Earthquake relocation and 3-dimensional crustal structure of P-wave velocity in cen-tral-western China

采用中国中西部地区(2l°～36°N，98°～112°E)193个地震台在1992～1999年间记录到的9988次地震的Pg和Sg震相走时的读数资料，用Roecker的SPHYPIT90程序，反演了该地区三维地壳P波速度结构，并用SPHREL3D90程序进行了地震的重新定位．反演结果揭示了中国中西部地区地震P波速度结构明显的横向不均匀性，这些不同深度上波速的横向变化多以该地区的活动断裂为分界线．可以看出活动断裂两侧存在明显的速度反差．通过重新定位，得到了6459次地震的震源参数，这些精确定位的地震震中明显沿该区活动断裂呈现条带状分布，其范围和尺度清晰地表示了这一地区地震活动与活动断裂的紧密关系．其中，82％重新精确定位的事件的震源深度在20km以内．这一结果与笔者用双差地震定位法得到的重新定位的震源深度分布相一致．

Imaging 3-D crustal P-wave velocity structure of western Yunnan with bulletin data

Western Yunnan is a region with intensive tectonic activity and serious earthquake risk. It is of significant importance to study three dimensional crustal structure of this region to understand the tectonic setting and disaster mechanism. Densification and digitalization of seismic networks in this region provides an opportunity to study the velocity structure with bulletin data. In this study, we collect P-wave data of 10 403 regional earthquakes recorded by 79 seismic stations from January 2008 to December 2010. In addition to first arrivals data （Pg with epieentral distance less than 200 km and Pn）, the Pg （or P） data with epicentral distance more than 200 km are also considered as later direct arrivals in the tomographic inversion. We also compare the quantity and the quality of the seismic data before 2010 and after 2010. The test results show that adding the follow-up Pg phase can effectively improve the inversion ability of crustal imaging, and quantity and the data quality are significantly improved since 2010. The tomographie results show that： （1） The Honghe fault zone, which is the major fault systems in this region, may cut through the entire crust, and the velocity contrasts between two sides at lower crust beneath the Honghe fault are estimated at higher than 10%, while the velocity difference below Nujiang fault zone extends only in the upper crust; （2） Most of the earthquakes in the region occurred at the interface of high-velocity media and low-velocity media, i.e., the areas with high velocity gradient, which has been validated in other areas.

Three-dimensional P-wave velocity structure of the crust beneath Hainan Island and its adjacent regions,China

Using over 3 500 first P arrival times recorded by nine digital seismic stations from Hainan Digital Seismic Net-work during 1999～2005,a 3-D P-wave velocity model of the crust under Hainan Island and adjacent regions has been determined. The results show that the pattern of velocity anomalies in the shallower upper crust is somewhat associated with the surface geological tectonics in the region. A relative low-velocity anomaly appears north of the Wangwu-Wenjiao fault zone and a relative high-velocity anomaly appears south of the Wangwu-Wenjiao fault zone,corresponding to the depressed areas in north Hainan Island,where many volcanoes are frequently active and geothermal values are relatively higher,and the uplifted and stable regions in central and south of the Hainan Is-land. In the middle and lower crust velocities are relatively lower in east Hainan than those in west Hainan,possi-bly suggesting the existence of the upwelling of hot materials from the mantle in east Hainan. The pattern of veloc-ity anomalies also indicates that NW faults,i.e.,the Puqian-Qinglan fault,may be shallower,while the E-W Wangwu-Wenjiao fault may be deeper,which perhaps extends down to Moho depth or deeper.

Earthquake relocation and 3-dimensional crustal structure of P-wave velocity in central-western China

A simultaneous inversion of earthquake relocation and three-dimensional crustal structure of P-wave velocity in central-western China (21癗~36癗, 98癊~112癊) were performed in this paper. The crustal P-wave velocity model and earthquake relocation for this region are obtained using Pg and Sg phase readings of 9 988 earthquakes from 1992 to 1999 recorded at 193 seismic stations within central-western China by SPHYPIT90 and SPHREL3D90 programs. A lateral inhomogeneous structure of P-wave velocity in this region was obtained. Ob-vious contrast of P-wave velocities was revealed on both sides of active fault zones. Relocated epicenters of 6 459 events show clear lineation along active faults, which indicated a close correlation between seismicity and the active faults in this region. Focal depths of 82% relocated events ranged from 0 to 20 km, which is in good agreement with that from double-difference earthquake location algorithm.

3-D velocity structure of P wave in the upper mantle beneath southwestern China and its adjacent areas

3-D velocity structure of P wave in the upper mantle beneath southwestern China and its adjacent areas (10°N [similar to] 36°N, 70°E [similar to] 110°E) down to the depth of 400 km has been studied by using 80 974 P-wave first arrival times recorded at 165 stations from 7 053 events both within the studying areas, selected from the ISC bulletin and the Bulletin of China and NEIC fundamental seismic network. With a resolution of grid spacing of 2°×2°, the velocity heterogeneity on the horizontal profile is obvious though it attenuates with the depth increasing. On the vertical profiles of velocity along the latitude of 16°N and 24°N, the collision and extrusion of India plate to Eurasia plate is displayed, and a remarkable velocity difference between India plate and Eurasia plate is shown. In the vertical profile along the longitude of 90°E, the subducting of India plate northward beneath Eurasia plate (Tibet plateau) is also obvious. On the horizontal profile at the depth of 90 km, a slow velocity stripe from Myitkyina, Myanmar to Donghai, Vietnam seems to be related to Honghe fault belt. An illustration method of describing the resolution more directly and exactly has been proposed and utilized in this paper.

3-D velocity structure in the central-eastern part of Qilianshan

The 3-D velocity tomography image of the central-eastern part of Qilianshan is obtained by the joint inversion of 3-D velocity structure and focal parameters based on the S-P data of micro-earthquakes recorded by the digital seismic network set up for a Sino-French cooperation program since 1996. The inversed velocity structure does primarily reflect some important features of the deep structure in the region and provide the scientific background for the further study of active tectonic structure and the calculation of earthquake parameters.

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

Based on the first arrival P and S data of 4 625 regional earthquakes recorded at 174 stations dispersed in the Yunnan and Sichuan Provinces, the 3-D velocity structure of crust and upper mantle in the region is determined, incorporating with previous deep geophysical data. In the upper crust, a positive anomaly velocity zone exists in the Sichuan basin, whereas a negative anomaly velocity zone exists in the western Sichuan plateau. The boundary between the positive and negative anomaly zones is the Longmenshan fault zone. The images of lower crust and upper mantle in the Longmenshan fault, Xianshuihe fault, Honghe fault and others show the characteristic of tectonic boundary, indicating that the faults likely penetrate the Moho discontinuity. The negative velocity anomalies at the depth of 50 km in the Tengchong volcanic area and the Panxi tectonic zone appear to be associated with the temperature and composition variations in the upper mantle. The overall features of the crustal and the upper mantle structures in the SichuanYunnan region are the lower average velocity in both crust and uppermost mantle, the large crustal thickness variations, and the existence of high conductivity layer in the crust or/and upper mantle, and higher geothermal value. All these features are closely related to the collision between the India and the Asia plates. The crustal velocity in the SichuanYunnan rhombic block generally shows normal value or positive anomaly, while the negative anomaly exists in the area along the large strike-slip faults as the block boundary. It is conducive to the crustal block side-pressing out along the faults. In the major seismic zones, the seismicity is relative to the negative anomaly velocity. Most strong earthquakes occurred in the upper-mid crust with positive anomaly or normal velocity, where the negative anomaly zone generally exists below.

3D Velocity Structure and Its Tectonic Implications in the East China Sea and Yellow Sea

3D structure of the crust and upper mantle in the studied area has been analyzed from surface wave tomography. The velocity distribution in the uppermost crust is symmetrical on two sides of the central line of the sea, and coincides with the structure of crystalline basement. The essential difference in tectonics between the East China Sea and the Yellow Sea mainly lies in that the velocity structures of their lower crust and upper mantle are identical to those of South China and North China respectively. In the upper mantle there exists a high-velocity zone with a nearly EW strike from the Hangzhou Bay, China, to the Tokara Channel, Japan, along about the latitude of 30°N. It is found that between the East China Sea and the Yellow Sea there are systematical differences in geomorphology, geology, seismicity, heat flow, quality factor and gravity and aeromagnetic anomalies, which is related to both left-lateral shear dislocation and right-lateral tear of the Benioff zone from the Hangzhou Bay to the Tokara Channel.It is inferred that the East China Sea was formed by Cenozoic back-arc extension. The boundary between the North China and South China crustal blocks stretches along the southern piedmont of Mts. Daba-Dabie-Hangzhou Bay-Tokara Channel, and the subduction zone at the Okinawa trench is the eastern boundary of the South China crustal block. The movements of the Pacific plate, Indian plate and upper mantle rather than the Philippine plate subduction have played a dominant role for the modern tectonic movements in East Asia.

CRUST AND UPPER STRUCTURE OF QINGHAI-TIBET PLATEAU AND ITS ADJACENT REGIONS FROM SURFACE WAVEFORM INVERSION

In this paper,218 long period Rayleigh wave records from 7 seismic station of CDSN are selected.We applied a partitioned waveform inversion to these data in order to construct a 3\|D model of shear velocity down to 400km depth in the crust and upper mantle of Qinghai\|Tibet plateau and Its Adjacent Regions (22°～44°N,70°～110°E).The first step of the waveform inversion used involved the matching of the waveforms of fundamental and highermost Ravleigh waves with waveforms synthesized from stratified models;in the second stage,the 3\|D model was constructed by solve linear constrains equation. The major structural features inferred from the surface waveform inversions can be summarized as follows:(1) There is a great contrast between surface waveform through Qinghai—Thibet plateau and the others.Main frequency of the former is lower than the latter, which indicate the crust depth of Qinghai—Tibet plateau is deeper than the others. In addition,the amplitude of about 30s period and 50s period is lower than both sides,which implied these exist lower velocity layer at about 25km depth and about 50km depth in Qinghai—Tibet plateau Crust.The former is common,the latter was argued because resolution of most method can not prove it.

3-D isotropic and anisotropic tomography of P-wave travel times from the Anhui airgun experiment in the Yangtze River

The Middle-Lower Yangtze River is a typical transition region between the nearly NW-oriented Tethys and NE-trending Pacific tectonic regimes.Structures of different periods and directions overlap strongly during these processes.The NE-trending Yangtze River compound structural belt and NW-trending Tongling-Hangzhou structural belt both control the magmatic activities and distributions of the metallogenic belts in the area.Here,we obtain 3-D high-resolution isotropic and azimuthally anisotropic velocity structures at depths of 1–10 km using the first arrivals from airgun sources.The velocity maps correspond well with the tectonic structures,with high-velocity anomalies distributed in ore-concentrated districts and low-velocity anomalies distributed along the Yangtze River.The fast directions are generally consistent with the fault strike,indicating that the azimuthal anisotropy is mainly dominated by the fault and fracture trends in the upper crust.The complicated fast directions near the Luzong and Tongling ore deposits reveal complex deformations in the upper crust,which are mainly caused by the intersection of the Yangtze River compound and Tongling-Hangzhou structural belts.The magma intrusion beneath the two ore deposits(Luzong and Tongling)are connected at depths of 5–10 km.

Fine three-dimensional P-wave velocity structure beneath the capital region and deep environment for the nucleation of strong earthquakes

A detailed 3-D P-wave velocity model of the crust and uppermost mantle under the capital region is de- termined with a spatial resolution of 25 km in the horizontal direction and 4—17 km in depth. We used 48750 precise P-wave arrival time data from 2973 events of local crustal earthquakes, controlled seismic explosions and quarry blasts. These events were recorded by 123 seismic stations. The data are analyzed by using a 3-D seismic tomography method. Our tomographic model provides new information on the geological structure and complex seismotectonics of this re- gion. Different patterns of velocity structures show up in the North China Basin, the Taihangshan and the Yanshan Mountainous areas. The velocity images of the upper crust reflect well the surface geological, topographic and lithologi- cal features. In the North China Basin, the depression and uplift areas are imaged as slow and fast velocity belts, re- spectively, which are oriented in NE-SW direction. The trend of velocity anomalies is the same as that of major structure and tectonics. Paleozoic strata and Pre-Cambrian basement rocks outcrop widely in the Taihangshan and Yanshan uplift areas, which exhibit strong and broad high-velocity anoma- lies in our tomographic images, while the Quaternary inter- mountain basins show up as small low-velocity anomalies. Most of large earthquakes, such as the 1976 Tangshan earthquake (M 7.8) and the 1679 Sanhe earthquake (M 8.0), generally occurred in high-velocity areas in the upper to middle crust. However, in the lower crust to the uppermost mantle under the source zones of the large earthquakes, low-velocity and high-conductivity anomalies exist, which are considered to be associated with fluids, just like the 1995 Kobe earthquake (M 7.2) and the 2001 Indian Bhuj earth- quake (M 7.8). The fluids in the lower crust may cause the weakening of the seismogenic layer in the upper and middle crust and thus contribute to the initiation of the large crustal earthquakes.

Crustal structure in Dabieshan UHP metamorphic belt and its tectonic implication

The model of Dabieshan crustal structure has been obtained on the basis of the deep seismic sounding data in thisarea. The 2-D crustal structure shows the feature of the collision orogens and provides some deep geophysicalevidences of the ultra-high pressure (UHP) metamorphic belt. The 3-D upper-crustal velocity struCture reveals thatthe velocity distribution at 2 km deep obviously relates to the surface geological setting and the UHP metarnorphicbelt has the higher velocity at 5~10 km deep. The observed data of Bouguer gravity anomalies reveal a largerrange of negative anomalies in Dabieshan area while the positive anomalies in the UHP metamorphic belt is calculated from the 3-D upper-crustal velocity structure. The 2-D crustal model along the seismic profile shows thatthe 'root' beneath the orogen is only 4-5 km thick and the velocity in the uppermost mantle changes a little in thelateral direction. The inconsistency between the observed and calculated Bouguer gravity anomalies mainly resultsfrom the crust, and at least the middle-upper crust should yield the negative anomalies. The material density of thecrust in the UHP metamorphic belt should be lower than that in the surrounding areas. This material with lowerdensity relates to the collision processes in which Yangtze crust subducted nor'thward to 100 km deep and thenreturned to the crust.

3D S-Wave Velocity Structure of the Crust and Upper Mantle

In this paper, 238 Rayleigh wave path data are selected and processed by the matched-filtering frequency-time analysis technique and the grid dispersion inversion method to obtain the 3D S-wave velocity structure of China mainland and its adjacent sea regions. The results show that the velocity structure relates to geotectonic division, Bouguer gravity anomaly is basically controlled by the relief of Moho discontinuity, the buried depth of LVL in upper mantle concerns the surface heat flow deeply. In this paper, authors indicate the main characteristics of the velocity structure in tectonic active and stable regions.

Crustal and uppermost mantle structure of Caucasus and surrounding regions

A 3-D P-wave velocity model is developed for the crust and uppermost mantle of Caucasus and the surrounding area by applying the tomographic method of Zhao et al. using 300 000 high-quality P-wave first arrivals from 43 000 events between 1964 and 2005. This tomographic method can accommodate velocity discontinuities such as the Moho in addition to smooth velocity variations. The spatial resolution is 1°× 1° in the horizontal direction and 10 km in depth. The velocity images of the upper crust correspond well with the surface geology. Beneath the southern Caucasus high velocity anomalies are found in the middle crust and low velocity anomalies are found in the uppermost mantle. Relatively low Pn velocities are located under the Lesser Caucasus, eastern Turkey, and northern Iran. Higher Pn velocities occur under the eastern portion of the Black Sea and the southern Caspian Sea, and also extend into the eastern edge of Azerbaijan. Tomographic model significantly reduces the travel-time residuals.

The 3-D structure of shear wave in South China and the southward extension of Tanlu fault

By processing the CSND Rayleigh wave data with the matched filter FTAN technique, Rayleigh wave dispersion for southeast China is obtained. The 4°×4°S wave dispersion of the pure path is calculated using random inversion scheme, and 3-D S wave velocity structure is set up. Incorporating the above-mentioned results with wide angle seismic sounding data, we studied structure framework and the extending of faults in this area, which demonstrates that the depth of Moho in South China varies from 30 to 40 km, shallower from west to east. The depth of Moho varies from 25 to 28 km for the offshore. The depth of the asthenosphere in upper mantle varies from 60 to 100 km. The depth difference of layers at the two sides of Tanlu fault is more than 10 km at the south part of the Yangtze River, and the fault extends downward more than 170 km. The fault exceeds the main land at Hainan Island and slips into the southern China Sea. Both Tanlu fault and the huge bend of gravity gradient anomaly are influenced by

Three-dimensional crustal velocity structure model of the middle-eastern north China Craton (HBCrust1.0)

Lithosphere thinning and destruction in the middle-eastern North China Craton(NCC), a region susceptible to strong earthquakes, is one of the research hotspots in solid earth science. All 42 seismic wide-angle reflection/refraction profiles have been completed in the middle-eastern NCC. We collect all the 2-D profiling results and perform gridding of the velocity and interface depth data, building a 3-D crustal velocity structure model for the middle-eastern NCC, named HBCrust1.0, by using the Kriging interpolation method. Our result shows that the first-arrival times calculated by HBCust1.0 fit well with the observations. The result demonstrates that the upper crust is the main seismogenic layer, and the brittle-ductile transition occurs at depths near interface C(the interface between upper and lower crust). The depth of interface Moho varies beneath the source area of the Tangshan earthquake, and a low-velocity structure is found to extend from the source area to the lower crust. Based on these observations, it can be inferred that stress accumulation responsible for the Tangshan earthquake may have been closely related to the migration and deformation of the mantle materials. Comparisons of the average velocities of the whole crust, the upper and the lower crust show that the average velocity of the lower crust under the central part of the North China Basin(NCB) in the east of the craton is obviously higher than the regional average. This high-velocity probably results from long-term underplating of the mantle magma.

340 GHz菱形曲折波导慢波结构

提出并研究了一种菱形曲折波导慢波结构。与传统的矩形曲折波导慢波结构相比,菱形曲折波导慢波结构在相同频带下拥有更大的尺寸,在相同尺寸下拥有更宽的带宽。同时提出了适用于这种慢波结构的输入-输出过渡结构和衰减器。在此基础上,设计了一种用于行波管的340 GHz菱形曲折波导慢波结构,并采用相速负跳变技术提高了其增益。模拟仿真结果表明,在加载电压为15.3 kV,电流为35 mA的圆形电子注的情况下,行波管在343 GHz的输出功率和增益分别达到8 W和33 dB,其3-dB带宽范围为330∼348 GHz。

P-wave tomography of crust and upper mantle under Southern California: Influence of topography of Moho discontinuity

We use 146 422 P-wave arrival times from 6 347 local earthquakes recorded by the Southern California SeismicNetwork to determine a detailed three-dimensional P-wave velocity structure at 0～35 km depth. We have takeninto account the Moho depth variations, which were obtained by seismological methods. Checkerboard tests sug-gest that our inversion results are reliable. Our models provide new information on regional geological structuresof Southern California. At shallow depths P-wave velocity structure correlates with surface geological features andexpresses well variations of surface topography of the mountains and basins. The velocity structure at each layer ischaracterized by block structures bounded by large faults. Ventura Basin, Los Angeles Basin, Mojave Desert, Pen-insular Ranges, San Joaquin Valley, Sierra Nevada, and Salton Trough show respectively all-round block. SanAndreas Fault becomes an obvious boundary of the region. To its southwest, the velocity is higher, and there arestrong heterogeneity and deeper seismicity; but to its northeast, the velocity is lower and shows less variation thanto the southwest, the seismicity is shallower. To investigate the effect of the Moho geometry we conducted inver-sions for two cases: one for flat Moho geometry, another for a Moho with lateral depth variations. We found thatthe topography of the Moho greatly affects the velocity structure of the middle and lower crust. When the Mohotopography is considered, a more reasonable tomographic result can be obtained and the resulting 3-D velocitymodel fits the data better.

中国大陆及海域Love波层析成像

收集了研究区域 ( 68°— 1 5 0°E ,5°— 5 5°N)内 33个数字地震台站记录的面波资料 ,利用多重滤波技术提取了 40 0 0余条路径上的Love波群速度频散曲线 .将研究区域划分成1°× 1°网格 ,采取Occam反演方法得到了 7.3— 1 84s共 43个周期的Love波群速度分布图 ;然后对网格结点进行S波速度结构反演 ,得出研究区域内 42 0km深度内的地壳上地幔三维速度结构 .并采用Checkerboard方法对分辨率进行检验 ,得到横向的分辨率约为 3°— 5° .研究结果表明 :中国大陆地壳上地幔结构的横向不均匀性非常明显 ,内部结构与地表特征的相关性可以达到 1 0 0— 1 5 0km深度 .大陆地区东西分带、南北分块 ,块体的边界反映比较清晰 .

川滇地区三维P波速度结构反演与构造分析

根据205个区域台站记录的近60000条地震初至P波走时资料,采用层析成像理论与伪弯曲射线追踪方法,反演了川滇地区地壳上地幔的三维P波速度结构.结合区域地质构造以及地球物理背景,分析和解释了三维速度结构图像反映的川滇地区不同深度的介质结构与构造特征.结果表明:(1)沉积盆地、高山山地等主要表现出速度负异常的特征,有的高山山体负异常可深达下地壳与上地幔,反映了新造山带的强烈构造隆升与相伴的重力均衡作用;(2)川滇块体周缘大型活动断裂带附近的中下地壳内普遍存在低速层,它们的存在为调节断裂和块体运动提供了深部解耦条件;(3)根据对P波速度结构图像的分析,识别和推断出川滇地区若干主要活动断裂的深部构造特征及它们倾向与下延深度.