Receiver function structures beneath the Haiyuan fault on the northeastern margin of the Tibetan plateau

We performed a receiver function analysis on teleseismic data recorded along two dense seismic profiles and from 4 broadband regional seismic stations across the northeastern Tibetan plateau. The crustal thickness and v_P/v_S ratio were measured by the H-κ domain search algorithm. The Moho discontinuity across the Haiyuan arc fault zone was also revealed by common conversion point (CCP) imaging. Our study results show that the crustal thickness and the v_P/v_S ratio were 42–56 km and 1.60–1.88, respectively. The crustal thickening on the northeastern margin indicates that the crust is shortening or that there was a superimposition of crusts during the collision of the Tibetan plateau with Eurasian block. Our results suggest that Haiyuan fault likely resulted from the interactions of high temperature and pressure conditions during the collision of the Indian and Asian continents. The Moho beneath the Haiyuan tectonic region exhibits an obvious offset and a vague discontinuity according to CCP imaging. This study suggests that the Haiyuan arc fault zone is a trans-crustal fault that cuts through the Moho in the northeastern Tibetan Plateau. Moreover, there are indications of strong deformation in the intensive crustal extrusion from the interior of the Tibetan Plateau to its northeastern margin.     

Study of crustal structure and geological implications of southwestern margin of Northeast India

It is noticed that few geophysical studies have been carried out to decipher the crustal structure of southwestern part of the Northeast India comprising of Tripura fold belt and Bengal basin as compared to the Shillong plateau and the Brahmaputra basin. This region has a long history of seismicity that is still continuing. We have determined first-order crustal features in terms of Moho depths (H) and average V_P/V_S ratios (κ) using H-κ stacking technique. The inversion of receiver functions data yields near surface thick sedimentary layer in the Bengal basin, which is nearly absent in the Shillong plateau and Tripura fold belt. Our result suggests that the crust is thicker (38–45 km) in the Tripura fold belt region with higher shear-wave velocity in the lower crust than the Shillong plateau. The distribution of V_P/V_S ratio indicates heterogeneity throughout the whole region. While low to medium value of Poisson’s ratio (1.69–1.75) indicates the presence of felsic crust in the Shillong plateau of the extended Indian Archean crust. The medium to high values of V_P/V_S ratio (> 1.780) in the Bengal basin and the Tripura fold belt region represent mafic crust during the formation of the Bengal delta and the Tripura fold belt creation in the Precambrian to the Permian age. The depth of the sediments in the Bengal basin is up to 8 km on its eastern margin, which get shallower toward its northeastern and southeastern margins.     

Crustal velocity structure in the southern edge of the Central Alborz (Iran)

Inversion of local earthquake travel times and joint inversion of receiver functions and Rayleigh wave group velocity measurements were used to derive a simple model for the velocity crustal structure beneath the southern edge of the Central Alborz (Iran), including the seismically active area around the megacity of Tehran. The P and S travel times from 115 well-located earthquakes recorded by a dense local seismic network, operated from June to November 2006, were inverted to determine a 1D velocity model of the upper crust. The limited range of earthquake depths (between 2 km and 26 km) prevents us determining any velocity interfaces deeper than 25 km. The velocity of the lower crust and the depth of the Moho were found by joint inversion of receiver functions and Rayleigh wave group velocity data. The resulting P-wave velocity model comprises an upper crust with 3 km and 4 km thick sedimentary layers with P wave velocities (V_p) of ～5.4 and ～5.8 km s^?1, respectively, above 9 km and 8 km thick layers of upper crystalline crust (V_p ～6.1 and ～6.25 km s^?1 respectively). The lower crystalline crust is ～34 km thick (V_p ～ 6.40 km s^?1). The total crustal thickness beneath this part of the Central Alborz is 58 ± 2 km.     

六盘山断裂带及其邻区地壳结构

新生代期间,中国大陆西部受印度一欧亚板块碰撞和青藏高原隆升影响,以地壳缩短、增厚、陆内造山和强烈地震活动等为主要特征.在青藏高原东北边缘,高原物质侧向移动被鄂尔多斯地块所阻,在六盘山地区发育了一系列左旋斜冲断裂.断裂带周缘构造变形强烈,地震活动频繁,是研究青藏高原横向扩展控制大陆内部弥散变形的理想场所.本文对穿越青藏高原东北缘一六盘山断裂带一鄂尔多斯地块的宽角反射与折射地震资料使用层析成像和射线反演算法进行成像,获得了研究区地壳速度结构模型,其结果反映出六盘山断裂带两侧地壳结构、构造特征差异显著:1)上地壳层析成像结果显示鄂尔多斯盆地一侧地壳上部速度较低,等值线呈近水平状,具有典型的沉积盆地特征,而青藏高原东北缘一侧上地壳速度相对较高,横向变化剧烈,呈褶皱状,二者的分界为海原一六盘山逆冲走滑断裂;2)全地壳射线反演结果显示鄂尔多斯地块地壳速度梯度大,下地壳底部速度高由铁镁质物质组成,具有典型稳定古老克拉通的特征,青藏高原东北缘地壳速度总体较低,主要由长英质及长英-铁镁质过渡物质组成,具有典型造山带的特征,而六盘山断裂带下方地壳速度结构复杂,层面呈拱形,部分层出现速度逆转,为两个构造单元的接触过渡带;3)青藏高原东北缘一侧地壳厚度~50 km,鄂尔多斯地块地壳厚度~42 km,六盘山断裂带下方莫霍面发生叠置,揭示出青藏高原东北缘、鄂尔多斯地壳在六盘山下汇聚,较薄且刚性的鄂尔多斯地壳挤入较厚且塑性的青藏高原东北缘地壳中的构造模式.     

The geophysical significance of oceanic plagiogranite

Rocks ranging in composition from trondhjemite to diorite (plagiogranites) have been recovered from ocean ridges and are common constituents of ophiolites. Velocities and densities of diorite and trondhjemite from the Mid-Atlantic Ridge are shown to differ significantly from similar properties of metadolerite and gabbro. Compressional (V_p) and shear (V_s) velocities of plagiogranites are relatively low (V_p = 4.78–5.91km/s at1kbar,V_s = 2.81–3.37km/s at1kbar), as are densities (2.57–2.64 g/cm³) and Poisson's ratios (0.24–0.27). These data lend strong support to the probable existence of a low-velocity/density zone within layer 3 of the oceanic crust. Based on observations in ophiolites, it is postulated that this zone can be up to 1 km in thickness and is laterally discontinuous.     

利用虚拟地震测深法约束青藏高原东北缘及周边地区地壳厚度

青藏高原东北缘作为高原向外扩张的最前缘地区,代表了高原最新的变形状态,是研究青藏高原变形加厚的关键地区.本文利用"中国地震科学台阵探测"项目在南北地震带北段布设的密集宽频带流动台阵资料,采用虚拟地震测深方法（VDSS）,对青藏高原东北缘及周边地区的地壳厚度进行了研究,以期为研究青藏高原东北向扩展的前缘位置,以及扩展的动力学模式等提供地球物理学依据.波形模拟的结果显示,研究区地壳厚度变化剧烈.其中,祁连和西秦岭地块内地壳厚度存在明显的东西向横向变化,以103°E为界,东部地区为45~50 km,而西部地区地壳已明显增厚,约达到55 km以上.与祁连造山带相邻的阿拉善块体南缘地壳也明显加厚,接近55 km,而阿拉善块体内部地壳厚度约为45~50 km.与其他研究地区相比,鄂尔多斯地块地壳相对要薄,但整体而言,鄂尔多斯地块地壳呈现南北薄（约45 km）、中央厚（约50 km）的形态特征.此外,在六盘山断裂带台站下方观测到复杂的SsPmp震相,推测为双Moho界面结构.结合其他地球物理学证据,我们认为青藏高原东北缘地区地壳增厚方式以均匀缩短增厚为主,且高原向北东扩展的前缘已越过祁连山北缘断裂,进入阿拉善块体南缘地区.     

The Himalayan foreland basin crust and upper mantle

Modeling of multimode surface wave group velocity dispersion data sampling the eastern and the western Ganga basins, reveals a three layer crust with an average V_s of 3.7 km s^?1, draped by ～2.5 km foreland sediments. The Moho is at a depth of 43 ± 2 km and 41 ± 2 km beneath the eastern and the western Ganga basins respectively. Crustal V_p/V_s shows a felsic upper and middle crust beneath the eastern Ganga basin (1.70) compared to a more mafic western Ganga basin crust (1.77). Due to higher radiogenic heat production in felsic than mafic rocks, a lateral thermal heterogeneity will be present in the foreland basin crust. This heterogeneity had been previously observed in the north Indian Shield immediately south of the foreland basin and must also continue northward below the Himalaya. The high heat producing felsic crust, underthrust below the Himalayas could be an important cause for melting of midcrustal rocks and emplacement of leucogranites. This is a plausible explanation for abundance of leucogranites in the east-central Himalaya compared to the west. The uppermost mantle V_s is also significantly lower beneath the eastern Ganga basin (4.30 km s^?1) compared to the west (4.44 km s^?1).     

Crustal Structure Along Sunda-Banda Arc Transition Zone from Teleseismic Receiver Functions

We analyzed receiver function of teleseismic events recorded at twelve Indonesian-GEOFON (IA-GE) broadband stations using nonlinear Neighbourhood Algorithm (NA) inversion and H-k stacking methods to estimate crustal thickness, V _p /V _s ratios and S-wave velocity structure along Sunda-Banda arc transition zone. We observed crustal thickness of 34–37 km in Timor Island, which is consistent with the previous works. The thick crust (> 30 km) is also found beneath Sumba and Flores Islands, which might be related to the arc-continent collision causing the thickened crust. In Timor and Sumba Islands, we observed high V _p /V _s ratio (> 1.84) with low velocity zone that might be associated with the presence of mafic and ultramafic materials and fluid filled fracture zone. The high V _p /V _s ratio observed at Sumbawa and Flores volcanic Islands might be an indication of partial melt related to the upwelling of hot asthenosphere material through the subducted slab.     

Nature of the upper crust beneath central Tibet

A wide-angle reflection/refraction seismic line obtained through Sino-French joint research near the Bangung-Nujiang Suture in Tibet [1] yielded crustal velocity estimates for both P and S waves. AV_p/V_s ratio of 1.65 was derived fo waves propagating in the upper ( < 25 km) crust. According to the high-pressure, high-temperature velocity measurements of Kern [2] and others, such a ratio implies the presence of quartz-rich rocks, e.g., granites and granitic gneisses, in the upper Tibetan crust. Because of the high geothermal gradient in Tibet ( > 50°C/km near the surface), a shallow low-velocity zone is expected to form in a granitic upper crust for both P and S waves. A two-dimensional ray-tracing study indicates that the observed first P and S waves [1] can be interpreted as diving waves above a low-velocity zone where a positive gradient exists. The low-velocity zone commences at a depth of 10 km and extends possibly to 20 km or deeper.     

Tomographic imaging of seismic velocity structure in and around the Onikobe volcanic area, northeastern Japan: implications for fluid distribution

We estimated three-dimensional P- (V_p) and S-wave velocity (V_s) and V_p/V_s structures in and around the Onikobe volcanic area, northeastern Japan, by local travel time tomography. We used travel time data from source and receiver pairs located within and outside the study area, which plays an important role in obtaining the optimum ray coverage and in elucidating the deeper structure more accurately. Detailed information on deeper structures is essential for imaging the complete volcanic system from the magmatic source zone through areas of shallow hydrothermal circulation. More than 50 000 travel time data for the P-waves and 35 000 for the S-waves were used to image the velocity structure. Our results show the following dominant features: (1) two conduits in the upper crust with low V_p and low V_s indicative of H₂O-rich fluid pathways: one lying beneath Naruko volcano, the other beneath the focal area of the 1962 Northern Miyagi earthquake (M6.5); (2) an underlying broad region in the lower crust with low V_p, low V_s and high V_p/V_s, suggestive of a zone of partial melt, from which the fluids in (1) are derived; and (3) low V_p/V_s areas near the surface of the Sanzugawa and Onikobe calderas, suggesting a diffuse vapor-saturated cap.     

Use of mode-converted waves in marine seismic data to investigate the lithology of the sub-bottom sediments in Isfjorden,Svalbard

The compression wavefield is efficiently converted to shear-wave energy at post-critical angles in areas of high impedance contrast at the sea floor. We have analysed mode-converted shear waves in a data set acquired with a hybrid marine/land geometry in Isfjorden, Svalbard. Through a kinematic 2D ray-tracing modellingV _p/V_s ratios for part of the uppermost 5km of the crust are obtained. Low values (V _p /V _s =1.65) are tentatively associated with the section of Devonian sandstones which appears to attain a minimum thickness of 1.5km below 3 km depth about 10km west of Kapp Thorden.     

Vp and Vp/Vs structures in the crust and upper mantle of the Taiwan region, China

A tomographic study of the V _p and V _p/V _s structures in the crust and upper mantle beneath the Taiwan region of China is conducted by simultaneous inversion of P and S arrival times. Compared with the previous tomographic results, the spherical finite difference technique is suitable for the strong heterogeneous velocity structure, and may improve the accuracy in the travel time and three-dimensional ray tracing calculations. The V _p and V _p/V _s structures derived from joint inversion and the relocated earthquakes can provide better constraints for analyzing the lateral heterogeneity and deep tectonic characters in the crust and upper mantle. Our tomographic results reveal significant relations between the seismic wavespeed structure and the tectonic characters. In the shallow depth, sedimentary basins and orogen show distinct wavespeed anomalies, with low V _p, high V _p/V _s in basins and high V _p, low V _p/V _s in orogen. As the suture zone of Eurasian Plate and Philippine Sea Plate, Longitudinal Valley is characterized by a significant high V _p/V _s anomaly extending to the middle-lower crust and upper mantle, which reflects the impact of rock cracking, partial melting, and the presence of fluids. In the northeast Taiwan, the V _p, V _p/V _s anomalies and relocated earthquakes depict the subducting Philippine Sea Plate under the Eurasian Plate. The high V _p of oceanic plate and the low V _p, high V _p/V _s atop the subducted oceanic plate extend to 80 km depth. Along the east-west profiles, the thickness of crust reaches 60 km at the east of Central Range with eastward dipping trend, which reveals the eastward subduction of the thickened and deformed crust of the Eurasian continental plate. Supported by Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KZCX3-SW-234-2), National Basic Research Program of China (Grant No. 2007CB411701), National High Technology Research and Development Program of China (Grant No. 2006AA09A101-0201) and National Natural Science Foundation of China (Grant Nos. 40804016, 40704013)     

Crust and uppermost mantle structure of the Ailaoshan-Red River fault from receiver function analysis

S-wave velocity structure beneath the Ailaoshan-Red River fault was obtained from receiver functions by using teleseismic body wave records of broadband digital seismic stations. The average crustal thickness, Vp/Vs ratio and Poisson’s ratio were also estimated. The results indicate that the interface of crust and mantle beneath the Ailaoshan-Red River fault is not a sharp velocity discontinuity but a characteristic transition zone. The velocity increases relatively fast at the depth of Moho and then increases slowly in the uppermost mantle. The average crustal thickness across the fault is 36―37 km on the southwest side and 40―42 km on the northeast side, indicating that the fault cuts the crust. The relatively high Poisson’s ratio (0.26―0.28) of the crust implies a high content of mafic materials in the lower crust. Moreover, the lower crust with low velocity could be an ideal position for decoupling between the crust and upper mantle.     

Crustal structure of northeastern Tibetan plateau and Ordos block: Waveform interpretation of the Maqen-Jingbian seismic refraction profile

The Maqen-Jingbian wide-angle seismic reflection and refraction experiment was carried out in 1998, which aims at determining detailed structure in the crust and top of the upper mantle and understanding structural relation between the northeastern Tibetan plateau and the Ordos block. The 1-D crustal models inferred by waveform inversion show strong variations in crustal structure, which can be classified into four different types: ① an Ordos platform with the Proterozoic crust and two high-velocity layers in the northeast section, ② a transitional crust between the northeastern Tibetan plateau and the Ordos block across the Haiyuan earthquake zone, ③ the Qilian orogenic zone in the central part, and 4 the Qinling orogenic zone in the southwestern section. The Moho depth increases from ~42 km to ~62 km from the NE part to the SW part of the profile. The crystalline crust consists of the upper crust and lower crust in northeastern Tibetan plateau. There is an obviously low P-wave velocity layer dipping northeastward, which is 12–13 km thick, at the bottom of the upper crust in Qinling orogenic zone and Haiyuan earthquake zone. The lower crust is characterized by alternating high and low P-wave velocity layers. Beneath Ordos block, i.e., the NE part of the profile, the crust shows quite a smooth increase in P-wave velocity down to the Moho at a depth of about 42 km.     

CrustalP-wave velocity and velocity-ratio study in northeast India by a microearthquake survey

The upper crustal (20 km)P-wave velocity beneath the Shillong Plateau and Nowgong area has been studied by the time-distance plot method. TheP-arrival data of the shallow (20 km) microearthquakes from three temporary networks are used, and the average velocity is found to be 5.55 km/s. The velocity ratio (V _p /V _s ) for the upper crust (0–20 km) as well as for the lower crust (21–40 km) are determined by the Wadati-plot method and station-by-station method. The average value obtained by the two methods is compatible; theV _p /V _s ranges between 1.74 to 1.76. A generalized seismic velocity model of the area is suggested by this study, which has been very useful for microearthquake location.     

Characterization of the crust of the Coast Mountains Batholith,British Columbia,from P to S converted seismic waves and petrologic modeling

The late Triassic to early Tertiary Coast Mountains Batholith (CMB) of British Columbia provides an ideal locale to study the processes whereby accreted terranes and subduction-related melts interact to form stable continental crust of intermediate to felsic composition and complementary ultramafic residuals. Seismic measurements, combined with calculated elastic properties of various CMB rock compositions, provide a window into the deep-crustal lithologies that are key to understanding the processes of continental growth and evolution. We use a combination of seismic observations and petrologic modeling to construct hypothetical crustal sections at representative locations across the CMB, then test the viability of these sections via forward modeling with synthetic seismic data. The compositions that make up our petrologic forward models are based on calculations using the free energy minimization program Perple_X to predict mineral assemblages at depth for the bulk compositions of exposed plutonic rocks collected in the study area. Seismic data were collected along two transects in west-central British Columbia: a southern line that crossed the CMB near the town of Bella Coola (near 52° N), and a northern line centered on the towns of Terrace and Kitimat (near 54° N). Along both transects, seismic receiver functions reveal high V_p/V_s ratios near the Insular/Intermontane terrane boundary and crustal thickness increasing from 26 ± 3 km to 34 ± 3 km (at the 1 sigma certainty level) from west to east across the Coast Shear Zone (CSZ). On the southern line, we observe an anomalous region of complex receiver functions and diminished Moho signals beneath the central portion of the CMB. Our petrologic and seismic profiles show that observed seismic data from much of the CMB can be well-matched in terms of crustal thickness and structure, average V_p/V_s, and amplitude of the Moho converted phase, without including ultramafic residual material in the lower crust.     

Velocities,elastic moduli and weathering-age relations for pacific layer 2 basalts

Compressional (V_p) and shear (V_s) wave velocities have been measured to 10 kb in 32 cores of basalt from 14 Pacific sites of the Deep Sea Drilling Project. Both V_pandV_s show wide ranges (3.70to6.38km/sec forV_pand1.77to3.40km/sec forV_sat0.5kb) which are linearly related to density and sea floor age, confirming earlier findings by Christensen and Salisbury of decreasing velocity with progressive submarine weathering based on studies of basalts from five sites in the Atlantic. Combined Pacific and Atlantic data give rates of decreasing velocity of ?1.89and?1.35km/sec per100my forV_pandV_s respectively. New analyses of oceanic seismic refraction data indicate a decrease in layer 2 velocities with age similar to that observed in the laboratory, suggesting that weathering penetrates to several hundred meters in many regions and is largely responsible for the extreme range and variability of layer 2 refraction velocities.     

Crustal seismic structure and depth distribution of earthquakes in the Archean Kuusamo region,Fennoscandian Shield

Two-dimensional crustal velocity models are derived from passive seismic observations for the Archean Karelian bedrock of north-eastern Finland. In addition, an updated Moho depth map is constructed by integrating the results of this study with previous data sets. The structural models image a typical three-layer Archean crust, with thickness varying between 40 and 52 km. P wave velocities within the 12–20 km thick upper crust range from 6.1 to 6.4 km/s. The relatively high velocities are related to layered mafic intrusive and volcanic rocks. The middle crust is a fairly homogeneous layer associated with velocities of 6.5–6.8 km/s. The boundary between middle and lower crust is located at depths between 28 and 38 km. The thickness of the lower crust increases from 5–15 km in the Archean part to 15–22 km in the Archean–Proterozoic transition zone. In the lower crust and uppermost mantle, P wave velocities vary between 6.9–7.3 km/s and 7.9–8.2 km/s. The average V_p/V_s ratio increases from 1.71 in the upper crust to 1.76 in the lower crust.The crust attains its maximum thickness in the south-east, where the Archean crust is both over- and underthrust by the Proterozoic crust. A crustal depression bulging out from that zone to the N–NE towards Kuusamo is linked to a collision between major Archean blocks. Further north, crustal thickening under the Salla and Kittilä greenstone belts is tentatively associated with a NW–SE-oriented collision zone or major shear zone. Elevated Moho beneath the Pudasjärvi block is primarily explained with rift-related extension and crustal thinning at ∼2.4–2.1 Ga.The new crustal velocity models and synthetic waveform modelling are used to outline the thickness of the seismogenic layer beneath the temporary Kuusamo seismic network. Lack of seismic activity within the mafic high-velocity body in the uppermost 8 km of crust and relative abundance of mid-crustal, i.e., 14–30 km deep earthquakes are characteristic features of the Kuusamo seismicity. The upper limit of seismicity is attributed to the excess of strong mafic material in the uppermost crust. Comparison with the rheological profiles of the lithosphere, calculated at nearby locations, indicates that the base of the seismogenic layer correlates best with the onset of brittle to ductile transition at about 30 km depth.We found no evidence on microearthquake activity in the lower crust beneath the Archean Karelian craton. However, a data set of relatively well-constrained events extracted from the regional earthquake catalogue implies a deeper cut-off depth for earthquakes in the Norrbotten tectonic province of northern Sweden.     

Receiver function study in northern Sumatra and the Malaysian peninsula

In this receiver function study, we investigate the structure of the crust beneath six seismic broadband stations close to the Sunda Arc formed by subduction of the Indo-Australian under the Sunda plate. We apply three different methods to analyse receiver functions at single stations. A recently developed algorithm determines absolute shear-wave velocities from observed frequency-dependent apparent incidence angles of P waves. Using waveform inversion of receiver functions and a modified Zhu and Kanamori algorithm, properties of discontinuities such as depth, velocity contrast, and sharpness are determined. The combination of the methods leads to robust results. The approach is validated by synthetic tests. Stations located on Malaysia show high-shear-wave velocities (V _S) near the surface in the range of 3.4–3.6 km s^− 1 attributed to crystalline rocks and 3.6–4.0 km s^− 1 in the lower crust. Upper and lower crust are clearly separated, the Moho is found at normal depths of 30–34 km where it forms a sharp discontinuity at station KUM or a gradient at stations IPM and KOM. For stations close to the subduction zone (BSI, GSI and PSI) complexity within the crust is high. Near the surface low V _S of 2.6–2.9 km s^− 1 indicate sediment layers. High V _S of 4.2 km s^− 1 are found at depth greater than 6 and 2 km at BSI and PSI, respectively. There, the Moho is located at 37 and 40 km depth. At station GSI, situated closest to the trench, the subducting slab is imaged as a north-east dipping structure separated from the sediment layer by a 10 km wide gradient in V _S between 10 and 20 km depth. Within the subducting slab V _S ≈ 4.7 km s^− 1. At station BSI, the subducting slab is found at depth between 90 and 110 km dipping 20° ± 8° in approximately N 60° E. A velocity increase in similar depth is indicated at station PSI, however no evidence for a dipping layer is found.     

Crustal thickness,VP/VS ratio,and shear wave velocity structures beneath Myanmar and their tectonic implications

The crustal structure in Myanmar can provide valuable information for the eastern margin of the ongoing Indo-Eurasian collision system. We successively performed H–k stacking of the receiver function and joint inversion of the receiver function and surface wave dispersion to invert the crustal thickness (H), shear wave velocity (V_S), and the V_P/V_S ratio (k) beneath nine permanent seismic stations in Myanmar. H was found to increase from 26 ?km in the south and east of the study area to 51 ?km in the north and west, and the V_P/V_S ratio was complex and high. Striking differences in the crust were observed for different tectonic areas. In the Indo-Burma Range, the thick crust (H ?～ ?51 ?km) and lower velocities may be related to the accretionary wedge from the Indian Plate. In the Central Myanmar Basin, the thin crust (H ?= ?26.9–35.5 ?km) and complex V_P/V_S ratio and V_S suggest extensional tectonics. In the Eastern Shan Plateau, the relatively thick crust and normal V_P/V_S ratio are consistent with its location along the western edge of the rigid Sunda Block.