blockage identified by previous studies to be better constrained. An important result is that at low frequencies (0.2 Hz), Sn propagates efficiently over the entire plateau, while at higher frequencies, Sn is blocked for a region of the northern plateau. The area of inefficient \rm Sn propagation extends farther southward as higher frequencies (between 0.2 and 1.0 Hz) are brought into consideration. The observation that low frequency Sn propagates efficiently across the whole plateau suggests that the lithosphere beneath Tibet is still intact and has not delaminated as previously proposed. The observation that high frequency Sn does not propagate beneath northern Tibet suggests that the Tibetan upper mantle has been heated and possibly contains partial melt.
T13E-1627
Joint inversion for crustal and Pn velocities and Moho depth for eastern margin of Tibetan Plateau
A classical problem in seismic tomography is the trade-off between a discontinuity location and the velocity of the medium. For that reason, crustal tomography and Pn tomography are typically done separately by isolating first arriving Pg and Pn waves. Although this avoids the contamination of secondary arrivals because of the existence of the Moho, lots of good data are discarded and, as a result, the lower crust is particularly poorly constrained. Here we design an iterative scheme to invert jointly for the velocities of the whole crust and the Pn waves as well as the Moho depth. We use a spherical pseudo bending ray tracing method for the heterogeneous crust and include secondary Pg waves at large distances. The advantage of a joint inversion approach is several fold. First, with accurate 2D Moho depth, different types of waves can be better separated, thus preventing the contamination of Pg from Pn waves. Secondly, with the resolution of 3D crustal velocity structure, Moho depth variation can be separated and simultaneously solved in the inversion. Thirdly, Pn waves and secondary Pg waves at large distances can also put constraint on crustal velocity, especially in the lower crust. We are applying the method to the eastern and southeastern margin of the Tibetan Plateau, where this method is particularly useful because of the rapid variation of crustal structure and Moho depth. Resolution of mid-lower crust structure is important for testing contested "channel-flow" model. Our data include arrival times from provincial, national, international bulletins, and our own hand picks as well as relative time measurements from cross-correlation between events. Preliminary results of our inversion will be reported.
T13E-1628
Seismicity of the Stanovoi Volcanic Field Region, Eastern Russia
The Stanovoi region of southern Yakutia is one of the most poorly understood seismically active regions in eastern Russia. Although a considerable number of earthquakes have been located in the region, the epicenters are likely poor as only distant stations were used in the locations. In addition, the active faults, type of faulting, and levels of microseismicity in the region were unknown. Within the Stanovoi region is a province of Cenozoic basaltic volcanism (0.5-1.0 Ma) that is near a cluster of previously located earthquakes. Up to now, it was unknown if the seismicity in the region had any connection to the volcanic field or to large faults visible in the satellite images and topography of the region. The proposed extension of the Tanlu fault into Russia from China may also terminate near the study area. We conducted a temporary deployment of five broadband seismic stations encircling the Stanovoi volcanic field. We also observed clear young fault scarps interpreted as southward-plunging low angle thrusts. Our seismicity results indicate that the volcanic field proper is aseismic while surrounding areas are very active. Combined analysis of seismicity and observed scarps and geomorphic features indicate that the Stanovoi region may be underlain by large, active thrust faults, which are generally consistent with north-northeastward movement of the Amur block into the Eurasian plate.
T13E-1629
Investigation of Multiple Geophysical Parameters in the Russian Far East
We investigate the structure of Eastern Russia using parameters extracted from the Michigan State University (MSU) Siberia database, compiled through cooperative efforts of MSU, Russian network operators and Los Alamos researchers. The database also includes information from teleseismic bulletins, such as those provided by the International Seismological Centre, U.S. Geological Survey and Alaska Earthquake Information Center. Selecting from the over 1.3 million phase arrival readings available in the database, we have inverted Pn arrivals to develop a tomographic image of upper mantle P-wave velocities using both direct and differential methods. Crustal P-wave travel times (Pg) are used to map the crustal velocities for the region. We are using catalog amplitude parameters to map laterally-varying compressional and shear wave attenuation features for the region from ~400,000 S and Sg amplitude readings and ~220,000 P and Pg amplitude readings. The results of our inversions will be compared with one another, and consistencies with known tectonic features in northeastern Russia will be compared, as well as the comparison with features derived for the region in previous work.
T13E-1630
3D Model of Vp/Vs Ratios in the Crust and Uppermost Mantle Beneath East Asia
We obtain a 3D model of the Vp/Vs ratio of the crust and uppermost mantle beneath East Asia by performing joint P- and S-wave travel-time tomography. Travel-time data from the extensive Annual Bulletin of Chinese Earthquakes (ABCE) and the International Seismological Centre (ISC/EHB) are used. The regional travel-time data from the ABCE is used to obtain the crust/uppermost mantle (0 to 100 km depth) velocity model. A total of 500,000 P-wave and 350,000 S-wave regional travel-times are used for the tomography. Two discontinuities (Conrad and Moho) are allowed in the inversion. The tomographic models provide three-dimensional (3-D) velocities in the crust and uppermost mantle, depth to the Moho, detailed Pn and Sn velocities, and Vp/Vs ratios. High Vp/Vs ratios exist beneath the East and South China Sea, Japan Sea and the Pacific Ocean. Along the east- west profiles crossing the Tibet, high Vp/Vs ratios in the crust beneath part of the Tibetan plateau suggest the presence of high temperatures and/or fluid. This observation is consistent with the results of previous studies in these areas based on seismic soundings and gravity surveys. Prominent high Vp/Vs ratios are observed beneath the active volcanic and geothermal sites in the Tengchong region, in the Changbai region, and in the Hainan region. Our Vp/Vs profiles clearly indicate the presence of high temperatures beneath these well-known volcanic sites. From this study, we conclude that the structure and Vp/Vs ratios we obtained for the crust and uppermost mantle are significant for better understanding of tectonic features in China and the surrounding area.
T13E-1631
Modeling Lateral Sn Velocity, Gradient, and Anisotropic Variations in the Upper Mantle
Various forms of Pn tomography have been developed to model the effect of two-dimensional (2-D) upper mantle variations on seismic travel times. The general abundance of Pn arrival times compared to Sn arrival times allows for higher resolution and more extensive path coverage. As seismic catalogs continue to increase in available data as well as pick quality, modeling upper mantle Sn velocity becomes more robust. Various researchers have modeled upper mantle velocity variations using Pn travel times. Phillips et al. (2007) also included solving for 2-D Pn gradient to the standard slowness solution. Based on the work of Zhao (1993) and Zhao and Xie (1993), the effect of upper mantle gradient on arrival times can be approximated by a simple term that can be extended to 2-D for tomographic studies. Pn gradient values matched those from previous studies for Tibet and North America. High gradients appeared to be associated with stable and convergent zones, low gradients with extensional zones. Recently, Pei et al. (2007) did a Pn and Sn seismic tomography study in China, also solving for velocity and anisotropy. Sn velocity variation was consistent with Pn variation, with overall variations consistent with tectonic activity. We merge arrivals from various local, regional, and global catalogs into one consistent, non-redundant catalog. Using the Bondár et al. (2004) ground truth (GT) criteria, we have ~22,000 GT25 and better events available for tomography in the Eurasia region. This data set includes ~3700 stations resulting in over 600,000 Sn arrivals. Using the tomography method from Phillips et al. (2007), we plan to continue the upper mantle 2-D modeling by solving for Sn slowness, gradient, and anistotropy in the Eurasia region.
T13E-1632
Imaging of P and S wave velocity structure beneath the Kii Peninsula in central Japan by combined analyses of P and S wave receiver functions
The Philippine Sea Plate (PHP) is subducting beneath the Kii Peninsula in central Japan, which generates great interplate earthquakes and low-frequency earthquakes there. It is important to closely investigate where such events occur around the PHP. Therefore we need to estimate the detailed configurations of the PHP and the Moho. Traditionally, we have so far used P wave radial receiver function (RF) to investigate S wave velocity structure by detecting P to S converted wave at seismic velocity discontinuities. In this study, we investigated P and S wave velocity structure also by detecting multiply-reflected P wave between surface and discontinuities. In the analyses, we used vertical RFs instead of auto-correlation and cross-correlation analyses. Additionally, we analyzed S wave RF of S to P and ScS to P converted waves to enhance reliability of P wave velocity structure. In this study, we used a dense seismic array that is located across the Kii Peninsula in southwest Japan from north to south. The array consists of 13 short-period seismic stations with around 5 km spacing. We used near earthquakes for detecting ScSp wave and teleseismic earthquakes for other waves. We analyzed multiply- reflected P waves by employing vertical RFs. In deconvolution, we first stacked vertical components of P waves observed at all array stations for each teleseismic event to remove local effects and improve S/N ratio. Then, we used the waveform portion with duration of 5 seconds after the P wave arrival. In addition, we analyzed Sp and ScSp waves by use of S wave RFs that are vertical components deconvolved by radial ones. In deconvolution, we used extended-time multi-taper RF estimation (Shibutani et al., 2006) instead of traditional water level methods. In the P wave RF analyses, we have investigated S wave structure by detecting Ps waves, and additionally we could investigate P wave structure by detecting multiply reflected P waves, Sp and ScSp phases. In the result of investigating P wave structure, we confirmed the phases reflected or converted at the Moho. However, we could not confirm the phase from PHP. Further newly-devised analyses would be needed to reveal the deep P and S wave structure.
T13E-1633
A Reference Crustal and Plate-Boundary Velocity Model of Japan
The study of velocity structure using earthquake data has been remarkably progressed by development of seismic observation networks, improvement of the methodology, and increase in processing power. Recent studies on 3D velocity structures have produced tomographic images with resolution of a few kilometers (e.g., Matsubara et al., 2005; Nakamichi et al., 2007). In addition to these traveltime analyses, waveform studies such as receiver function analyses have been developed to image the configuration of continental Moho and oceanic plate boundaries (e.g., Yamauchi et al., 2003; Shiomi et al., 2004). Reflection and refraction surveys with controled sources have also been providing information on 2D and 3D velocity structures (e.g., Sato et al., 2005; Special Project for Earthquake Disaster Mitigation in Urban Areas). Thus, plenty of structural property models exist over the Japan islands, but the validity of an individual model is confined to its study area. Therefore, it is essential to build a reference crustal and plate-boundary velocity model for the whole Japan by combining them all together. If this sort of reference velocity model over the Japan islands is available under a unified criterion, it will be valuable for many fields of seismology and Earth sciences. We here construct a reference crustal and plate-boundary velocity model of Japan by integrating 2D models from seismic profiling and receiver functions, 3D models of seismic tomography, and other geophysical data such as gravity anomalies. The goal of this study is to construct a 3D laterally heterogeneous seismic velocity structure model, which clarifies the topography of the Conrad and Moho discontinuities and shapes of the oceanic plates, like the SCEC Unified Velocity Model (e.g., Magistrale et al., 1996). We first make a preliminary Japan model by compiling information on the topography of the Conrad and Moho discontinuities and subducting plates. To this end, we collect 2D seismic velocity models obtained by seismic profiling of reflection/refraction surveys and receiver function analyses, and then integrate them to a 3D velocity model, using complementary information on 3D structures, such as 3D boundary shapes obtained by travel time and gravity anomaly analyses. At this stage, maintaining local structural continuity is a key challenge in the process. We also have to clarify major tectonic features such as the Median Tectonic Line and the Itoigawa-Shizuoka Tectonic Line, and consider the continuity between land-based and offshore models. The integrated 3D reference model will improve the reliability of strong ground motion prediction. Improvements will be the most critical for large-scale ground motion simulations for plate-boundary earthquakes in the Nankai and Tokai regions, which can cause strong shaking in major metropolitan areas of Japan. So, this modeling should be one of the most essential parts of earthquake damage mitigation in Japan.
T13E-1634
Seismological Features of the Subducting Slab Beneath the Kii Peninsula, Central Japan, Revealed by Receiver Functions
We report seismological evidence that the subducting Philippine Sea slab (PHS) beneath the Kii Peninsula, central Japan, can be divided into three segments. Offshore the Kii Peninsula, the "Tonankai" and "Nankai" fault segments suffer mega-thrust earthquakes that repeat every 100 to 150 years. The structure of the young, thin, contorted PHS is important to the seismo-tectonics in this region. We apply the receiver function (RF) analysis to 26 Hi-net short-period and 4 F-net broad-band seismographic stations. In the case that dipping velocity discontinuities and/or anisotropic media exist beneath seismometer, both radial RFs and transverse RFs contain useful information to estimate underground structure. For isotropic media with a dipping-slab interface, back- azimuthal variation in RFs depends largely on three parameters, the downdip azimuth, dip angle and the depth of the interface. We stack both radial and transverse RFs with allowance a time-shift caused by the dipping interface, searching for optimal parameters based on the grid-search technique at each station. At some stations located near the eastern coastline of the Kii Peninsula, the dip angle of the interface inferred from RF stacking is much steeper than that estimated by the local seismicity. This discrepancy arises from the interference of two slab-converted phases, suggesting a layer atop the slab. In these cases we refine the stack to distinguish two slab phases and estimate three parameters of each dipping interface separately. Two interfaces with the same dip direction and low dip angle are estimated at these stations, with depth difference near 6 km. Thus, the shallower interface may be related to the layer within the oceanic crust and the deeper one is the slab Moho. These double-layered interfaces are detected only at stations located up-dip of a belt-like distribution of non- volcanic low-frequency tremor. Comparing the interface dips estimated in this study with the direction of slab motion determined by the GPS observation, we can classify the slab beneath the Kii Peninsula into three segments: eastern, central and southern. Within the eastern part, the RF-estimated dip direction is shifted about 30° clockwise from the GPS- estimated slab motion, and the dip angle is small. The difference of RF-estimated slab dip and GPS-estimated slab motion is small in the central part. Discrepancy between slab dip and motion estimates increases to 40°--90° in the western part. At the some stations located in the western and eastern part of the Kii Peninsula, the dip angle of the interface is much steeper than another model estimated by the local seismicity. Since the local seismicity distribution reflects well the larger-scale geometry of the slab, the RF-estimated dip angle may imply anisotropic wavespeeds within strongly sheared media above the slab. The boundary between the eastern and central regions of the Kii Peninsula corresponds to the region where (1) a topographic high in the slab interface is subducting and (2) the intraslab seismicity shows double-layered activity. The boundary between central and southern regions is coincident with the segment boundary of megathrust earthquakes in the Nankai region. The structural features revealed by RF-stacking may be an important key to the seismotectonics around the Kii Peninsula.
T13E-1635
S-wave Velocity Structure in the Kanto Basin from Inverting the HZ Ratios of Rayleigh Waves
We are carrying out detailed modeling of velocity structure in the Tokyo metropolitan area in order to upgrade strong ground motion prediction. We have proposed an integrated velocity structure model by compiling refraction/reflection, borehole, microtremor, and gravity data by joint inversion method (Tanaka et al., 2005). We here perform further modeling of velocity structure of sedimentary layer up to the depth of seismic basement, by the tuning of S-wave velocity model using HZ ratios of Rayleigh waves extracted from continuously observed data of broadband seismographs. The velocity structure model of Tanaka et al. (2005) was estimated by the refraction/gravity joint inversion method with refraction data obtained by artificial exploding events and gravity data in the whole Kanto basin. The model consists of three sediment layers (Shimosa, Kazusa, and Miura layers).The depths of Kazusa/Miura and sediment/basement interfaces, and the basement velocity distribution are obtained to minimize the residuals of travel times and gravity data. Based on the above velocity model, we applied the H/V spectral peak frequency matching method (Suzuki et. al., 2005) to the tuning of velocity structure model. This method modifies the model to adjust theoretical H/V spectral ratio calculated from the model to observed one averaged from the data of moderate earthquakes. We however found the method had some problems; 1) Extracted spectra contain waves not only the assumed Rayleigh waves, 2) Some stations have not-obvious peak and did not show good match with observed spectra, 3) Locations of large earthquakes around the Kanto basin are unevenly distributed. We then applied the Rayleigh wave HZ ratio inversion method (Tanimoto and Alvizuri, 2006), as the method of extracting Rayleigh waves from long-term microtremor data, and inverting HZ ratio calculated from the whole spectral waveform, for the tuning of structure model including S-wave velocity structure. We show some improvements in this approach; 1) Picking correctly extracted Rayleigh wave with phase-shift characteristics, 2) Trying to fit the shape of Rayleigh wave HZ ratio with much information of spectra from previous method, 3) Using microtremors observed at each stations, and large earthquakes around Kanto basin covers azimuthally from each site. We applied the inversion of the HZ ratio of Rayleigh wave for the modeling of 1-D S-wave velocity structure under the F-net sites in the Kanto basin. We used 1-year data in the term of 1998-2006 from continuously observed data by broadband seismographs of F-net. As a starting model of HZ ratio inversion, we extracted 1-D velocity structure at each site from the result of joint inversion. We show the obtained S-wave structure model contains slightly higher velocity than the initial model. We will apply this method for the earthquake data as opposed to microseisms at highly distributed stations in the Kanto basin.
T13E-1636
Investigate the aseicmic zone in Central Range of Taiwan by analyzing the short-period waveform data
Many scientists are interested in the aseismic zone beneath central Taiwan. However, there are no optimum explanations to prove it. This research is talking about the earthquakes in the aseismic zone beneath Central Range of Taiwan. According to the seismicity from 1992 to 1996 in Taiwan, it is obviously few earthquakes in Central Range. The temporal seismic stations were set up by the Institute of Earth Science (IES) of the Academia Sinica in Taiwan. The continuous short period waveform are powerful enough to provide evidences for whether there are small and local earthquakes or not. The continuous waveform data were collected from January to March in 2007 for stations DT01, DT02 and DT03. We also collected the waveform data, closed to the aseismic zone, recorded by the stations in TAIGER project and Central Weather Bureau Seismic Network (CWBSN). After preliminary processes, there are indeed lots of earthquakes occurred beneath Central Range in aseismic zone. We use those data of events to relocate the earthquakes. We analyze the travel times and waveform of those earthquakes to retrieve tectonic structure beneath central Taiwan.
T13E-1637
Estimations of the Shallow Velocity Structures in the Taipei Basin, Taiwan, Using Inversion of the Receiver Function
It is essential to estimate the S-wave velocity structure of sediments on the bedrock for the strong-motion simulation. Based on the receiver function method, we investigate the shallow S-wave velocity structures of the Taipei basin using the strong-motion records of TSMIP in Taiwan. A receiver function of a complex Fourier spectral ratio (R/V), radial over vertical component, is originally applied to estimate S-wave velocity structures of crust and mantle using the far-field long-period P-waves (Langston, 1979). In this study, the PS-phase conversion is applied to detect the depths of the seismic bedrock. We calculate the receiver functions at the frequencies of roughly 1-5Hz. According to the inversion of the receiver functions, the depths of the bedrock gradually increase from southeast to northwest. The result is in good agreement with the available geological information in the Taipei basin
T13E-1638
Estimations of the S-Wave Velocity Structures at the Puli Area, Taiwan, Using the Array Records of Microtremors
In Puli, the collapse of many structures during the Chi-Chi earthquake was very closely related to the site effects (Huang and Tarng, 2005). The Shallow Shear-wave velocities have widely been used for earthquake ground- motion site characterization. Thus, the S-wave velocity structures of the Puli area are investigated using the array records of microtremors at three sites (CUH, NAK and PUL). The dispersion curves at these sites are calculated using the F-K method (Capon, 1969); then, the S-wave velocity structures at the Puli area are estimated by employing the surface wave inversion technique (Herrmann, 1991). Compared with CUH and NAK, PUL has higher phase velocities at frequencies of 2-6 Hz and lower values at frequencies of 0.3-1.3Hz. If the S-wave velocity of bedrock is assumed to be 2000m/sec, the depths of the alluvium at the Puli area are between 450m (NAK) and 660m (PUL). Besides, three distinct interfaces (330-420m, 570-630m, and 800-840m) exist in the shallow velocity structure. The above results are in good agreement with those using the seismic exploration method (Wang et al., 2002).
T13E-1639
Crustal Structure Of The Himalayan Foreland Basin Inferred From Surface Wave Group Velocity Analysis
The Himalayan foreland basin is one of the largest and most dynamic terrestrial basins on the earth's surface. It stretches between the syntaxial belt of NW and NE Himalaya and arc southward into the Arabian Sea in the west and the Bay of Bengal in the east. The foreland can be broadly sub- divided into the Gangetic and the Indus basins. The Gangetic foreland basin fronts the eastern and central Himalaya, while the Indus foreland is situated to the south of the NW Himalaya forming the western edge of the Indian subcontinent. The Aravalli craton divides the Himalayan foreland with the Gangetic basin to its east and the Indus basin to the west. We study the crustal fabric of the two basins through surface wave group velocity modeling. We use several local and regional earthquakes from Himalaya and Pakistan recorded on the broadband seismographs located on the southern edge of the Ganga basin. Most of the data set provides pure path analysis of the crustal structure of these basins. Inversion of fundamental and first higher mode Rayleigh wave group velocity dispersion for the Ganga Basin reveal a ~5 km thick sedimentary layer with Vs 2.75 km/s followed by a two layer crust with average Vs of 3.6 km/s and an Sn velocity of 4.2 km/s. A comparison with the shear wave velocity models of the Indus basin show that the Gangetic basin is underlain by felsic crust similar to those of Bundelkhand craton, north India, while the Indus basin has a more mafic, higher velocity crust. The crustal thickness beneath both basins is similar (~40 km), but the Sn velocity beneath the Indus basin (~4.5 km/s) is much higher than that of the Ganga basin.