中国东部及其邻区上地幔顶部Pn波速度结构及各向异性

中国东部地处欧亚板块东南部,紧邻西太平洋板块俯冲带,有着复杂的地质构造和深部结构,是国内外学者研究的重点区域.本文采用Pn波层析成像方法反演得到了中国东部及其邻区上地幔顶部Pn波速度结构及各向异性.研究中应用的Pn到时数据主要来自多种地震观测报告,并特别补充了东北流动台阵、华北流动台阵以及区域地震台网记录的地震事件,拾取了大量高精度的Pn到时数据,最终挑选出2049个台站记录的24072次地震的240814条Pn波到时数据.结果显示,中国东部地区上地幔顶部平均速度为8.06 km·s^-1,速度变化范围为7.81~8.32 km·s^-1.东北地区东部表现为显著的低速异常,华北克拉通中、东部Pn呈现低速,而西部地区则表现为高速异常,华南地块主体表现为高速.反演结果还揭示江汉盆地、下辽河盆地、二连盆地及海拉尔盆地都显示出高速,而四川盆地和松辽盆地内部则呈现出不均匀的结构特征.四川盆地的高速异常显示出明显的分块现象,这可能是该盆地在沉积前具有不同的基底物质;松辽盆地的北部呈现为高速,而南部却表现为低速异常,这一特征与松辽盆地南、北部分别为高、低热流相对应,暗示盆地南部的岩石圈已经历了改造.研究进一步揭示Pn波低速区和高、低速过渡带的各向异性也较为强烈,而大部分强震都发生在这些区域之上的地壳内,说明这些部位容易发生变形而应力集中或产生应力差.     

THE 3-D VELOCITY STRUCTURE OF CRUST AND UPPERMOST MANTLE AND ITS TECTONIC IMPLICATIONS IN FUJIAN PROVINCE

It is important to detect the fine velocity structures of the crust and uppermost mantle to understand the regional tectonic evolution, earthquake generation processes, and to conduct earthquake risk assessment. The inversion of uppermost mantle velocity and Moho depth are strongly influenced by crustal velocity heterogeneity. In this study, we collected first arrivals of Pg and Pn and secondary arrivals of Pg wave from the seismograms recorded at Fujian provincial seismic network stations. New 3-D P-wave velocities were inverted by multi-phase joint inversion method in Fujian Province. Our results show that the fault zones in Fujian Province have various velocity patterns. The shallow crust is characterized by high velocity that represents mountains, while the mid-lower crust shows low velocities. The anomalous velocities are correlated closely with tectonic faults in Fujian Province. Velocity anomalies mainly show NE-trending distribution, especially in the mid-lower crust and uppermost mantle, which is consistent with the NE-trending of the regional main fault zones. Meanwhile, a part of velocity patterns show NW trending, which is related to the secondary NW-oriented faults. Such velocity distribution also shows a geological structural pattern of "zoning in east-west direction and blocking in north-south direction" in Fujian area. In the crust, a low velocity zone is found along Zhenghe-Dapu fault zone as mentioned by previous study, however our result shows the low velocity exists at depth of 20~30km in mid-lower crust. Compared with previous study, this low velocity zone is larger and deeper both in range and depth. The crustal thickness of 28~35km from our joint inversion is similar to the results from the receiver functions of previous studies. The thinnest crust(28km)is observed at offshore in the north of Quanzhou; while the thickest crust(35km)is located west of Zhangzhou near the Zhenghe-Dapu fault zone. Generally, thinner crustal thickness is found in offshore of Fujian Province, and thicker crustal thickness is in the mainland. However, we also found that crustal thickness becomes thinner along the east side of Yongan-Jinjiang Fault. The values of Pn velocities in the region vary from 7.71 to 8.26km/s. The velocity distribution of the uppermost mantle presents a large inhomogeneity, which is correlated with the distribution of the fault zone. High Pn velocity anomalies are found mainly along the west side of the Zhenghe-Dapu fault zone(F₂), and the east side of the Shaowu-Heyuan fault zone(F₁), which is strip-shaped throughout the central part of Fujian. Low Pn velocity anomalies are observed along the coast and Taiwan Straits, including the Changle-Zhaoan fault zone, the coastal fault zone, and the Fuzhou Basin. We also found a low Pn velocity anomaly zone, which extends to the coast, in the Shaowu-Heyuan fault zone at the junction of the Fujian, Guangdong and Jiangxi Provinces. In the west of Taiwan Straits, both high and low Pn velocity anomalies are observed. Our results show that the historical strong earthquakes(larger than magnitude 6.0) are mainly distributed between positive and negative anomaly zones at different depth profiles of the crust, and similar anomalies distribution also exists at the uppermost mantle, suggesting that the occurrence of strong earthquakes in the region is not only related to the anomalous crustal velocity structure, but also affected by the velocity anomaly structure from the uppermost mantle.     

QLg tomography in Colombia

The crustal attenuation of Lg waves in Colombia was estimated and analyzed using local seismological data from the National Seismological Network of Colombia (RSNC). The selected dataset comprises 510 crustal earthquakes with a total of 2928 ray paths. This large dataset allowed us to invert for a regional average of Q_Lg, tomographic images for Colombia, and simultaneously, source size for each event and site term at the seismological stations. The computed regional average for Q_Lg in the frequency band 0.5–5.0 Hz was found to agree with the previously reported values in neighboring regions. In order to resolve the lateral variation in the attenuation of Lg waves in the crust, independent tomographic inversions for 26 frequencies between 0.5 and 5.0 Hz were conducted. The resulting maps confirm that heterogeneities in the crust exist and that they are related to the large-scale tectonic features in the country. The highest attenuation in the region is linked to the presence of active volcanic arcs, where the crust is weakened by partial melting. Relatively high attenuation is also found in zones where the crust is composed of accreted oceanic rocks and in regions with near-surface low-velocity sedimentary layers.     

利用远震接收函数研究江西省地震台站下方莫霍面深度及泊松比分布

利用三分量远震记录资料,计算获得了江西省13个数字地震台下方的体波接收函数,利用H-kappa叠加方法反演得到这些台站下方的地壳厚度及泊松比。结果表明,研究区域东西向莫霍面起伏平缓,南北向地壳厚度变化稍大,全区域内地壳平均厚度为31 km,最大深度为九江的35 km,最小深度为赣州的28 km。泊松比的分布在研究区内从0.2到0.3之间变化,最大为会昌的0.27,最小为南昌的0.21。赣南各台站泊松比分布明显高于赣北,这可能与该区域地幔组分及地质演化过程有关。     

Discussion on origin of Pn velocity variationin China and adjacent region

IntroductionInrecentyears,theresultsfromresearchesonwavevelocitystructureofuppermostmantleusingMohorefractionwavesindicatethattheclearlateralvelocityvariationandanisotropyexistintheuppermostmantle(Hearn,1996,1999;Mele,etal,1998;WANG,etal,2002).Astotheoriginofvelocitylateralvariation,thegeneralconsiderationisthatitmayberelatedtothemate-rialcomposition,pressureandtemperatureintheuppermostmantle(Hearn,1984,1999;Beghouletal,1993).Theseismicanisotropyoftheuppermostmantleisnowanestablishedfactan…     

中国大陆Pn波速度结构与强震孕育的深部背景

在作者以往工作的基础上，增加了中国地震年报（1997～1998年）、朝鲜以及我国各区域地震台网的数据，使Pn到时数据增加到近14万条，从中选取符合反演条件的8043次地震在581个地震台站上记录到的Pn射线99440条,重新作了中国大陆Pn波速层析成像,空间分辨率达到3°×3°. 文中将Pn波速度横向变化与活动地块的划分进行了对比,结果表明: Pn波速度横向变化与Ⅰ级活动地块区之间关系密切，大致以105°E为界，西部地区速度以正异常为主，其中西域地块区速度正异常最大，其次是青藏地块区，而滇缅地块区为负异常区；东部地区速度以负异常为主，华北地块区速度负异常最大，其次是东北亚地块区. 西部的地块区上地幔顶部Pn波速度各向异性的平均快波速度方向大体为NWW—SEE向;东部的地块区大体为NNW—SSE向，大致与相应的张应力优势方向一致. Pn波速度横向变化与强震活动间的关系也较明显，所有MS≥7.0的强震大部分发生在上地幔顶部Pn波速度的低速异常区上面的地壳内，少数发生在高、低速区域的过渡地带或两个高速区之间的相对低速部分.     

利用接收函数方法研究中国东北东南部地区不同构造体的地壳特征

基于我们布设的探测深俯冲的中国东北地震台阵NECsaids台阵和固定地震台长时段的观测记录及NECESSArray流动台阵共计152个台站数据提取得到的33752条P波接收函数,采用H-κ叠加分析和共转换点（CCP）叠加成像等方法进行统一分析处理,并汇集他人接收函数研究结果得到中国东北东南部地区迄今为止最高分辨率的地壳厚度和平均波速比分布图像.对中国东北东南部地区不同构造体的地壳特征综合分析研究表明：研究区不同陆块的地壳属性存在明显差别,张广才岭地块中南部的地壳厚度和波速比与华北克拉通东北缘相当,地壳厚度同地表地形之间显示有明显的正相关关系;松辽地块东南缘地壳最薄、波速比最高,地壳厚度同壳内波速比之间显示出明显的负相关关系;兴凯地块西部地区的地壳结构表现为稍厚的地壳厚度和研究区内最低的壳内波速比,其地壳厚度同壳内波速比之间亦显示出明显的负相关关系;佳木斯地块西南缘在具有"正常"的壳内波速比同时地壳最厚.研究区内的郯庐断裂带北延段在切穿其下Moho面的同时表现出南北分段的特征：北段（44.4°N—47°N）两分支之下的Moho面整体下凹,而南段（41.5°N—43.3°N）两分支之下的Moho面则整体上隆.长白山天池火山下方表现为Moho面下凹沉落及高壳内波速比特征,推测其壳内岩浆囊很可能存在于火山口东北隅至少10 km的范围内.     

SsPmp震相地壳探测方法

SsPmp波是远震S波经地表反射转换的P波在莫霍面发生反射后被地表台站接收得到的震相.震中距在30°~50°之间的远震S波震相经地表反射转换的P波射线参数较大,在莫霍面发生全反射,使得台站接收的SsPmp波具有较强的能量,能够从地震记录中清楚地识别出来,为探测台站附近的莫霍面形态提供新的途径.本文通过合成理论地震图分析了SsPmp震相与地壳厚度、射线参数和Pn波速度之间的关系.结果表明:对于水平界面,地壳厚度只影响SsPmp与Ss波之间的相对到时差;Pn波速度只影响SsPmp的相位;射线参数既对SsPmp波的相对到时有影响,也会引起SsPmp波的相位变化.对于复杂的界面,SsPmp反映的深度与速度梯度最大的深度接近,而反映的Pn波速度与实际的Pn波速度一致.     

青藏高原中东部地壳和上地幔顶部P波层析成像

为获取青藏高原中东部地壳和上地幔顶部的精细结构,本文基于1万4 484条天然地震的P波（Pg和Pn）到时数据,对青藏高原中东部地壳和上地幔顶部进行P波三维速度结构层析成像,获取了该区域内地壳P波、上地幔顶部Pn波的速度结构和地壳厚度信息。层析成像结果显示,青藏高原中东部地壳P波速度范围为5.2—7.2 km/s,上地幔顶部Pn波速度范围为7.7—8.4 km/s,地壳厚度范围为48.0—68.6 km,地壳和上地幔顶部存在强烈的横向不均匀性,与地质块体分布有较好的对应关系。地壳P波速度结构显示,研究区中、下地壳分布有较大范围的低速区,上地壳与中下地壳P波分布存在明显的差异:羌塘地块和巴颜喀拉地块在上地壳主要表现为高速异常,随着深度增加逐渐表现为低速异常;而柴达木地块在上地壳主要表现为低速异常,下地壳则表现为高速异常;柴达木地块和拉萨地块在上地幔顶部表现为较高的Pn波速度,最高约为8.4 km/s,而在巴颜喀拉地块和羌塘地块东部,Pn波总体上表现为低速,最低约为7.7 km/s。研究区内地壳厚度的总体特征表现为南厚北薄,其中羌塘地块东部和拉萨地块的地壳较厚,而柴达木地块和巴颜喀拉地块东部的地壳相对较薄,羌塘地块西部存在局部的地壳变薄现象,反映了印度板块对欧亚板块北向俯冲作用下的岩石圈变形特征。      

Mapping Crustal Heterogeneity Using Lg Propagation Efficiency Throughout the Middle East, Mediterranean, Southern Europe and Northern Africa

—?In this paper we describe a technique for mapping the lateral variation of Lg characteristics such as Lg blockage, efficient Lg propagation, and regions of very high attenuation in the Middle East, North Africa, Europe and the Mediterranean regions. Lg is used in a variety of seismological applications from magnitude estimation to identification of nuclear explosions for monitoring compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). These applications can give significantly biased results if the Lg phase is reduced or blocked by discontinuous structure or thin crust. Mapping these structures using quantitative techniques for determining Lg amplitude attenuation can break down when the phase is below background noise. In such cases Lg blockage and inefficient propagation zones are often mapped out by hand. With our approach, we attempt to visually simplify this information by imaging crustal structure anomalies that significantly diminish the amplitude of Lg. The visualization of such anomalies is achieved by defining a grid of cells that covers the entire region of interest. We trace Lg rays for each event/station pair, which is simply the great circle path, and attribute to each cell a value equal to the maximum value of the Lg/P-coda amplitude ratio for all paths traversing that particular cell. The resulting map, from this empirical approach, is easily interpreted in terms of crustal structure and can successfully image small blockage features often missed by analysis of raypaths alone. This map can then be used to screen out events with blocked Lg prior to performing Q tomography, and to avoid using Lg-based methods of event identification for the CTBT in regions where they cannot work.¶For this study we applied our technique to one of the most tectonically complex regions on the earth. Nearly 9000 earthquake/station raypaths, traversing the vast region comprised of the Middle East, Mediterranean, Southern Europe and Northern Africa, have been analyzed. We measured the amplitude of Lg relative to the P-coda and mapped the lateral variation of Lg propagation efficiency. With the relatively dense coverage provided by the numerous crossing paths we are able to map out the pattern of crustal heterogeneity that gives rise to the observed character of Lg propagation. We observe that the propagation characteristics of Lg within the region of interest are very complicated but are readily correlated with the different tectonic environments within the region. For example, clear strong Lg arrivals are observed for paths crossing the stable continental interiors of Northern Africa and the Arabian Shield. In contrast, weakened to absent Lg is observed for paths crossing much of the Middle East, and Lg is absent for paths traversing the Mediterranean. Regions that block Lg transmission within the Middle East are very localized and include the Caspian Sea, the Iranian Plateau and the Red Sea. Resolution is variable throughout the region and strongly depends on the distribution of seismicity and recording stations. Lg propagation is best resolved within the Middle East where regions of crustal heterogeneity on the order of 100?km are imaged (e.g., South Caspian Sea and Red Sea). Crustal heterogeneity is resolvable but is poorest in seismically quiescent Northern Africa.     

南岭-武夷交汇区的深部背景及地壳泊松比

为研究南岭-武夷交汇区深部动力学过程提供深部背景资料和科学依据,本文利用远震P波接收函数H-κ叠加和共转换点（CCP）叠加两种方法获取了研究区66个宽频带流动台站及24个固定台站下方的地壳厚度、泊松比和Moho面起伏形态,揭示了扬子地块与华夏地块地壳结构及泊松比变化特征,给出了南岭和武夷之间一条莫霍凸起带的高分辨图像.
结果显示：（1）研究区内地壳厚度平均值为31.2 km,泊松比平均值为0.23,总体呈现薄地壳、低泊松比的特点.地壳厚度从西北往东南由厚变薄,与区域地壳伸展特征相一致.（2）在韶关-赣州-吉安-南昌一线存在条带状薄地壳结构,平均值为28 km,呈南西-北东向展布,对应的泊松比值略微升高.推测地壳减薄带的形成可能与来自南海方向的地幔热流上涌有关.（3）江南造山带的泊松比整体偏低,存在两处平均值小于0.21的区域.华夏地块内地壳厚度与泊松比之间存在弱的负相关,表示随着地壳厚度的增大,铁镁质的下地壳厚度在整个地壳厚度中所占的比例减小.     

Volcano spacings and lithospheric/crustal thickness in the Archaean

There is a linear relationship between the spacing of Pliocene-Pleistocene volcanoes and the thickness of the lithosphere and attenuated crust in the East African rift valley. Assuming that the physical-chemical properties of the Archaean and Cenozoic lithosphere and crust were broadly similar, we use the spacing of volcanic centres in the Abitibi greenstone belt of southern Canada to determine lithospheric and crustal thickness in the Archaean. The abitibi volcanoes have been deformed and so have elliptical cross-sections. In order to arrive at their original form we have removed the effects of tectonic strain by two alternative mechanisms of pure and simple shear which give comparable results. A mean original volcano spacing of 84–88 km suggests that the lithsophere was 80–90 km thick and that the crust was probably 35–45 km thick in this greenstone belt about 2700 m.y. ago. The crustal values are comparable with those determined by geochemical parameters and are consistent with the suggestion that greenstone belts formed in extensional marginal basins between crustal-thickened continental masses, deep sections of which are now seen in Archaean high-grade regions.     

Seismic Velocity and Q Structure of the Middle Eastern Crust and Upper Mantle from Surface-wave Dispersion and Attenuation

—Observed velocities and attenuation of fundamental-mode Rayleigh waves in the period range 7–82 sec were inverted for shear-wave velocity and shear-wave Q structure in the Middle East using a two-station method. Additional information on Q structure variation within each region was obtained by studying amplitude spectra of fundamental-mode and higher-mode Rayleigh waves. We obtained models for the Turkish and Iranian Plateaus (Region 1), areas surrounding and including the Black and Caspian Seas (Region 2), and the Arabian Peninsula (Region 3). The effect of continent-ocean boundaries and mixed paths in Region 2 may lead to unrealistic features in the models obtained there. At lower crustal and upper-mantle depths, shear velocities are similar in all three regions. Shear velocities vary significantly in the uppermost 10 km of the crust, being 3.21, 2.85, and 3.39 km/s for Regions 1, 2, and 3, respectively. Q models obtained from an inversion of interstation attenuation data show that crustal shear-wave Q is highest in Region 3 and lowest in Region 1. Q’s for the upper 10 km of the crust are 63, 71, and 201 for Regions 1, 2, and 3, respectively. Crustal Q’s at 30 km depth for the three regions are about 51, 71, and 134. The lower crustal Q values contrast sharply with results from stable continental regions where shear-wave Q may reach one thousand or more. These low values may indicate that fluids reside in faults, cracks, and permeable rock at lower crustal, as well as upper crustal depths due to convergence and intense deformation at all depths in the Middle Eastern crust.     

新疆和周边地区地壳厚度和Vp/Vs比值变化的接收函数约束

我们用修订后的接收函数H-K叠加算法估计了新疆和周边地区90个地震台站下的地壳厚度和波速比Vp/Vs.该地区平均地壳厚度约为52 km,变化范围约为33～79 km.最厚地壳位于西昆仑山的TAG台站,最薄地壳在塔拉斯-费尔干那断层附近的NRN台站.对每个台站的点地壳厚度估计结果进行了线性各向同性变差模型的标准克里金空间...     

Response spectral relationships for rock sites derived from the component attenuation model

The seismological model was developed initially from the fundamental relationship between earthquake ground motion properties and the seismic moment generated at the source of the earthquake. Following two decades of continuous seismological research in the United States, seismological models which realistically account for both the source and path effects on the seismic shear waves have been developed and their accuracy rigorously verified (particularly in the long and medium period ranges). An important finding from the seismological research by Atkinson and Boore and their co‐investigators is the similarity of the average frequency characteristics of seismic waves generated at the source between the seemingly very different seismic environments of Eastern and Western North America (ENA and WNA, respectively). A generic definition of the average source properties of earthquakes has therefore been postulated, referred to herein as the generic source model. Further, the generic ‘hard rock’ crustal model which is characteristic of ENA and the generic ‘rock’ crustal model characteristic of WNA have been developed to combine with the generic source model, hence enabling simulations to be made of the important path‐related modifications to ground motions arising from different types of crustal rock materials. It has been found that the anelastic contribution to whole path attenuation is consistent between the ENA and WNA models, for earthquake ground motions (response spectral velocities and displacements) in the near and medium fields, indicating that differences in the ENA and WNA motions arise principally from the other forms of path‐related modifications, namely the mid‐crust amplification and the combined effect of the upper‐crust amplification and attenuation, both of which are significant only for the generic WNA ‘rock’ earthquake ground motions. This paper aims to demonstrate the effective utilization of the latest seismological model, comprising the generic source and crustal models, to develop a response spectral attenuation model for direct engineering applications. The developed attenuation model also comprises a source factor and several crustal (wave‐path modification) component factors, and thus has also been termed herein the component attenuation model (CAM). Generic attenuation relationships in CAM, which embrace both ENA and WNA conditions, have been developed using stochastic simulations. The crustal classification of a region outside North America can be based upon regional seismological and geological information. CAM is particularly useful for areas where local strong motion data are lacking for satisfactory empirical modelling. In the companion paper entitled ‘response spectrum modelling for rock sites in low and moderate seismicity regions combining velocity, displacement and acceleration predictions’, the CAM procedure has been incorporated into a response spectrum model which can be used to effectively define the seismic hazard of bedrock sites in low and moderate seismicity regions. This paper and the companion paper constitute the basis of a long‐term objective of the authors, to develop and effectively utilize the seismological model for engineering applications worldwide.     

Geophysical models for the tectonic framework of the Lake Vostok region, East Antarctica

Aerogeophysical and seismological data from a geophysical survey in the interior of East Antarctica were used to develop a conceptual tectonic model for the Lake Vostok region. The model is constrained using three independent data sets: magnetic, seismic, and gravimetric. A distinct change in the aeromagnetic anomaly character across Lake Vostok defines a crustal boundary. Depth to magnetic basement estimates image a 400-km-wide and more than 10-km-deep sedimentary basin west of the lake. Analysis of teleseismic earthquakes suggests a relatively thin crust beneath Lake Vostok consistent with predictions from kinematic and flexural gravity modelling. Magnetic, gravity, and subglacial topography data reveal a tectonic boundary within East Antarctica. Based on our kinematic and flexural gravity modelling, this tectonic boundary appears to be the result of thrust sheet emplacement onto an earlier passive continental margin. No data presently exist to date directly either the timing of passive margin formation or the subsequent shortening phase. The preserved thrust sheet thickness is related to the thickness of the passive margin crust. Because a significant amount of time is required to erode the thrust sheet topography, we suggest that these tectonic events are Proterozoic in age. Minor normal reactivation of the thrust sheet offers a simple mechanism to explain the formation of the Lake Vostok Basin. A low level of seismicity exists in the vicinity of this tectonic boundary. The existence of a crustal boundary in the Antarctic interior provides new constraints on the Proterozoic architecture of the East Antarctic craton.     

利用H-K叠加方法和CCP叠加方法研究中国东北地区地壳结构与泊松比

利用中国东北布设的流动地震台阵（116个）以及国家和区域台网（121个）的宽频带台站记录的824个远震事件,采用P波接收函数CCP叠加和H-K叠加两种方法获得了研究区详尽的地壳厚度图像.研究结果显示,两种方法获得的地壳厚度分布特征具有很好的一致性,中国东北下方地壳厚度存在明显的东西横向差异.重力梯度带西侧和佳木斯地块的台站下方地壳较厚,介于36~41 km之间,而在兴蒙槽地褶带中重力梯度带往东从36 km减薄至34 km左右.松辽盆地北侧、东侧和南侧地壳厚度较薄,为29~34 km,反映了该区复杂的地壳变形过程.CCP剖面显示郯庐断裂深切地壳,敦化—密山断裂下方莫霍面出现错断.H-K叠加得到的地壳平均泊松比显示,东北地区绝大部分台站下方的泊松比值较大,0.24~0.29.长白山、松辽盆地东部、燕山台隆东部和大兴安岭北部,泊松比值达到0.27~0.30,可能有幔源物质上涌,下地壳铁镁组分含量增加.     

The lithosphere structure and geodynamics of the West and East Black Sea Basins

The velocity structure of the Black Sea lithosphere has been studied using the local seismic tomography method based on the Backus-Gilbert approach and applied for a quite large amount of seismological data. As seismic sources, we used the earthquakes that occurred within the Black Sea and adjoining regions and generated seismic waves recorded by seismic stations around the Black Sea. This yielded information on the 3D distribution of P-waves within the most heterogeneous and poorly investigated uppermost layer of the Black Sea region (down to a depth of 60 km). The interpretation of these results, together with new data on the velocity structure of the crust of the Black Sea led to the conclusion about the different lithosphere beneath the West and the East Black Sea Basins. This can be explained by origination of the depressions in the western and eastern parts of the Black Sea at different microplates and by specific features in their development at rifting and post-rifting stages.     

Crustal thickness and Poisson’s ratio variations across the northwest Himalaya and eastern Ladakh

Crustal thickness and Poisson’s ratios are estimated across the northwest (NW) Himalaya and eastern Ladakh applying H-k stacking method on receiver functions of teleseismic earthquakes recorded at 16 broadband seismological stations. The results show significant lateral variation of crustal thickness from the Lesser and Higher Himalaya (～50 km thick) to Ladakh (～80 km thick) through the Indus Tsangpo Suture Zone (ITSZ). The Indian Moho is continuously traceable across the ITSZ which is consistent with the underthrusting of the Indian plate beyond the surface collision boundary. The estimated Poisson’s ratios in the Lesser and Higher Himalaya are low (0.249–0.253), suggesting felsic composition of the crust. The Poisson’s ratio is intermediate in the Tethyan Himalaya (0.269–0.273) and high beneath Ladakh (0.280–0.303), indicating the effect of aqueous fluid/partial melt present in the crust.     

Fine structure of Pn velocity beneath Sichuan-Yunnan region

We use 23298 Pn arrival-time data from Chinese national and provincial earthquake bulletins to invert fine structure of Pn velocity and anisotropy at the top of the mantle beneath the Sichuan-Yunnan and its adjacent region. The results suggest that the Pn velocity in this region shows significant lateral variation; the Pn velocity varies from 7.7 to 8.3 km/s. The Pn-velocity variation correlates well with the tectonic activity and heat flow of the region. Low Pn velocity is observed in southwest Yunnan, Tengchong volcano area, and the Panxi tectonic area. These areas have very active seismicity and tectonic activity with high surface heat flow. On the other hand, high Pn velocity is observed in some stable regions, such as the central region of the Yangtze Platform; the most pronounced high velocity area is located in the Sichuan Basin, south of Chengdu. Pn anisotropy shows a complex pattern of regional deformation. The Pn fast direction shows a prominent clockwise rotation pattern from east of the Tibetan block to the Sichuan-Yunnan diamond block to southwest Yunnan, which may be related to southeastward escape of the Tibetan Plateau material due to the collision of the Indian Plate to the Eurasia Plate. Thus there appears to be strong correlation between the crustal deformation and the upper mantle structure in the region. The delay times of events and stations show that the crust thickness decreases from the Tibetan Plateau to eastern China, which is consistent with the results from deep seismic sounding.