Middle and upper crust shear-wave velocity structure of the Chinese mainland

In order to give a more reliable shallow crust model for the Chinese mainland, the present study collected many short-period surface wave data which are better sensitive to shallow earth structures. Different from traditional two-step surface wave tomography, we developed a new linearized surface wave dispersion inversion method to directly get a 3D S-wave velocity model in the second step instead of inverting for 1D S-velocity profile cell by cell. We convert all the regionalized dispersions into linear constraints for a 3D S-velocity model. Checkerboard tests show that this method can give reasonable results. The distribution of the middle-and upper-crust shear-wave velocity of the Chinese mainland in our model is strongly heterogeneous and related to different geotectonic terrains. Low-velocity anomalies delineated very well most of the major sedimentary basins of China. And the variation of velocities at different depths gives an indication of basement depth of the basins. The western Tethyan tectonic domain (on the west of the 95°E longitude) is characterized by low velocity, while the eastern Tethyan domain does not show obvious low velocity. Since petroleum resources often distribute in sedimentary basins where low-velocity anomaly appears, the low velocity anomalies in the western Tethyan domain may indicate a better petroleum prospect than in its eastern counterpart. Besides, low velocity anomaly in the western Tethyan domain and around the Xing’an orogenic belt may be partly caused by high crustal temperature. The weak low-velocity belt along ~105°E longitude corresponds to the N-S strong seismic belt of central China.     

The physical implication of coda amplitude ratio and its possible application

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Q value of anelastic S-wave attenuation in Yunnan region

The study of seismic attenuation property is a major subject in seismology. Seismic waves recorded by seismic stations （seismographs） contain source effect, seismic wave propagation effect, site response of seismic stations and instrumental response. The path effect of seismic wave propagation, site response of seismic stations and instrumental response must be taken out in the study of source property with seismic data. The path effect of seismic wave propagation （seismic attenuation） involves an important influential factor, the anelastic attenuation of medium, which is measured with quality factor Q, apart from geometric attenuation with the distance. As a basic physical parameter of the Earth medium, Q value is essential for quantitative study of earthquakes and source property （e.g. determination of source parameters）, which is widely used in earthquake source physics and engineering seismology.     

Three dimensional shear wave velocity structure of crust and upper mantle in South China Sea and its adjacent regions by surface waveform inversion

IntroductionHistorically, surface waves provided much of our understanding of the lateral heterogeneity in the crust and the upper mantle, especially in oceans or desert areas. Traditional method utilizes the great circle theorem for surface waves. It assumed that surface wave propagates along the great circle connecting epicenter with station. Firstly, traditional surface wave analysis was carried out by using multi-frequency-filter dispersion analysis to acquire the group or phase velocity d…     

Three-dimensional seismic velocity tomography of the upper crust in Tengchong volcanic area, Yunnan Province

Introduction Tengchong is one of youngest volcanic areas in Chinese mainland. Since Pliocene, the volcanoes have erupted several times. Nowadays, the thermal activity is very intensive there. The possibility of re-eruption and the reserve of geothermal energy in the area are the questions to which the publics pay much attention, and the volcanists and seismologists dedicated. HAN, et al (1996) reviewed all of the studies carried out in Tengchong area. In the late 1990s, an integrative volca…     

Velocity structure of the crust and upper mantle in Xingtai earthquake region and its adjacent area

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Study and review on crust-mantle velocity structure in Bohai Bay and its vicinity

Introduction Bohai Bay, along with its adjacent areas, is one of the seismically active areas in North China. Understanding its crust/upper-mantle structural characteristics and lateral heterogeneity of the medium in this area is of great significance to the study of seismogenic environment, thus improvimg the level of earthquake prediction. For years, scientists have studied the area by gravity and magnetic methods (FENG, et al, 1989), geothermal field (WU, et al, 1988; TIAN, ZHANG, 19…     

Group velocity distribution of Rayleigh waves and crustal and upper mantle velocity structure of the Chinese mainland and its vicinity

Introduction A lot of results have been achieved on the study of crust and upper mantle structure in Chinese mainland by the surface wave dispersion. The seismometer can be classified into three types based on the development from the analog to digital instrument. FENG, et al (1981) measured the dispersion curves of group and phase velocity along 28 paths, with 19 earthquakes recorded by 15 base analog seismic stations, subdivided the Chinese mainland into 5 blocks and obtained the average c…     

The velocity structure of crust and uppermantle in the Wudalianchi volcano area in—ferred from the receiver function

The Wudalianchi volcano is a modern volcano erupted since the Holocene.Its frequent occurrence of the small earthquake is considered to be indicator of active dormancy volcano.The S wave velocity structure is inferred from the receiver function for the crust and upper mantle of the Wudalianchi volcano area.The results show that the low velocity structure of Swave is widely distributed undemeath the volcano area and part of the low-velocity-zone located at shallow depth in the Wudalianchi volcano area.The low velocity structure is related to the seismicity.The Moho interface is not clear undemeath the volcano area,which may be regard to be an nec-essary condition for the lava upwelling.Therefore,we infer that the Wudalianchi volcano has the deep structural condition for the volcano activity and may be alive again.     

A study on 3-D velocity structure of crust and upper mantle in Sichuan -Yunnan region, China

Many evidences indicate that the collision of two plates deformed strongly the crust of the SYR, and the deformation has been continued up to the present. In addition, the SYR is in the south segment of the South-North Seismic Zone of China, which is one of the regions in the Chinese mainland, where the seismic activity is very high, and the strong earthquakes frequently occurred. Since the 1970s, a series of large earthquakes with magnitude M>7.0 occurred in SYR, such as the 1970 Tongha…     

Poisson's ratio of eclogite: Implications for lower crustal delamination of orogens

Eclogite is essentially a bi-mineralogic high-grade metamorphic rock consisting of garnet and omphacite and is the product of high-to-ultrahigh pressure metamorphism of basaltic rocks due to the subduction of oceanic crust or the thickening/subduction of …     

Seismic velocity and Poisson’s ratio tomography of the crust beneath southwest Anatolia: an insight into the occurrence of large earthquakes

The western part of Anatolia is one of the most seismically and tectonically active continental regions in the world, and much of it has been undergoing NS-directed extensional deformation since the Early Miocene. In this study, we determine 3-D tomographic images of the crust under the southwestern part of the North Anatolian Fault Zone by inverting a large number of arrival time data of P and S waves. From the obtained P- and S-wave velocity models, we estimated the Poisson’s ratio structures for a more reliable interpretation of the obtained anomalies. Our tomographic results confirmed the major tectonic features detected by previous studies and revealed new structural heterogeneities related to the active seismotectonics of the studied area. High P-wave velocity anomalies are recognized near the surface, while at deeper crustal layers, low P-wave velocities are widely distributed. The crustal S-wave velocity and Poisson’s ratio exhibit more structural heterogeneities compared to the P-wave velocity structure. Microearthquake activity is intense along highly heterogeneous zones in the southwestern part, which is characterized by low to high P-wave velocity, low S-wave velocity, and high Poisson’s ratio anomalies. Large earthquakes are also concentrated in zones dominated by low velocities and low to high Poisson’s ratios. Results of the checkerboard and synthetic tests indicate that the imaged anomalies are reliable features down to a depth of 25 km. Moreover, they are consistent with many geological and geophysical results obtained by other researchers along the southwestern part of the North Anatolian Fault Zone. An erratum to this article can be found at     

基于瑞雷波法的成都盆地场地剪切波速结构确定

在2008年的汶川地震中获得了大量的有价值数据和资料,使得成都盆地成为了研究的热点.为了能够获得成都盆地场地剪切波速结构,为成都盆地及汶川地震的进一步研究提供基本数据支持,本文利用多道面波分析方法在成都盆地(E:103°～105°,N:30°～32°)布置了40个面波测点进行场地剪切波速结构和覆盖层厚度的调查,测点间距...     

2-D crustal Poisson’s ratio from seismic travel time inversion in Changbaishan Tianchi volcanic region

Based on the inversion method of 2D velocity structure and interface, the crustal velocity structures of P-wave and S-wave along the profile L ₁ are determined simultaneously with deep seismic sounding data in Changbaishan Tianchi volcanic region, and then its Poisson’s ratio is obtained. Calculated results show that this technique overcomes some defects of traditional forward calculation method, and it is also very effective to determine Poisson’s ratio distribution of deep seismic sounding profile, especially useful for study on volcanic magma and crustal fault zone. Study result indicates that there is an abnormally high Poisson’s ratio body that is about 30 km wide and 12 km high in the low velocity region under Tianchi crater. Its value of Poisson’s ratio is 8% higher than that of surrounding medium and it should be the magma chamber formed from melted rock with high temperature. There is a high Poisson’s ratio zone ranging from magma chamber to the top of crust, which may be the uprise passage of hot substance. The lower part with high Poisson’s ratio, which stretches downward to Moho, is possibly the extrusion way of hot substance from the uppermost mantle. The conclusions above are consistent with the study results of both tomographic determination of 3D crustal structure and magnetotelluric survey in this region. Foundation item: Key Project from China Earthquake Administration and the Project (95-11-02-01) from Ministry of Science and Technology (2001DIA10003). Contribution No. RCEG200401, Geophysical Exploration Center, China Earthquake Administration.     

Time-space evolution characteristics of abrupt variation of wave velocity ratio in the seismogenic process of recent strong earthquakes in Yunnan area

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Shear wave velocity structure of the crust and upper mantle underneath the Tianshan orogenic belt

From April, 2003 to September, 2004, a passive broadband seismic array consisting of 60 stations was deployed over the Tianshan orogenic belt by State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration. Among them, 51 stations make up an about 500-km-long profile across the Tianshan Mountains from Kuytun to Kuqa. The receiver function profile and S-wave velocity structure of the crust and upper mantle down to 100 km deep are obtained by using the re-ceiver function method (Liu et al. 1996, 2000). The main results can be summarized as follows: (1) A clear mountain root does not exist beneath the Tianshan Mountains, and the crust-mantle boundaries underneath the stations mostly have transitional structures. This implies that the material differentia-tion between the crust and mantle is not yet accomplished and the orogenic process is still going on. (2) The crust beneath the Tianshan Mountains has laterally blocked structures in direction perpendicular to the mountain strike, and the crust-mantle boundary has a clear dislocation structure. Both of them correspond to each other. (3) The offsets of the Moho discontinuity are highly correlated to the tectonic borders on the surface and that corresponding to the frontal southern Tianshan fault reaches to 14 km. This manifests that large vertical divergent movement took place between different blocks. This sup-ports the discontinuous model of the Tianshan orogeny, and the Tarim block subduction is restricted only to the southern side of the South Tianshan. (4) Inside the upper and middle crust of the Tianshan Mountains exist several low-velocity bodies correlated with high seismicity located on the moun-tain-basin jointures on both sides of the mountain and between different blocks, and the low-velocity bodies on the mountain-basin jointures are inclined obviously to the mountain. This implies that the low-velocity bodies may be correlated closely to the thrust and subduction of the basins on both sides of the mountain, the splicing of adjacent blocks and the fast uplift of the Tianshan Mountains.     

Three-dimensional velocity images in Yunnan and its neighboring district

Using the techniques of seismic tomography three-dimensional velocity images at crust and upper mantle in Yunnan province and its adjacent region have been successfully reconstructed. The results of image are: (1) The image of the velocity in the upper crust is closely related to the well-known geological structure of the surface, the Kangdian earth axis is a distinct high velocity area, and a high velocity rock stratum, which appoaching the surface of the earth, has been formed. (2) There is a low-velocity layer between 26°–31°N and 100°–104°E in deep crust, the depth of Moho discontinuity in Sichuan bass in is less than 50 km. (3) The results of seismological tomography not only reveal the lateral heterogeneity in the researched region, but also find approximately the strike of Honghe fault from the image at bottom of crust, and the velocity in both side of the fault are different obviously. (4) There is a low-velocity column within 25 km to 110 km in Tengchong region, which may be occured by upward moving of the basalt in the mantle. (5) In studied area, the thickness of the crust in west part is thicker than in southeast part. (6) From the image at bottom of the crust we can find that earthquakes with magnitude greater than 5 occurred in big velocity gradient zones, especially in transition zone between high and low velocity. There are a few earthquake in the low-velocity area. (7) We can see from Figure 6 that there still clearly exists lateral heterogeneity at 450 depth. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,15, 61–67, 1993.     

Determination of S-wave velocity structure using microtremors and spac method applied in Thessaloniki (Greece)

Array measurements of microtremors at 16 sites in the city of Thessaloniki were performed to estimate the V_s velocity of soil formations for site effect analysis. The spatial autocorrelation method was used to determine phase velocity dispersion curves in the frequency range from 0.8–1.5 to 6–7 Hz. A Rayleigh wave inversion technique (stochastic method) was subsequently applied to determine the Vs profiles at all the examined sites. The determination of V_s profiles reached a depth of 320 m. Comparisons with V_s values from cross-hole tests at the same sites proved the reliability of the SPAC method. The accuracy of the V_s profiles, the ability to reach large penetration depths in densely populated urban areas and its low cost compared to conventional geophysical prospecting, make Mictrotremor Exploration Method very attractive and useful for microzonation and site effects studies. An example of its application for the site characterization in Thessaloniki is presented herein.     

The P and S wave velocity structures of the crust and upper mantle beneath Tibetan Plateau

Using the P-and S-wave arrivals from the 150 earthquakes distributed in Tibetan Plateau and its neighboring areas, recorded by Tibetan seismic network, Sichuan seismic network, WWSSN and the mobile network situated in Tibetan Plateau, we have obtained the average P-and S-wave velocity models of the crust and upper mantle for this region:

Interpretation of S-wave data from Tai’an-Xinzhou DSS profile and its relationship with Xingtai earthquakes

On the basis of S wave information from Tai’an-Xinzhou DSS profile and with reference to the results from P-wave interpretation, the 2-D structures, including S-wave velocity V _s, ratio γ between V _p and V _s; and Poisson’ s ratio σ, are calculated; the structural configuration of the profile is presented and the relevant inferences are drawn from the above results. Upwarping mantle districts (V _s≈4.30 km/s) and sloping mantle districts (V _s≈4.50 km/s) of the profile with velocity difference about −4% at the top of upper mantle are divided according to the differences of V _s, γ and σ in different media and structures, also with reference to the information of their neighbouring regions; the existence of Niujiaqiao-Dongwang high-angle ultra-crustal fault zone is reaffirmed; the properties of low and high velocity blocks (zones) including the crust-mantle transitionalzone and the boudary indicators of North China rift valley are discussed. A comprehensive study is conducted on the relation of the interpretation results with earthquakes. It is concluded that the mantle upwarps, thermal material upwells through the high-angle fault, the primary hypocenter was located at the crust-mantle juncture 30.0∼33.0 km deep, and additional stress excited the M _S=6.8 and M _S=7.2 earthquakes at specific locations around 9.0 km below Niujiaqiao-Dongwang, the earthquakes took place around the high-angle ultra-crustal fault and centered in the brittle media and rock strata with low γ and low σ values. This subject is part of the 85-907-02 key project during the “8th Five-Year Plan” from the State Science and Technology Commission.