The Menyuan M_S6.9 earthquake occurred on January 8, 2022, which is the third strong M_S>6 earthquake on the western part of the Lenglongling fault following two Menyuan M_S6.4 earthquakes that took place in 1986 and 2016. In order to explore the fault deformation and stress states of different timescales before the M_S6.9 Menyuan earthquake and the dynamic environment of frequent strong earthquakes in the area nearby the epicenter, with GPS velocities of 1991—2015 and 2017—2021 as boundary constraints, a fine three-dimensional viscoelastic finite element model was established. The model included the impacts of tectonic units, the layered structure of the crust-mantle, the inhomogeneity of the medium, the interactions of many different faults, and the shape of the faults. It also refined the key faults in the region and their geometric characteristics. The basic pattern of stress accumulation in the Qilian Mountain tectonic region under the long-term tectonic movement environment, the long-term slip rate and stress accumulation rate of faults and their change characteristics during the five years before the Menyuan M_S6.9 earthquake are calculated and analyzed. Combining the results of the source mechanism solution and cross-fault level observation, the following conclusions are obtained:

(1)According to the simulation results for a longer period of 1991—2015, the stress field in the study area gradually rotates clockwise, with NNE-SSW extrusion and NWW-SEE tension to NE-SW extrusion and NW-SE tension from west to east. The direction of the principal compressive stress is mostly perpendicular to the fault strike. The region near the epicenter of the Menyuan M_S6.9 earthquake has been subjected to long-term NE-SW extrusion and NW-SE tensional stress. The maximum shear stress accumulates faster than the surrounding area. The above stress accumulation characteristics overall promote NW-oriented shear and NE-oriented extrusion movement of faults, which contribute to the generation and occurrence of strike-slip and thrust earthquakes on the NWW-oriented Lenglongling Fault.

(2)The simulation results show that most NWW-orientated faults exhibit a left-lateral strike-slip and thrust nature. In contrast, NNW-orientated faults display a right-lateral strike-slip and extrusion nature. The fault’s stress nature corresponds with its movement nature. Spatially, the overall trend of fault movement in the study area is that the extrusion rate gradually decreases from west to east, and the slip rate gradually increases from west to east. This indicates that the Qilianshan tectonic belt plays a significant role in transforming and adjusting the tectonic deformation of the northeastern margin of the Qinghai-Tibetan plateau.

(3)The fault movement and its stress distribution show significant segmentation, indicating the crucial role of fault geometry in fault movement. The western segment of the Lenglongling Fault has a geometric inflection pattern, causing stress accumulation variability and uncoordinated movement between different segments. Compared to the surrounding fault segments, this fault segment has a higher rate of stress accumulation yet experiences hindered movement in space which causes a lower slip rate. fault zones that exhibit motion deficits and rapid energy accumulation are more susceptible to earthquakes.

(4)Compared to the period between 1991 and 2015, the simulation outcomes obtained during 2017—2021 demonstrated noticeable differences and irregularities in the distribution of motion and stress increment fields along the fault, which were segmental in nature. Within~5 years before the Menyuan M_S6.9 earthquake, the strike-slip rate at the western segment of the Lenglongling fault is further reduced, the accumulation rate of shear stress was significantly increased; the extrusion rate was significantly weakened, and the rate of positive stress accumulation was slowed down. These recent changes in fault motion and stress are conducive to promoting left-lateral slip-strike earthquakes on this fault segment.

(5)From a hydrostatic perspective, the above studies demonstrate that the epicenter region had accumulated high stress for a long time before the earthquake, and as the earthquake approached, the positive stress on the seismic fault surface increased slowly, and the friction increased synchronously, leading to the weakening and deficit of movement on the local fault segment.

In conclusion, the western segment of the Lenglongling fault has a strong stress background and favorable conditions for the occurrence of strong earthquakes, and the risk of strong earthquakes is still predicted to exist in the future.

The paper collects the seismic waveforms of the M_S5.6 earthquake that occurred in southern Nima, central Tibe on July 24, 2009 recorded by Tibet seismic network and the mobile seismic networks of the orresponding period, i.e. Western Tibet/Y2 and TITAN. The seismic waveform data were preprocessed by rglitches, rmean, rtrend, taper, transfer and filtering. Then we hand-picked the arrival times of the P-and S-waves(0.05~2Hz for P wave, and 0.05~0.5Hz for S wave). The Hypo2000 method was applied to accurately relocate the earthquake.

Because the earthquake occurred in the hinterland of Tibetan plateau, there are few local seismic stations available. Since the seismic stations and seismic phase information used in processing by different institutions are different, the epicenter location and focal mechanism determined by various institutions are different. Compared with the result(31.30°N, 86.10°E)relocated by Tibet seismic network, our result(31.08°N, 86.05°E)is more reliable due to the uniform distribution of stations used in our study, which is roughtly identical to the GCMT result(31.05°N, 86.10°E)inverted by the moment tensor method.

Based on the relocated result, we apply the Cut-and-Paste(CAP)inversion method to invert the focal mechanism and focal depth. The waveform is decomposed into Pn1 and surface wave to perform cross-correlation fitting of theoretical waveform and actual waveform, respectively. To suppress the noise and influence of the source region medium, the bandpass filter is selected as 0.05~0.15Hz for body wave and 0.05~0.1Hz for surface wave. We set the earthquake source time function as 5s and search for the best focal depth at the depth of 1~30km, and the search step is 1km concerning the magnitude of the earthquake. The result shows that the earthquake has a best-fitting focal depth of 19.3km from the mean sea level and is of strike-slip faulting(the nodal plane Ⅰ: 220°/82°/-17° and nodal plane Ⅱ: 314°/73°/-171°).

The shear stress and normal stress of the two nodal planes of the earthquake are calculated according to the stress field characteristics of the earthquake area. The generation of the earthquake is consistent with the stress field characteristics of NS compression and EW extension in the region. Referring to the near-EW strike-slip fault zone constrained by the EW-trending Wozang Fault and the NWW-trending Zhala Fault in the 1︰250000 regional geological survey map near the epicenter area, it is inferred that the earthquake is of EW-trending dextral strike-slip faulting.

Most of the earthquakes that occurred along the 31°N belt near this earthquake area are EW-trending strike-slip ones, even in the interior of the Tangra-Yumco Rift. Considering the physical properties beneath Tibetan plateau, the low-velocity and high-conductivity layers are widely distributed in the depth range of 20km to 30km in the thick crust. According to surface geology and deep structures revealed by regional geophysics(receiver function, magnetotellurics, and tomography)of the region, the earthquake occurred on the top of the brittle-ductile transition zone with a low seismic velocity between the middle and upper crust beneath the south boundary faults of the Seng-ge Kambab-Lhaguo Tso-Yongzhu-Jiali ophiolite mélange zone(SYMZ), 30km away from the Tangra-Yumco Rift to the west. The occurrence of the earthquake indicates that SYMZ, which formed in the Late Jurassic, was reactivated in an EW-trending strike-slip manner during the quick uplift of the plateau. This cognition is of great significance to understand the geodynamic mechanisms of the EW-trending extension within the Tibetan plateau.

The coseismic displacements are required to characterize the earthquake rupture and provide basic data for exploring the faulting mechanism and assessing seismic risk in the future. Detailed field investigation is still an important way to acquire reliable coseismic displacements comparing to geodetic measurements. Combining with previous research on other earthquakes, this study tries to discuss distributed deformation along the strike rupture and its implications. The M_W7.4 Madoi earthquake ruptured the southeast section of the Kunlun Shankou-Jiangcuo Fault on May 22, 2021, in Qinghai Province. It is a typical strike slip event, and its epicenter locates at~70km south of the East Kunlun Fault, which is the north boundary of the Bayan Har block. Field investigation results show that the surface rupture extends along the piedmont. The deformation features mainly include compression humps, extensional and shear fissures, and scarps. After the earthquake, we used the unmanned aerial system to survey the rupture zone by capturing a swath of images along the strike. The swath is larger than 1km in width. Then we processed the aerial images by commercial software to build the orthoimage and the digital elevation model(DEM)with high resolutions of 3~5cm. We mapped the surface rupture in detail based on drone images and DEM along the western section. Meanwhile, we also got the commercial satellite images captured before the earthquake, on 2nd January 2021. The images were processed with geometrical rectification before comparison. The spatial resolution of satellite images before earthquake is about 0.5m.
At the south of the Eling Hu(Lake), the clear offset tire tracks provide an excellent marker for displacement measurement. We located the positions of tracks precisely based on remote sensing images, and compared between the tracks lines after earthquake and the corresponding positions before earthquake, then extracted distance difference, which is defined as coseismic displacements. The results show that the total displacement is about 3.6m, which contains the distributed deformation of about 0.9m. The off-fault deformation is about 33% of the on-fault and about 25% of the total deformation. The ratios are similar to previous studies on earthquake worldwide. The fault zone width is probable about 200m. The total horizontal displacement measured by this study is similar to the slip in depth by InSAR inversion, which implies that there is no slip deficit at the west rupture section of the earthquake.
The results also present the asymmetry of distributed deformation that most distributed deformation occurs at the south of the surface rupture zone. Comparing with other earthquakes in the world, it is likely that the asymmetrically distributed deformation is common in strike-slip earthquakes and the asymmetric feature is not related to the property of the material. The characteristics of distributed deformation might be related to fault geometry at depth or local stress state. More work is needed to resolve this question in the future. This study implies that we probably underestimated the slip rates resulting from ignoring distributed deformation in the past. In order to avoid underestimation of slip rates, we can correct the previous results by the ratio of distributed deformation to total slip. It is also suggested that the study sites should be on the segment with narrow deformation and simple geometry.

Previous studies have shown that M≥8 earthquakes and more than 80% M≥7 earthquakes occurred in the boundary zones of active blocks. Therefore, studies on the slip rate and stress distribution of the boundary faults can provide the basis for assessing the risk of strong earthquake. It also can help us understand the regional tectonic deformation, motion and dynamic process. Based on current cognition of the division of active block and fault system in the Sichuan-Yunnan region, we build a two-dimensional finite-element contact model, which includes ten small blocks and the primary block boundary faults, such as East Kunlun Fault, Minjiang Fault, Huya Fault, Xianshuihe-Xiaojiang Fault and Red River Fault. Slip rate and stress distribution of the primary block boundary faults are obtained by using long-term GPS observation data from 1991 to 2015 and “block-loading” method. This loading method can reflect interaction between the block and the boundary. Compared with the direct loading of GPS results, it can avoid local distortion caused by the large single-point error. Comparing GPS observation results with simulation results, the residual error less than 1mm accounts for 66%, and the error less than 2mm accounts for 86%. The direction angle residual error less than 5° accounts for~56%, and that less than 10° accounts for 82%, which means that simulation results of this study are reasonable. In addition, by collecting the relevant information on seismic activity and focal mechanism solutions in the Sichuan-Yunnan region, and combining with the simulation results, we discuss the relationship between slip rate distribution, transfer and stress transformation in large left-lateral strike-slip fault zones, the tectonic mechanism with normal fault type, as well as the probable cause of the seismic discrepancy between the northern and southern segments of the Red River Fault. The main conclusions are as follows:
(1)As the strike of the left-lateral strike-slip East Kunlun and Xianshuihe-Xiaojiang fault zones turns sharply from NW to near north-south-direction, the strike-slip component is partially absorbed by the fault-bend parts and then converted into strain accumulation, resulting in high stress distribution in the fault-bend areas. Among them, the area from the easternmost end of East Kunlun Fault to Huya Fault absorbs a strike-slip rate of~0.15mm/a. The accumulative rates of compressional stress are 3 711.7Pa and 699.3Pa, respectively. And the area from southeastern end of Xianshuihe Fault to Anninghe and Daliangshan Faults absorbs a strike-slip rate of~1mm/a. The accumulative rates of compressional stress are 3 051.7 Pa and 2 844.6 Pa, respectively.
(2)Affected by the left-lateral shear of Xiaojiang Fault, the south-central segment of the Red River Fault is dominated by right-lateral strike-slip with weak compression. The right-lateral strike-slip rate is 1.20~2.68mm/a. The right-lateral strike-slip rate of north segment of Red River Fault is 0.71~1.54mm/a. This indicates that right-lateral strike-slip in the northern segment of Red River Fault is caused by traction of the south-central segment. The Red River Fault constitutes a right-lateral shear deformation zone arranged in right-step en echelon pattern with the Jinsha River Fault and Deqin-Zhongdian Fault. In the vicinity of Deqin-Zhongdian Fault, the Yulong snow mountain eastern piedmont fault, the southern segment of the Lijiang-Xiaojinhe Fault and the Ninglang-Yongsheng-Binchuan Fault, form a tectonic pull-apart zone. The normal focal mechanisms are predominantly distributed within this zone. This deformation pattern is not consistent with imbricated thrust conversion-limited extrusion model, which suggests that the current movement mode of Jinsha River and Lijiang-Xiaojinhe fault zones and their effect on regional deformation may have changed.
(3)The north segment of the Red River Fault appears to be slightly tensional, while the south segment is weakly compressional. According to Coulomb's criterion, the shear stress required for fault rupture in the northern section should be lower than that in the southern section. As a result, the north section is more likely to rupture and the seismic activity is significantly stronger than that of the south-central part.

Based on the broadband records of the digital seismic networks of Qinghai, the focal mechanisms of the Dingqing, Xizang earthquakes(M_S≥3.0) are of the obtained with Cut-and-Paste(CAP)inversion method and from USGS, seven of them are normal fault type with a little strike-slip component. The dominant direction of the fault strikes is near SN, the dominant distribution of dip angles is 58°~69°, and the dominant distribution of rake angle is -81°~-103°. The dominant direction of P axis is SWW, and that of T axis is SEE. The best double couple solution of the M_S5.5 earthquake in 2016 is 12°, 58° and -103° for strike, dip and rake angles, respectively, the second nodal plane solution is 216°, 34° and -70°, the centroid depth is 7.3km, and its moment magnitude is 5.3. For the M_S5.1 earthquake in 2020, the solution is 9°, 57°, -101° for strike, dip and rake angles, respectively, the second nodal plane solution is 209°, 35° and -74°, the centroid depth is 6.8km, and its moment magnitude is 4.9.
The double difference relative positioning method(HypoDD)is used to relocate the Dingqing earthquakes from February 1, 2015 to March 5, 2020. Broadband data of 9 seismic stations of Qinghai seismic network, Tibet seismic network and scientific array within about 400km around the epicenter are used, and the relocation of 217 earthquakes is obtained. After relocation, the Dingqing earthquake sequence is more clustered than before, with zonal distribution along NE-SW direction, which is in agreement with the fault strike of focal mechanism solutions, but not consistent with the major strike-slip faults in the region. The focal depths of the Dingqing earthquakes are close to the normal distribution, 75 percents of them range from 8 to 12km. The focal depths of earthquakes in 2015-2018 are confined in the range of 5~15km, and that in 2018—2020 are mainly from 7km to 12km, the range of focal depths is significantly reduced after 2018. After the occurrence of M_S5.5 earthquake in 2016, the earthquakes ruptured rapidly to the west and south, and most of the aftershocks were of magnitude 3 or below, and the sequence attenuation was fast, which may be because that the mainshock released most of the energy in the sequence. The aftershocks of the M_S5.1 earthquake in 2020 mostly ruptured along the horizontal direction or to the deep. The earthquakes occurring from 2019 to March 2020 are located in the middle of the sequence in spatial distribution, and there are two dominant directions of NE-SW and SSE in the spatial distribution of epicenters, showing an L-shape distribution. The reason may be that the earthquake encountered obstacles in the rupture along the NE-SW direction, the strain energy was not fully released, and then turned to the SSE faults after stress adjustment to induce subsequent aftershocks. In the NE direction of the “L-shape”, in addition to the M_S5.1 earthquake on January 25, 2020, there were also earthquakes with M_S5.5 on May 11, 2016 and M_S4.5 on October 12, 2017, while only a few earthquakes with M_S3.4 and below occurred in SSE direction, indicating that the NE-trending faults are the dominant area of Dingqing earthquakes activity in recent years.
Since the focal mechanism solutions of M_S5.5 earthquake in 2016 and M_S5.1 earthquake in 2020 are both of normal fault type, the dominant distribution direction of aftershocks is NE, according to the analysis of relocation, focal mechanism and geological structure background, it is inferred that the seismogenic structure of M_S5.5 earthquakes in 2016 and M_S5.1 in 2020 may be of a same normal fault type with NE direction. The fault plane may be nodal plane Ⅰ, i.e. the nodal plane with strike of 12°, dip angle of 58°, rate angle of -103° and strike of 9°, dip angle of 57° and rate angle of -101°. Because the Dingqing earthquakes occurred in the hinterland of the Qinghai-Tibet Plateau, the related research data on the distribution and attitude of small-scale faults is very scarce, so it is difficult to determine the seismogenic faults of the Dingqing earthquakes.

Tianshan is one of the longest and most active intracontinental orogenic belts in the world. Due to the collision between Indian and Eurasian plates since Cenozoic, the Tianshan has been suffering from intense compression, shortening and uplifting. With the continuous extension of deformation to the foreland direction, a series of active reverse fault fold belts have been formed. The Xihu anticline is the fourth row of active fold reverse fault zone on the leading edge of the north Tianshan foreland basin. For the north Tianshan Mountains, predecessors have carried out a lot of research on the activity of the second and third rows of the active fold-reverse faults, and achieved fruitful results. But there is no systematic study on the Quaternary activities of the Xihu anticline zone. How is the structural belt distributed in space?What are the geometric and kinematic characteristics?What are the fold types and growth mechanism?How does the deformation amount and characteristics of anticline change?In view of these problems, we chose Xihu anticline as the research object. Through the analysis of surface geology, topography and geomorphology and the interpretation of seismic reflection profile across the anticline, we studied the geometry, kinematic characteristics, fold type and growth mechanism of the structural belt, and calculated the shortening, uplift and interlayer strain of the anticline by area depth strain analysis.
In this paper, by interpreting the five seismic reflection profiles across the anticline belt, and combining the characteristics of surface geology and geomorphology, we studied the types, growth mechanism, geometry and kinematics characteristics, and deformation amount of the fold. The deformation length of Xihu anticline is more than 47km from west to east, in which the hidden length is more than 14km. The maximum deformation width of the exposed area is 8.5km. The Xihu anticline is characterized by small surface deformation, simple structural style and symmetrical occurrence. The interpretation of seismic reflection profile shows that the deep structural style of the anticline is relatively complex. In addition to the continuous development of a series of secondary faults in the interior of Xihu anticline, an anticline with small deformation amplitude(Xihubei anticline)is continuously developed in the north of Xihu anticline. The terrain high point of Xihu anticline is located about 12km west of Kuitun River. The deformation amplitude decreases rapidly to the east and decreases slowly to the west, which is consistent with the interpretation results of seismic reflection profile and the calculation results of shortening. The Xihu anticline is a detachment fold with the growth type of limb rotation. The deformation of Xihu anticline is calculated by area depth strain analysis method. The shortening of five seismic reflection sections A, B, C, D and E is(650±70) m, (1 070±70) m, (780±50) m, (200±40) m and(130±30) m, respectively. The shortening amount is the largest near the seismic reflection profile B of the anticline, and decreases gradually along the strike to the east and west ends of the anticline, with a more rapidly decrease to the east, which indicates that the topographic high point is also a structural high point. The excess area caused by the inflow of external material or outflow of internal matter is between -0.34km² to 0.56km². The average shortening of the Xihubei anticline is between(60±10) m and(130±40) m, and the excess area caused by the inflow of external material is between 0.50km² and 0.74km². The initial locations of the growth strata at the east part is about 1.9~2.0km underground, and the initial location of the growth strata at the west part is about 3.7km underground. We can see the strata overlying the Xihu anticline at 3.3km under ground, the strata above are basically not deformed, indicating that this section of the anticline is no longer active.

Coulomb stress change on active faults is critical for seismic hazard analysis and has been widely used at home and abroad. The Sichuan-Yunnan region is one of the most tectonically and seismically active regions in Mainland China, considering some highly-populated cities and the historical earthquake records in this region, stress evolution and seismic hazard on these active faults capture much attention.
    From the physical principal, the occurrence of earthquakes will not only cause stress drop and strain energy release on the seismogenic faults, but also transfer stress to the surrounding faults, hence alter the shear and normal stress on the surrounding faults that may delay, hasten or even trigger subsequent earthquakes. Previously, most studies focus on the coseismic Coulomb stress change according to the elastic dislocation model. However, the gradually plentiful observation data attest to the importance of postseismic viscoelastic relaxation effect during the analysis of seismic interactions, stress evolution along faults and the cumulative effect on the longer time scale of the surrounding fault zone. In this paper, in order to assess the seismic hazard in Sichuan-Yunnan region, based on the elastic dislocation theory and the stratified viscoelastic model, we employ the PSGRN/PSCMP program to calculate the cumulative Coulomb stress change on the main boundary faults and in inner blocks in this region, by combining the influence of coseismic dislocations of the M≥7.0 historical strong earthquakes since the Yongsheng M7.8 earthquake in 1515 in Sichuan-Yunnan region and M≥8.0 events in the neighboring area, and the postseismic viscoelastic relaxation effect of the lower crust and upper mantle.
    The results show that the Coulomb stress change increases significantly in the south section of the Xianshuihe Fault, the Anninghe Fault, the northern section of the Xiaojiang Fault, the southern section of the Longmen Shan Fault, the intersection of the Chuxiong-Jianshui Fault and the Xiaojiang Fault, and the Shawan section of the Litang Fault, in which the cumulative Coulomb stress change exceeds 0.1MPa. The assuming different friction coefficient has little effect on the stress change, as for the strike-slip dominated faults, the shear stress change is much larger than the normal stress change, and the shear stress change is the main factor controlling the Coulomb stress change on the fault plane. Meanwhile, we compare the Coulomb stress change in the 10km and 15km depths, and find that for most faults, the results are slightly different. Additionally, based on the existing focal mechanism solutions, we add the focal mechanism solutions of the 5 675 small-medium earthquakes(2.5≤M≤4.9)in Sichuan-Yunnan region from January 2009 to July 2019, and invert the directions of the three principal stresses and the stress shape factor in 0.1°×0.1° grid points; by combining the grid search method, we compare the inverted stress tensors with that from the actual seismic data, and further obtain the optimal stress tensors. Then, we project the stress tensors on the two inverted nodal planes separately, and select the maximum Coulomb stress change to represent the stress change at the node. The results show that the cumulative Coulomb stress change increase in the triple-junction of Sichuan-Yunnan-Tibet region is also significant, and the stress change exceeds 0.1MPa.
    Comprehensive analysis of the Coulomb stress change, seismic gaps and seismicity parameters suggest that more attention should be paid to the Anninghe Fault, the northern section of the Xiaojiang Fault, the south section of the Xianshuihe Fault, the southern section of the Longmen Shan Fault and the triple-junction of the Sichuan-Yunnan-Tibet region. These results provide a basis for future seismic hazard analysis in the Sichuan-Yunnan region.

Studies on the effect of near-surface overburden soil layers on seismic motion have shown that the overburden soil layers have a significant impact on the seismic effect of the site due to the formation age, genetic type, thickness difference, structure, and dynamic characteristics of the soil layers. In this paper, the one-dimensional seismic response analysis of a nuclear power plant site containing a thick hard interlayer was conducted to discuss the influence of the hard interlayer thickness on the site seismic response, so as to provide a basis for determining the seismic motion parameters for seismic design of similar sites. Based on the engineering geological data of a nuclear power plant site, five models of one-dimensional soil-layer seismic response analysis were built, and the equivalent linear method of the one-dimensional site seismic response was applied to analyze the effect of the interlayer thickness on the peak acceleration and the acceleration response spectra of the site seismic response. The seismic response characteristics of the site and influence rules of the hard interlayer thickness are summarized as follows:1)Under different input seismic motion levels, the peak acceleration at the top of the hard interlayer was less than the input peak acceleration, and the peak acceleration at the ground surface of site was greater than the input peak acceleration. 2)Under the same input seismic motion, the ratios of the peak accelerations at the top of hard interlayer to the input peak accelerations were smaller than the ratios of the peak accelerations at the ground surface to the input peak acceleration, and these ratios first decreased and then increased gradually with the increase of the hard interlayer thickness; while for the same hard interlayer thickness, these ratios gradually decreased as the input peak acceleration increasing. 3)For the same input seismic motion, the ratios of the peak accelerations at the ground surface of site to those at the top of the hard interlayer increased gradually as the hard interlayer thickness increased; however, corresponding to different hard interlayer thicknesses, the variation characteristics of ratios which are the peak accelerations at the ground surface of site to those at the top of the hard interlayer were inconsistent with the increase of the input peak acceleration. 4)The hard interlayer had a significant influence on the short-period acceleration response spectrum and the thicker the hard interlayer was, the wider the influence frequency band would be; while for a special hard interlayer thickness, the influence frequency band is certain, and the hard interlayer had little effect on the acceleration response spectrum coordinates outside this frequency band, the longer the period is, the less the influence of the hard interlayer on the acceleration response spectrum coordinates. The seismic response characteristics of the site and influence rules of the hard interlayer thickness indicate that the hard interlayer thickness has a significant impact on the peak acceleration and the acceleration response spectra of the site seismic response, and the hard interlayer has obvious isolation effect at the seismic motion, and the increase of its thickness reduces the nonlinear effect of the site and leads to the wider influence frequency band. Meanwhile, the higher the input peak acceleration is, the stronger the nonlinear effect of the site, and it's remarkable that the soft layer overlying the hard interlayer has a significant amplification effect on the seismic motion.

The NE-trending regional deep fault, i.e. the Jintan-Rugao Fault, is a boundary fault between the Subei depression and Nantong uplift, and its research has always received broad attention because of its importance and complexity. For the absence of definite proof, there is little consensus regarding the structure and spatial distribution of the fault among geoscientists, and its latest active time is ambiguous. The study of Quaternary activity characteristics of the Jintan-Rugao Fault is of great significance for earthquake trend prediction and engineering safety evaluation, and for earthquake prevention and disaster reduction in Jiangsu Province. In order to investigate the spatial location, characteristics and tectonic features and redefine the activity of the NE-segment of the Jintan-Rugao Fault, and on the basis of likely location and marker beds derived from petroleum seismic exploration sections, we collect and arrange 4 shallow seismic exploration profiles crossing the fault to conduct high-resolution seismic reflection imaging, following the working concept of ‘from known to unknown, from deep to shallow’. In this study, an observation system with trace intervals of 4~6m, shot intervals of 12~18m, and channels of 90~256 and 15~36 folds is used. In addition, by introducing different tonnage vibroseis to suppress the background noise, the raw data with high SNR(signal-noise ratio)can be obtained. By using the above working method and spread geometry, we obtained clear imaging results of the subsurface structure and fault structure in the coverage area of the survey lines. This exploration research accurately locates the NE-segment of Jintan-Rugao Fault, and further shows that it is not a single fault but a fault zone consisting of two normal faults with N-dipping and NE-striking within the effective detection depth. The shallow seismic profiles reveal that the up-breakpoint on the south branch with stronger activity is at depth of 235~243m, which offsets the lower strata of lower Pleistocene. Combining drilling data around the survey lines, we infer the activity time of this fault is early Pleistocene. The results of this paper provide reliable seismological data for determining the location and activity evaluation of the NE-segment of Jintan-Rugao Fault. In eastern China, where the sedimentary layer is thicker, the latest active age of faults can not be determined entirely according to the latest faulted strata. For a fault passing through the thicker area of new deposits, its latest active age should be based on the tectonic background, seismic activity, present tectonic stress field, topographic deformation, structural micro-geomorphological characteristics, sedimentary thickness of new strata, controlling effect of faults on new strata and the latest strata of faults, and combined with upper breakpoints, morphology, structure and occurrence of faults, the active state of the target concealed faults should be analyzed. If the activity of the fault is judged only by the upper faulted point, it may lead to overestimating the age of the fault activity.

The M_W6.6 Arketao earthquake occurred on November 25, 2016 in Muji Basin of the Kongur extensional system in the eastern Pamir. The region is the Pamir tectonic knot, one of the two structural knots where the India plate collides with the Eurasian plate. This region is one of the most active areas in mainland China. The seismogenic structure of the earthquake is preliminarily determined as the Muji dextral-slip fault which locates in the north of Kongur extensional system. Based on field surveys of seismic geological hazard, and combined with the characteristics of high altitude area and the focal mechanism solution, this paper summarizes the associated distribution and development characteristics of sandy soil liquefaction, ground fissures, collapse, and landslide. There are 2 macroscopic epicenters of the earthquake, that is, Weirima village and Bulake village. There are a lot of geological hazards distributed in the macroscopic epicenters. Sand liquefaction is mainly distributed in the south of Kalaarte River, and area of sand liquefaction is 1 000m². The liquefaction material gushed along the mouth of springs and ground fissures, because of the frozen soil below the surface. More than 60% of soil liquefactions are formed in the mouth of springs. According to the trenching, these liquefactions occurred in 1.8 meters underground in the gray green silty clay and silty sand layers. The ground fissures are mainly caused by brittle failure, and the deformation of upper frozen soil layer is caused by the deformation of lower soil layer. The ground fissures at Weirima village are distributed in a chessboard-like pattern in the flood plain of Kalaarte River. In the Bulake village, the main movement features of the ground fissure are tension and sinistral slip, and the directions of ground fissures are 90°~135°. The collapse and landslide are one of the important geological disasters in the disaster area. The rolling stones falling in landslide blocked the roads and smashed the wire rods, and the biggest rolling stone is 4 meters in length. We only found a small landslide in the earthquake area, but there are a large number of unstable slopes and potential landslides in the surroundings. The ground fissures associated with sand liquefaction are an important cause of serious damage to the buildings.

The historical document record is of vital significance to determine the volcanic eruption history age in the volcanology research and it cannot be replaced by ¹⁴C dating and other methods. The volcanoes are widely distributed in the northeast area of China, but there is lack of relevant historical records. However, there are the records of the volcanic eruption in the historical documents of Goryeo Dynasty(AD918-1392)and Joseon Dynasty(AD1391-1910)in the Korean Peninsula which is separated by a river with China only. Some of the records have been widely used as important information to the research of Changbaishan Tianchi volcano eruption history by researchers both at home and abroad, but they have different opinions. On the basis of the historical documents in the Korean Peninsula, that is, the History of Goryeo Dynasty and the Annals of the Joseon Dynasty so on, the phenomena of volcanic eruptions, including the intuitive eruptive events and the doubtful volcanic eruption phenomenon such as "the ash fall", "the white hair fall", "the sky fire", "the dust fall" are investigated and put in order systematically in this paper. The results are as follows:1)The intuitive eruptive events are the 1002AD eruption of Mt. Halla volcano on Jeju Island, Korea Peninsula, and the 1007AD volcanic eruption offshore to the west of Jeju Island, Korea Peninsula, as well as the 1597AD eruption of Mt. Wangtian'e volcano in Changbai County, Jilin Province, China; 2)"The ash fall" is airborne volcanic ash, and those "ash falls" happening in 1265, 1401-1405, 1668, 1673 and 1702AD are possibly the tephra of Changbaishan Tianchi volcano; 3)"The white hair fall" is Pele's hair and it is speculated that the "white hair fall "happening in 1737AD is related to Changbaishan Tianchi volcanic eruption; 4)If regarding "the sky fire" as the volcanic eruption phenomenon, "the sky fire" happening in 1533AD is possibly the Changbaishan volcanic eruption event, and "the sky fire" in 1601-1609AD may be the eruptive event of the Longgang volcano in Jilin Province, China or Changbaishan Tianchi volcano; 5)"The dust fall" is recorded in many historical documents. However, "the dust fall" is not the volcanic ash fall but the phenomenon of loess fall. So, it is improper to determine the eruptive events of Changbaishan Tianchi volcano on the basis of "the dust fall".

As the northeast boundary of the Tibetan plateau, the Haiyuan-Liupan Shan fault zone has separated the intensely tectonic deformed Tibetan plateau from the stable blocks of Ordos and Alxa since Cenozoic era. It is an active fault with high seismic risk in the west of mainland China. Using geology and geodetic techniques, previous studies have obtained the long-term slip rate across the Haiyuan-Liupan Shan fault zone. However, the detailed locking result and slip rate deficit across this fault zone are scarce. After the 2008 Wenchuan M_S8.0 earthquake, the tectonic stress field of Longmen Shan Fault and its vicinity was changed, which suggests that the crustal movement and potential seismic risk of Haiyuan-Liupan Shan fault zone should be investigated necessarily.
Utilizing GPS horizontal velocities observed before and after Wenchuan earthquake(1999~2007 and 2009~2014), the spatial and temporal distributions of locking and slip rate deficit across the Haiyuan-Liupan Shan fault zone are inferred. In our model, we assume that the crustal deformation is caused by block rotation, horizontal strain rate within block and locking on block-bounding faults. The inversion results suggest that the Haiyuan fault zone has a left-lateral strike-slip rate deficit, the northern section of Liupan Shan has a thrust dip-slip rate deficit, while the southern section has a normal dip-slip rate deficit. The locking depths of Maomao Shan and west section of Laohu Shan are 25km during two periods, and the maximum left-lateral slip rate deficit is 6mm/a. The locking depths of east section of Laohu Shan and Haiyuan segment are shallow, and creep slip dominates them presently, which indicates that these sections are in the postseismic relaxation process of the 1920 Haiyuan earthquake. The Liupan Shan Fault has a locking depth of 35km with a maximum dip-slip rate deficit of 2mm/a. After the Wenchuan earthquake, the high slip rate deficit across Liupan Shan Fault migrated from its middle to northern section, and the range decreased, while its southern section had a normal-slip rate deficit.
Our results show that the Maomao Shan Fault and west section of Laohu Shan Fault could accumulate strain rapidly and these sections are within the Tianzhu seismic gap. Although the Liupan Shan Fault accumulates strain slowly, a long time has been passed since last large earthquake, and it has accumulated high strain energy possibly. Therefore, the potential seismic risks of these segments are significantly high compared to other segments along the Haiyuan-Liupan Shan fault zone.

The M_W6.6 Arketao earthquake,which occurred at 14:24:30 UTC 25 November 2016 was the largest earthquake to strike the sparsely inhabited Muji Basin of the Kongur extension system in the eastern Pamir since the M 7 1895 Tashkurgan earthquake.The preliminary field work,sentinel-1A radar interferometry,and relocated hypocenters of earthquake sequences show that the earthquake consists of at least two sub-events and ruptured at least 77km long of the active Muji dextral-slip fault,and the rupture from this right-lateral earthquake propagated mostly unilaterally to the east and up-dip.Tectonic surface rupture with dextral slip of up to 20cm was observed on two tens-meter long segments near the CENC epicenter and 32.6km to the east along the Muji Fault,the later was along a previously existing strand of the Holocene Muji fault scarps.Focal mechanisms are consistent with right-lateral motion along a plane striking 107°,dipping 76° to the south,with a rake of 174°.This plane is compatible with the observed tectonic surface rupture.More than 388 aftershocks were detected and located using a double-difference technique.The mainshock is relocated at the Muji Fault with a depth of 9.3km.The relocated hypocenters of the 2016 Arketao earthquake sequence showed a more than 85km long,less than 8km wide,and 5~13km deep,NWW trending streak of seismicity to the south of the Muji Fault.The focal mechanism and mapping of the surface rupture helped to document the south-dipping fault plane of the mainshock.The listric Muji Fault is outlined by the well-resolved south-dipping streak of seismicity.The 2016 Arketao M_W6.6 and 2015 Murghob M_W7.2 earthquakes highlight the importance role of strike-slip faulting in accommodating both east-west extensional and north-south compressional forces in the Pamir interior,and demonstrate that the present-day stress and deformation patterns in the northern Pamir plateau are dominant by east-west extension in the shallow upper crust.

The Tibetan plateau is bounded by Altyn Tagh Fault in its northern edge, this well known for the characteristic of left-lateral strike-slip from late Quaternary, but its magnitude of left-lateral slip rate measured by geological way, either on a high level (20~30mm/a) or on a lower level (~9mm/a), is hotly debated, and that is central to reviewing the existent deformation mechanism of Tibet plateau. The present-day fault slip rate along Altyn Tagh Fault has revealed by Global Positioning System (GPS), however the limited GPS stations and its poor distribution might increase the uncertainties of the predicted fault slip rates, especially to this so mega fault with 1500km length approximately.
A dense GPS velocity field (from 2009 to 2013) has obtained along Altyn Tagh Fault and its vicinity, which provides us a good opportunity to study its slip rates along its different fault segments in detail. In this paper, we use the spherical linear elastic block theory constrained by new geodetic observations from GPS stations we have mentioned, to estimate fault slip rates along the Altyn Tagh Fault and other major faults in its vicinity. Our 3D geometric block model is based on the previous researches of active block. Then the optimal locking depths of Altyn Tough Fault are fixed by trail tactics, the result of optimal locking depths shows that it is from 10km to 15km in the southern part of Altyn Tagh Fault, in its middle and north segment the locking depths are deeper than its southern part, and in its north terminal the locking depth is 11km. Then the fault slip rates along Altyn Tough Fault are obtained as well as other fault slip rates of major faults in its vicinity. The left-lateral fault slip rates of different segments along Altyn Tough Fault are (7.8±0.2)mm/a (south of Qaidam Basin), (7.5±0.1)mm/a (south of Subei), (5.3~5.5)mm/a (from Subei to Changma) and(1.0±0.4)mm/a (north of Changma), which trend to decreasing from south to north along the fault strike, and the decreasing of the slip rate is mainly confined within the Qilian Range, and converted to the crustal contraction in this area.

Xiangjiaba Reservoir is currently China's third largest reservoir and began impounding water at the end of 2012. After the impoundment, the water level rose to 71m, while seismic activity near the dam was not significantly increased. At the end of June 2013, the reservoir began impounding water again, the water level continued to rise and flooded the tail region of the reservoir. In the reservoir area and the Xiluodu reservoir area in the upstream, a reservoir seismic network including 35 seismic stations was set up which can roundly record earthquakes in this area. According to the records of the reservoir seismic network from September 2007 to June 2013, only 38 earthquakes with M_L≥1.0 occurred, 0.66 times a month on average, while in July-September 2013, 186 earthquakes with M_L≥1.0 occurred, with an average of 62 events a month, nearly 100 times of that in the past. So, most of the earthquakes are induced earthquakes. At the same time 553 earthquakes with M_L≤1.0 were also recorded in this area. A large number of small earthquakes occurring in the strong earthquake background area have caused a big stir. The source location of these earthquakes are rechecked based on 3D velocity model, 94% of the rechecked focal depth is less than 5km. Based on observations of the reservoir seismic network and vertical P- and S-wave's maximum amplitude ratio method, we inversed 9 focal mechanisms before the impoundment and 69 focal mechanisms after the impoundment in the tail region of the reservoir. Using these focal mechanisms, the stress field of the northern part and southern part of the study area is calculated in order to analyze the characteristic and cause of the induced earthquakes. The results indicate that most of the 69 focal mechanisms are strike-slip type, there is more transitional type, and less normal type and thrust type. The focal mechanisms spatial orientation is complex, fracture types are diverse, which may indicate that the stress state is uneven and the control of regional stress field to small earthquakes is weak. The stress field in the south and north is quite different and not consistent with regional stress field. The north shows compressive stress state while the south shows a state of weak extension. The Yaziba Fault, which passes through the tail region of reservoir, is an active fault, but does not control the induced seismicity, which may indicate that the reservoir storage inhibits the reverse fault activity. Carbonate rocks, limestone and karst cave are developed in the tail region. Analysis believes that reservoir water flows into the caves, penetrates into cracks and joints, leading to increased pore pressure, reducing the frictional strength and fracture strength and increasing reservoir water load which cause elastic deformation, so, it is believed that the combined action of all the above factors is the cause for the induced earthquakes.

There are carbonate rock, limestone and caves in the reservoir head area of Xiluodu Reservoir, which is the third largest reservoir in the world. After the impoundment, the water level has risen to about 140 meters, and consequently, more than 6 000 micro-earthquakes occurred on the reservoir head region, with magnitude of the vast majority being less than 1 and the maximum magnitude M_L3. These micro-earthquakes concentrated within an area of 10km in width from the reservoir banks, 5km in depth, and 40km in length along the reservoir basin. These earthquakes did not affect the safety of the reservoir and dam. We inverted 700 focal mechanisms by using the waveforms recorded by the reservoir's digital seismic network before and after the impoundment, and further inverted the stress field of the whole reservoir head region and the sub-regions. The results show a complex orientation of focal mechanism, different rupture types, and uneven and unstable stress state, which is not in consistency with other regional stress fields obtained by a lot of natural earthquakes, indicating the reservoir induced seismicity is not strictly controlled by the regional stress field. According to the analysis, the reservoir water flows into caves, penetrating into cracks and joints, leading to increase of pore pressure, reducing the friction and fracture strength of rocks, and generating elastic deformation caused by the increased load of reservoir water. The joint actions of these may be the cause of the earthquakes. The accumulated regional stress and local stress were released first, then, the additional stress produced by the reservoir water loading was dominating. There are no major active faults in the reservoir head area. Reservoir water level will rise again by tens of meters in 2014. With the penetration of cracks, the adjustment of stress field, and the backflow of water which will inundate the upstream region of the reservoir basin, the possibility of occurrence of moderate earthquakes cannot be ruled out. The seismic fortification criteria are high for the dam of Xiluodu Reservoir, so these earthquakes will not cause safety problems. We suggest carrying out detailed hydro-geological, geophysical explorations during the continuous active period of the reservoir-induced seismicity to obtain accurate scientific data for determining the causes of induced seismicity and searching for the technical approaches for controlling the induced seismicity. These measurements will mitigate the impact of emergencies and play an exemplary role for the other similar reservoirs.

The Wangtian'e volcano is situated in the middle part of the Changbai Korean Autonomous County in Jilin Province,China,whose summit elevation is 2051.4m and 35km to the Changbaishan Tianchi(Mt. Baekdu Sky Lake)volcano. There are the historical records concerning the earthquake and volcano eruption occurring on October 6,1597 in the Korea historical document 'the Annuals of the Choson Dynasty’.The paper investigates the historical materials based on the old map of Korea and the local chronicles of the Samsu County of Hamgyong Province in Choson Dynasty,and suggests that the 1597 volcano eruption occurred in a chain of mountains at the bottom of the Wangtian'e basaltic lava platform located between the Shisandaogou Village and the Shisidaogou Town in the Changbai Korean Autonomous County,Jilin Province,China. The geographic position of the 6 October 1597 Wangtian'e volcano eruption is about 30km to the summit of Mt. Wangtian'e and about 60km to the Mt. Chanbai(Mt. Baedu)Tianchi Lake.

In our country,the water level and discharge flow rate are observed simultaneously when observing the dynamic water level in the seismic ground fluid observation network.Thus,two types of dynamic variations are obtained generally,and they are handled and analyzed separately.Two contrary results are obtained sometimes in the information of aquifer stress state change.Facing this irrational status,the relationship of water level to flux in dynamic water level observation well was explored based on the fluid dynamics theory.The equivalent static water level was put forward,synthesizing the two observation terms into one unified physical quantity.The calculation means to synthesize the water level and flux into one quantity was proposed and tested using the observation data of wells Dongshui-3 and Liaogu-1.

In this paper,hydrogeological condition of Liaogu-1 well was introduced; the present condition of extraction of geothermal resources around Liaocheng seism-hydrochemistry station was researched comprehensively. The effect of the extraction of geothermal resources on underground fluid observation and behaviors were analyzed basing on the research. It is believed that the extraction of geothermal resources in the well area resulted in the dry-up of Liaogu-1 well and the emergence of abnormal behaviors in various measurements. The extraction of karst water in carbonate rock around Liaogu-1 well affected the underground fluid observation and behaviors,the disturbance became more serious especially in winter when groundwater was mined for heating. In this paper,for the sake of removing or reducing the effect,the artificial self-flow was carried out by transformation of wellhead equipment. As a result,obvervation items were insured,obvervation condition of part of items was improved,the dynamic data of many obvervation items was continuable,only few items were affected seriously,and actual effect was obtained. But the essential way out is to protect the observation system and stop extraction of geothermal resources in same stratum.

From the teleseismic P-waveform data recorded at the dense mega seismic array deployed in the western Sichuan area by the State Key Laboratory of Earthquake Dynamics,Institute of Geology,China Earthquake Administration,we investigate the crustal anisotropy beneath the stations using waveform correlation method and weighted stacking method. As a preliminary result,we measured the fast polarization azimuth and time delay of the Ps converted wave in the receiver functions at 4 stations on both sides of Longmen Shan Faults. Our results show:1)The waveform correlation method is better than the weighted stacking method and it turns out not only the fast polarization azimuth,but also the time differences between the fast-and slow-wave; The results obtained by using the weighted stacking method are something undetermined due to that the symmetric axis of the crustal anisotropy medium is unclear previously; Application of both methods will be in favor of judging the reliability of the results. 2)The fast polarization azimuths are consistent each other at the stations in Sichuan Basin,suggesting the crust beneath Sichuan Basin has well integrality and a weak lateral deformation. 3)Taking the epicenter of the Wenchuan earthquake as a boundary,the fast polarization azimuth is parallel with the Longmen Shan Faults on the north side of the Sonpan-Ganzi block,and perpendicular to the faults on the south side. This suggests that under the obstruction of the Sichuan Basin,the soft lower crust beneath the north side of the Sonpan-Ganzi block has a NE direction extended deformation along the Longmen Shan Faults,and the crust on its south side is in the status of compressive deformation perpendicular to the faults. Our results can be used for interpreting the single-side rupture of the Wenchuan earthquake and the aftershock evolution.

In the western Sichuan(100°～105°E,26°～32°N),a mobile array consisting of 297 broadband seismic stations has been deployed by the State Key Laboratory of Earthquake Dynamics,Institute of Geology,China Earthquake Administration since October of 2006.Until June of 2008,a total of 690 teleseismic events(m_b>5.0,30°≤Δ≤90°)have been recorded.The May 12 Wenchuan earthquake(M_S8.0)provides an opportunity to test the western Sichuan array.The preliminary data analysis of the May 12 Wenchuan earthquake and its larger aftershocks has been carried out in this study.Our results show: 1)The event parameters of the May 12 Wenchuan earthquake sequence need to be modified and their location error reaches to 8～24km.A more reasonable estimation of the location of the main shock is possibly at the depth of 19km.2)the wavefield analysis of the Lixian earthquake(M_S5.9)of May 16,2008 manifests that the surface waves of this event are not fully developed,and thus its source depth should not be very shallow.The peak values of the ground-motion velocity on the vertical and horizontal component have an abnormal increase by 4 times and more of the normal attenuation,which is related closely to the faults within the range of 200～250km,when the topography and site effects are not considered.3)The preliminary analysis of the crustal and upper mantle structure beneath the Sichuan basin and the Songpan-Ganzi block manifests that the crust beneath the Sichuan basin thickens along the western direction and its lower crust displays the hard structure.The crustal thickness in the northeast of Chengdu City reaches 46km.The crustal structure beneath the Songpan-Ganzi block has complex lateral variations.The crustal thickness in the Wenchuan earthquake source region reaches 52km.In the depth range of 14～20km,its crust has a complex high-velocity structure with the averaged velocity larger than 4.0km/s.The Wenchuan earthquake should be located within the area with high-velocity medium.In the lower middle crust,a low-velocity layer exists with the S-wave velocity of～3.6km/s,which could provide a relaxed boundary condition for the upper crust movement-deformation.This observation is consistent with the abnormal attenuation of ground motion with the epicenter distance obtained from the wavefield measurements.

The potential hazards associated with earthquakes are liquefaction, slope instability, foundation subsidence, and ground collapse. For the nuclear power plant siting, the investigations in the preliminary and the detailed stages should be carried out in the connection with the composite evaluation of seismic intensity and engineering geological condition for the site area.

The concepts of capable fault and surface faulting are studied in this paper, and an emphasis is given on the extent of investigation and requirements for the investigation in concrete seismological geologic siting for nuclear power plants.

The features of crustal deformation are discussed in the region of Three Gorges in this paper. It is shown that differential motion of Huangling fault block with respect to its peripheral area exists, with a maximum rate of 5-10 mm in year. The results of short leveling across faults indicate that the activity of the faults is characterstic of inheritance, with an order of magnitude mm/a. The horizontal deformation nets show that the Xiannu Mountain fault zone has undergone left-lateral compression-shear movement, and the Tianyangping fault zone right-lateral slip in the recent years.

The scarps on the Mackay segment are divided into three groups based on the different amount of displacement and their topographic location. The three groups of scarps suggest three paleoearthquakes. The paleoearthquakes on the Mackay segment had magnitudes of about 7 based on comparing the amount of the displacement and length of scarps. Using the diffusion equation to model scarps on the Mackay segment, the age estimetes suggest that the recurrence interval of magnitud 7 earthquakes is about 5-11 Ka.