Based on continuous waveform, surface wave and body wave data from teleseismic events recorded by broadband seismic network stations in the southeastern margin of the Tibetan Plateau, we retrieved 6-120 s Rayleigh wave phase velocity and 6-50 s group velocity dispersion curves for station pairs, as well as receiver functions beneath all of the stations. The Rayleigh wave phase velocity dispersions at overlapping periods based on ambient noise empirical Green's functions were found to be congruent with those based on teleseismic surface wave data. We then performed inversion to obtain a reliable 3-D shear wave velocity structure by using both the surface wave dispersion measurements and receiver functions. The resulting shear wave velocity distribution revealed that low-velocity zones (LVZs) are ubiquitous in the mid-lower crust beneath the Chuandian block, the Songpan-Ganze fold belt, and some parts of the western Yangtze platform. This result differs from those of previous studies, which indicated that LVZs are mainly concentrated in the lower crust beneath the Chuandian block. We also found that the LVZs vary substantially in terms of occurrence depth, thickness, and anomaly strength. They are also influenced by the major fault zones, especially the Lijiang-Xiaojinhe fault, and the tectonic blocks in the study area. Extremely high Poisson's ratios (sigma > 0.3) and low shear wave velocities (V-s < 3.4 km s(-1)) coincide in the northwestern margin of the Chuandian block and the region to the south of the Sichuan basin, indicating the existence of partial melt in the mid-lower crust in that area. The mantle LVZs exhibit spatial consistency with crustal LVZs to some degree in the eastern Songpan-Ganze fold belt, Shangrila sub-block, and Yajiang sub-block, implying the existence of a weak and low-density layer in the upper-most mantle.

Data from controlled explosions provide a unique advantage to study velocity models as the source locations are precisely known. A series of explosion were conducted in Fujian Province in 2010-2012. In this study, we hand-picked first Pg and Pn arrivals and secondary Pg arrivals from the explosion seismograms recorded at provincial stations. Using a joint inversion method, we inverted for a new 1-D P-wave velocity model, Fujian explosion model (FJEM). The FJEM model shows a 45% improvement in fitting the travel-time data over the current routine model. Similar velocity models inverted from subsets of explosion data suggest a relative stable and simple velocity structure in Fujian province. Relocations of the explosions suggest that the traditional South China model has small horizontal location errors (average 0. 52 +/- 0. 45 km), but errors in depth are significant (average 4. 7 +/- 1. 2 km), while the FJEM model has similar horizontal location capability but much smaller errors in depth (average 1. 3 +/- 1. 1 km). Synthetic tests using real earthquake bulletins show similar results for the South China model : (1) for earthquakes in the interior of Fujian with good azimuthal coverage, the horizontal mislocation is small; (2) the horizontal mislocations for earthquakes along the coast and in Taiwan Strait with poor azimuthal coverage can be very large; and (3) mislocations in depth can be large regardless station distribution, which have strong trade-off with origin time errors. Essentially, the horizontal location is largely affected by station azimuthal coverage and much less by the reference model. But the improved FJEM model not only is closer to the real Earth (fit data better) but also can reduce depth mislocation. We propose that South China model be replaced and the FJEM model be used in routine earthquake locations in Fujian and perhaps neighboring regions as well.

Based on the real data from joint inversion of ambient noise, surface wave data, and P wave receiver functions of 170 broad-band seismic stations of national and regional networks of the southeastern margin of Tibetan Plateau and its adjacent areas, we preformed the recovering tests to the presumed initial model of southeastern margin of Tibetan Plateau. We calculated pure path dispersion curves on the basis of the initial model, then retrieved the Rayleigh wave dispersion curves between station pairs and receiver functions beneath each station. Finally, the recovering tests were taken to measure the recovery ability to the initial model based on different seismic data alone and joint inversion of multiple types of seismic data. Our result reveals that joint inversion of receiver function, dispersion cures based on empirical Green’s functions （EGFs） of ambient noise and on teleseismic surface wave data can take full advantage of the resolution of each seismic data, and can resolve the crustal and upper mantle LVZs perfectly. Additionally, we analyzed the resulting reliability on the condition of adding calculation error or random noise. From these tests, surface wave dispersions with 1% error in joint inversion can resolve the low velocity zone （LVZ） commendably, while with 5% error can cause some differences from the initial model. Though receiver functions with 4% random noise in joint inversion can reduce the resolution of the upper and lower boundaries of the mantle LVZ, they can commendably recover the LVZs in terms of occurrence depth and velocities.

Crustal earthquake focal mechanisms are investigated in the southeastern margin of the Tibetan Plateau, where the Tibetan Plateau and stable South China Block merge. An updated database of focal mechanisms has been compiled by selecting 132 Global Centroid Moment Tensor solutions and by adding the 173 new solutions (3.5 ≤ Ms ≤ 7.4) estimated by waveform inversion in this study. A total of 305 mechanisms are included in this database. These solutions show regionally specific distributions with dominant strike-slip faulting and some normal and reverse faulting. Focal mechanism solutions have also been inverted for the stress tensor orientation to obtain the principal stress axes over the study region. Results show that the horizontal maximum principal σ 1 axes rotate clockwise with a wider range than the geodetically measured surface motion in the east, which is not limited to the Xianshuihe–Xiaojiang fault, but has some overlap with the Zhaotong–Lianfeng fault. Localized normal faulting stress regimes are observed in the Jinshajiang–Litang fault areas and the Baoshan sub-block. The minimum principal axes are oriented with a gradually changing trend from north–south to northwest–southeast, from north to south, indicating diverse compression stress patterns. Significant changes in the crustal stress field after the Wenchuan earthquake are preliminarily observed in the Baoshan sub-block where orientations of two principal axes have changed, and in the Jinggu–Ximeng sub-block areas where the strike-slip faulting stress pattern has transformed to normal faulting.

This paper discusses the spatiotemporal pattern of seismicity during the impoundment of the Xiaowan reservoir area, Yunnan Province, China, in the period from May 21, 2005 to December 31, 2012. The filling operations took place in five phases, starting on December 16, 2008. A notable increase in seismicity was only observed during the third filling phase, starting on July 15, 2010. Seismicity increase was mostly localized within two clusters, located to the northwest and west of the dam. Seismicity rates in these clusters showed a significant correlation with the water level increase, with the seismicity starting to increase when the water level reached the area covered by the two clusters, which additionally support they were induced by the reservoir impoundment. We further investigate source parameters for 44 pre-impoundment earthquakes and 164 post-impoundment earthquakes with M L ≥ 2.0. Corner frequencies, seismic moments, and stress drops are obtained based on the spectral analysis of regional data, upon corrections for geometrical spreading, frequency-dependent Q, and site effects. Our results show that during the post-impoundment phase reservoir-induced seismicity (RIS) inside the two clusters have systematically lower stress drops with respect to those occurring at further distance, and surrounding natural tectonic earthquakes, by a factor of about two to three times, suggesting a possible source characteristic that differentiate reservoir-induced seismicity from natural tectonic earthquakes. However, temporal stress drops changes within the clusters cannot be resolved. To compare the difference in stress drop between RIS and natural tectonic earthquakes, not only the difference in stress drop due to different region, but also the temporal change of background stress level even in the same region should be taken into account.

We perform high-resolution 3D shear-wave structures of the crust and upper mantle using continuous wave data recorded at 170 stations from national and regional networks in the southeastern Tibetan plateau. First, empirical Green's functions are obtained between all the possible station pairs. Then Rayleigh-wave group velocity and phase velocity dispersion curves are retrieved using the frequency-time analysis method. The Rayleigh-wave group velocity images at periods from 8 to 50 s and phase velocity images at periods from 8 to 40 s are inverted on a 1 degrees x 1 degrees grid. Finally, the 1D shear-wave structure beneath each grid and the 3D shear-wave velocity structure are acquired by linear interpolation. Our results indicate that low-velocity layers (LVLs) are ubiquitous in the middle-lower crust beneath the southeastern Tibetan plateau but vary in depth (20-35 km), thickness (5-20 km), and strength (5.4%-10.8% velocity reduction) in different blocks. The 3D geometry of the LVLs is complex and cannot be interpreted as a model with a uniform flow channel. The shear velocity in the Chuandian block is unevenly distributed, indicating that the block is inhomogeneous. The 12 May 2008 Wenchuan and 20 April 2013 Lushan earthquake sequences (M-s >= 3.0) all occurred in the crustal layers in which LVLs are inhibited by the strong Sichuan basin and in which the topography and crustal thickness vary sharply. This formed the earthquake preparation environment of the two earthquakes. The Ningnan-Qiaojia pull-apart region is characterized by low shear-wave velocity in the shallow crust, which agrees well with the sedimentary features of the upper crust. A relatively high velocity is observed in the central part of the low-velocity Sichuan basin in the shallow layers, indicating the basement beneath the Sichuan basin may be uplifted around the central part of the basin.

Ten earthquakes (ML ≥ 4) occurred in Fujian and its adjacent areas from 2007 to 2009. These frequently occurred moderate earthquakes actually affected the local people's life. It is necessary to strengthen the study of the earthquakes. In this paper, focal mechanism solutions of four earthquakes (ML > 4.2) have been calculated by using broadband digital data of Fujian earthquake network and the seismic moment tensor inverse software TDMT-INVC developed by Douglas Dreger of the Berkeley Seismological Laboratory. The results of focal mechanism solutions are analyzed. The focal depths are determined in the inversion.

We present a 3D model of shear velocity of crust and upper mantle in China and surrounding regions from surface wave tomography.We combine dispersion measurements from ambient noise correlation and traditional earthquake data.The stations include the China National Seismic Network,global networks,and all the available PASSCAL stations in the region over the years.The combined data sets provide excellent data coverage of the region for surface wave measurements from 8 to 120 s,which are used to invert for 3D shear wave velocity structure of the crust and upper mantle down to about150 km.We also derive new models of the study region for crustal thickness and averaged S velocities for upper,mid,and lower crust and the uppermost mantle.The models provide a fundamental data set for understanding continental dynamics and evolution.The tomography results reveal significant features of crust and upper mantle structure,including major basins,Moho depth variation,mantle velocity contrast between eastern and western North China Craton,widespread low-velocity zone in midcrust in much of the Tibetan Plateau,and clear velocity contrasts of the mantle lithosphere between north and southern Tibet with significant E–W variations.The low velocity structure in the upper mantle under north and eastern TP correlates with surface geological boundaries.A patch of high velocity anomaly is found under the eastern part of the TP,which may indicate intact mantle lithosphere.Mantle lithosphere shows striking systematic change from the western to eastern North China Craton.The Tanlu Fault appears to be a major lithosphere boundary.

The temporal and spatial characteristics of 1924 post-impoundment earthquakes (M-L -0.3 to 3.1) recorded by 26 temporary stations from 16 March 2009 to 14 July 2010 in the Three Gorges reservoir area, Hubei Province, China, are investigated in this paper. The epicenters are mainly concentrated in three clusters and located along the Yangtze river within the range of 10 km from the reservoir waterfront. The hypocentral depths range from 0 to 15 km and appear to increase with the epicenters approaching the dam. Source parameters for 97 selected reservoir-induced earthquakes of M-L >= 1.5 were estimated after applying corrections for geometrical spreading, frequency-dependent Q, and site effects. The results show that the seismic moments (M-0) are between 3.96 x 10(11) and 4.07 x 10(13) N.m, whereas static stress drops (Delta sigma) are 0.01-0.26 MPa. We find the static stress drops in this area vary with seismic moment for our data range, approximately as Delta sigma proportional to M-0(0.46). Apparent stresses we obtained lie between 0.0019 and 0.049 MPa and also increase with increasing seismic moment, indicating that large earthquakes radiate more energy per seismic moment than smaller ones in the Three Gorges reservoir area and do so more efficiently. Our results show that reservoir-induced earthquakes appear to have systematically lower stress drop with respect to natural tectonic earthquakes, by about one order of magnitude. This may be attributed to the high pore pressures of the underground medium, and the presence of water decreasing the coefficient of friction.

<正>川滇地区位于中国大陆西南部,其西部是活跃的青藏高原,东部是稳定的扬子地台,地质构造活动与印度板块和欧亚板块的强烈碰撞作用密切相关,是世界上最复杂的构造区之一。由于复杂的地质构造,该地区也是中国大陆地震活动性最强的区域之一,历史上发生7.0级以上地震44次,8级以上地震4次。川滇地区作为青藏高原与扬子板块之间的过渡带一直是科学家研究的重点区域。剪切波对于地下流体、深部温度变化和介质性质十分敏感,因此,

A pronounced increase in seismicity started in and around Longtan reservoir, southwestern China after October 1, 2006 when it began the impoundment, and by the end of May 14, 2010, about 3,233 earthquakes with −0.6 ≤ M L ≤ 4.2 had been located. This seismicity which occurred in five clusters mainly concentrated in the areas where few earthquakes had occurred before the first filling. There were four water filling periods in the Longtan reservoir, and the observed reservoir-induced seismicity (RIS) shows a strong correlation with the filling cycles. After the first filling, there appears to be an instant undrained response due to an elastic response to the reservoir load in the third and fourth cluster. Then, this seismicity is followed by a delayed, drained response due to pore pressure diffusion, with the seismicity migrating outwards in one or more directions in the second and third filling period. The seismic diffusivity (α s) we estimated is about 4.54 × 105 cm2/s. The activity levels in the five clusters are different due to differences in the structures and permeabilities of the faults. The delayed seismic response to the filling in the third cluster was due to the combined effects of the lack of local fault intersecting the reservoir and lower permeability of the rock. The b value we obtained for reservoir-induced events was significantly different and higher than that of pre-impoundment natural tectonic earthquakes in the Longtan reservoir. The results of relocated earthquakes based on double difference earthquake location algorithm showed that their focal depths were mainly shallower than about 10 km and the distribution of relocated RIS in four clusters had no relation with these intersecting faults in the Longtan reservoir except the fifth cluster. All these characteristics of RIS in the Longtan reservoir indicate that they may relate to the coupled poroelastic response that includes both pore pressure diffusion and an undrained response, but the pore pressure diffusion and the water permeation appear to play a more important role on inducing the earthquakes in Longtan reservoir.

We investigate the relationship between the impoundment and seismicity in the Longtan reservoir, southwestern China and find evidence that the seismicity was reservoir induced. After the reservoir impoundment, a pronounced increase in seismicity was observed in five clusters mainly concentrated in the areas where few earthquakes had occurred before the first filling. The observed induced seismicity shows a strong correlation with the filling cycles. The activity levels in the five clusters are different due to differences in the structures and permeabilities of the faults. Source parameters for 1,616 earthquakes with M L 0.1–4.2 recorded by 24 fixed and temporary stations deployed around the reservoir were calculated after applying corrections for geometrical spreading, frequency-dependent Q, and site effects. The static stress drop and apparent stress in this area both appear to increase with increasing seismic moment over the entire magnitude range. Our results show that reservoir induced earthquakes have ten times lower average stress drop than natural tectonic earthquakes. These results may indicate that the reservoir induced seismicity can occur with a lower tectonic stress due to the high pore pressures of the underground medium, and that the effect of the water decreases the coefficient of friction.

We retrieved Rayleigh wave Green functions between all possible station pairs after pre-processing, cross-correlation and stacking the data, which are 21 months' continuous ambient seismic noise recorded by 100 evenly distributed broadband seismic stations mainly in the National Seismic Network in central and eastern Chinese mainland. The quality of the empirical Green functions (EGFs) and the dispersion curves along selected paths were estimated to measure the reliability and stability of the EGFs. Our results display that the EGFs are correlated with the earthquake signals, suggesting the reliability of the EGFs. In addition, we counted the number of dispersion measurements with SNR>10 retrieved from the time-series of different lengths (3—21 months). The result shows that, to obtain enough reliable EGFs for velocity tomography, at least 12 months' data should be used and 12 months’ data can make sure that dispersion curves under 30 s are temporally stable.

High-resolution three-dimensional V P and V P /V S images in the Longtan reservoir area were obtained from local earthquake data by using 3,178 events with total 24,153 P-wave and 23,987 S-wave arrivals collected from 23 seismic stations. The tomographic images show that significant V P heterogeneities can be seen at layers of different depth in the Longtan reservoir area. Low-V P anomalies both beneath and around the main rivers in the reservoir area may be related to the composition of rocks which are mainly deposit carbonate and arenaceous shale, which contributes to water saturation. We deduced that the high porosity rocks beneath the main rivers may be fully saturated with water. The phenomenon that V P is relatively high in the area which is 10–20 km away from the rivers indicates that horizontal saturation of water is limited within a small range of area that is about 10–20 km from the main rivers. The characteristic is significant that seismicity in the Longtan reservoir area is coincident with the distribution of the low-V P area. V P /V S tomographic images show that V P /V S ranges from 1.8 to 2.05 in shallow layers above 4 km depth beneath the Longtan reservoir, suggesting the properties of the rocks are limestone and shale. At the depth of 7 km, the distribution of V P /V S image varies quite remarkably, especially in the dam area. This demonstrates that the range of influence by the saturation of water in the media below the reservoir surface can reach 4–7 km depth in the dam area.

By using the seismic wave data from the Digitalized Remote Seismic Station Network of Chengdu,the relationship between the quality factor Q value and frequency for Sichuan province with the method of Atkinson,Q(f)=203.1f 0.834,is got.Then we use the Moya inversion method to calculate the site responses of a seismic station.The comparison of the earthquake source spectral parameters respectively from both method of Moya and Atkinson are made.The earthquake sources parameters from both methods of Moya and Atkinson are consistent with each other.Our calculated parameters show that the relationship between seismic moment and seismic magnitude is logM0=17.27+1.18 ML.The corner frequency decreases with the increasing of seismic moments and the stress drop is from 10 to 400 Pa.The stress drop do not show clearly connected with the seismic moment.

Study shows that the focal mechanisms of foreshock sequence are more consistent than those of general small earthquake swarm.Due to the intricacy of calculating the focal mechanism of small earthquakes,we use correlation coefficient of spectral amplitude to study the consistency of focal mechanism of small earthquakes,so as to analyze the possibility of determining the type of small earthquake swarm.We apply the method to five earthquake swarms in the northwest of Yunnan Province where small earthquake occurs frequently.The results show that the mean correlation coefficient of spectral amplitude of foreshocks of the Shidian MS5.9 earthquake in 2001 is 0.88,the Yongsheng MS6.0 earthquake in 2001 is 0.93,while the general small earthquake swarms in Yangbi(2002),Eryuan(2003),Binchuan(2005) is 0.48,0.68,and 0.63 respectively.The results suggest that the correlation coefficient of spectral amplitude of foreshock sequence is larger,closer to 1.0 than that of general small earthquake swarm,which leads to more similarity of focal mechanisms of foreshock sequence.In this paper,the theoretical expectations conform to the actual calculated results in these earthquake cases.This feature preliminarily implies that the correlation analysis of spectral amplitude could serve as a method that has clear physical significance and practical utility in determining the type of the earthquake swarm,and thus be worthy of further study.

Using the digital waveform data recorded by six temporary digital seismic stations which was closed to the epicenters of the Yao’an M5.9 and M6.5 earthquakes in Jan.15,2000,applying the methods of the spectrum analysis and Brune’theory,we obtained source spectrum parameters and source parameters of Yao’an earthquake sequence.The result shows that the seismic moments of Yao’an earthquake sequence are between 108N·m and 1014 N·m,the stress drops are between 0.005 MPa and 10 MPa,and the source radius is between 50 m and 660 m,and we obtain the relationship between seismic moment M0 and local magnitude ML.With the seismic moment increases the corner frequency reduces,but the stress drop increases.The source radius increases with the magnitude increases,but there is no evidence relationship between source radius and stress drops.

In this text,we firstly discussed the relationship between the water filling and seismicity in Longtan reservoir and found that the characteristic of reservoir induced seismicity was very apparent in Longtan reservoir and these epicenters mainly distributed in 5 clusters.Digital seismic data recorded by 24 fixed and temporary stations located around the reservoir were used to study the characteristics of anelastic attenuation in this region and to obtain the site responses.After that,source parameters of 1616 earthquakes with magnitude bigger than 0.1 were calculated and the differences existing in characteristics of source parameters between reservoir induced seismicity and tectonic earthquake were discussed.Our results show that(1) Seismicity in Longtan reservoir has strong relationship with the water filling of reservoir and the activity status of seismicity for these five clusters are various in different water filling period.The response of seismicity to the water filling process is affected by the development of local faulted structure and the water permeability of the rock.(2) Seismic moment M0increases with increasing magnitude ML in Longtan reservoir and their statistical relationship is LogM0=1.07ML+10.17.Relationship between stress drop and magnitude is consistent with the result gained by Nuttli that intraplate earthquake follows the ISD model,with a statistical relationship LogΔσ=0.71ML-2.89.(3) Radiated seismic energy and apparent stress in Longtan reservoir all increase with increasing magnitude,indicating that the big earthquake is a radiator of seismic energy with more efficient rather than small earthquake.(4) In general,there are some differences in stress drop for different cluster.The distribution of stress drops has a good consistence with the water depth of reservoir,with high value of stress drop in deeper water depth.(5) Stress drops of reservoir induced seismicity are smaller than that of tectonic earthquake with same magnitude apparently,about 10 times.These results maybe indicate that the reservoir induced seismicity can occur with a lower tectonic stress due to the high hole pressure of the underground medium and the effect of the water lubrication.

Radiated seismic energy for 80 earthquakes were estimated using the data of Yushu Ms 7.1 earthquake sequence in Qinghai recorded by 4 temporary stations disposed after the mainshock and 3 fixed stations around the source region with the deadline on 19 May 2010,after the calibration of wave travel path effect and the site response.We compare the difference among the results calculated by various methods and take into account the underestimate of the radiated seismic energy for moderate and small earthquake due to the limited recording bandwidth and the energy compensation.Our results show that(1) Radiated seismic energy of moderate and small earthquakes is estimated by Andrews' method.The actual spectrum's method is based on actual spectrum(handled separately pre-and post-the energy compensation) and the theory's method extends the upper integration limit to 10 times the corner frequency of the event we studied are in same level,showing that the results have high reliability and stability.Among them,the values of radiated energy derived from Andrews' method are lowest and that of the theory's method are highest,results calculated by the actual spectrum's method are between them.(2) The results of radiated energy calculated by Gutenberg empirical formula are higher than that of aforesaid 4 results and come up to 4 times that of the theory's method.With the increase in magnitude,the tendency of overestimate of the radiated seismic energy become more apparently.(3) As lack of energy compensation,the results of Andrews' method seem to be lower.The two results of radiated seismic energy after compensated by various methods achieve to be approach and the theory's method is considered to be more rational used for calculating the radiated energy for moderate and small earthquake because the complexity existed in the determination for upper bound of recording bandwidth.(4) The results for radiated energy after the energy compensation are bigger than that of before the energy compensation and the increase in radiated energy due to energy compensation seems to be more apparently for small earthquake.Within the data we studied,scale energy calculated by various methods increasing with the increasing seismic moment,indicating that the radiated efficiency for big earthquake is bigger than that of small earthquake in Yushu earthquake sequence.