Co-seismic near-surface rupture is one of the keys to the recognition of earthquake fault and the defense to seismic hazard. However, conventional investigation methods such as outcrop mapping and trenching, are often disturbed by the variation of capping formation. Besides, it’s difficult to apply these methods under the sea water. Drawing on the idea of time-lapse seismic techniques in the petroleum industry, we suggest identifying the buried co-seismic ruptures using two reflection seismic data sets acquired before and after earthquake, respectively. In this paper, a case study is presented.
On Nov. 26^th, 2018, a M_S6.2 earthquake occurred in the south Taiwan Straits. The focal mechanism of this event is dextral strike slip with a slight dip-slip component, and the aftershock distribution is E-W oriented. West of the epicenter, multi-channel seismic profiling was carried out twice under the direction of Fujian Earthquake Agency in 2017 and 2019. To avoid the influence of the difference in acquisition conditions, before comparing we reprocessed and cross-equalized the two data sets with the same de-noise method, illumination, migration algorithm and velocity field. The profile correlation in 20~50Hz shows that the dominant reflecting wave groups are coincident with the time-lapse ones, which means the two sections are in phase.
The comparison results show that: at about 25km west of the epicenter, the reflection profile met the earthquake fault inferred by focal mechanism, and the morphology of Fault F₁ did not change significantly after the earthquake, but at depths greater than 400m, the remarkable reflections near the strike-slip fault plane changed significantly. From 2017 to 2019, the strongest reflection in the hanging wall reduced in amplitude and shifted from near horizontal to a jagged fold in shape, besides, the polarity of two remarkable reflections reversed, and a piece of the basement reflection in the heading wall close to the fault plane subsided about 8milliseconds measured in two-way travel time. The remarkable reflections on the rest of the profile were aligned accurately as the control. These phenomena can be interpreted as a fluid migration through sandstone fractures model perfectly, which is also consistent with the petrological features in the study area. Since the gap between the two acquisition dates is only 26 months and there was no human activity affecting subsurface structure in the vicinity, the pore fluid migration is inferred to be related to the 2018 event.
This study demonstrates that although the vertical co-seismic displacement is smaller than the resolution of reflection seismic profiles in most cases, the near-surface fluid migration which accompanied the co-seismic rupture may cause significant impedance changes near the fault plane, and such changes can be reliably identified on time-lapse seismic profiles. Compared with the conventional investigation methods for co-seismic ruptures, the time-lapse seismic method can overcome the displacement absorption of capping formation and expand the identifiable scope of co-seismic ruptures. This method is practicable, especially for marine earthquake researches, because the acquisition repeatability and surface consistence of the marine reflection seismic data are relatively better than that of the land reflection seismic data. This study provides a new idea for recognizing the earthquake fault and slip distribution of shallow source earthquakes, which is especially important for the study of marine earthquakes with fewer geology and geodesy data available.

Weihe Basin, which is wide in the east and narrow in the west, deep in the south and shallow in the north, is one of the typical Cenozoic grabens in Asia continent, connecting the Ordos block in the north, Qinling fold belt in the south, adjacent to the arcuate fault belt in the northeast margin of Tibet Plateau in the west and the Shanxi rift zone in the east. The Weihe Basin has experienced strong faulting and sedimentation since early Cenozoic, with many buried active faults developed. The nearly E-W-trending Taochuan-Huxian Fault is one of these faults. The middle-deep depth seismic profiling shows that the buried segment of Taochuan-Huxian Fault in Weihe Basin is located between the Qinling north margin fault and the Weihe Fault and it is a fundamental fault that cuts through the Palaeozoic stratum and divides the Xi'an depression into two parts. To explore and know the location and structural characteristics of the Taochuan-Huxian fault segment hidden in the Weihe Basin and its activity in the Late Quaternary is of important significance for the researches of seismo-tectonic structure and seismic hazard of strong earthquakes in the study region. For this purpose, we deployed 7 profiles for shallow seismic reflection surveys, relied on the “Xingping Active Fault Project”. Based on these surveys, we determined the existence and hidden positions of the Taochuan-Huxian Fault and its branches in the Weihe Basin by combining with the data from some existing seismic reflection profiles of shallow-depths and middle-deep depths. Our research suggests that the Taochuan-Huxian Fault(F₈)is connected to the southern margin fault of the Taibai Basin in the west, and eastward, passes through the northern margin of the Qinling Mountains and enters into the Weihe Basin at the town of Tangyu, Zhouzhi County, and then is concealed under the loose sediment in the Weihe Basin. The strike direction of this fault is northeast when crossing obliquely through the town of Zhouzhi County, then gradually turns to a nearly east-west direction between Zhouzhi and Huxian, showing a northward convex bend in the fault trace buried in the basin. Further eastward, the Taochuan-Huxian Fault(F₈)connects to the Tieluzi Fault near the town of Yinzhen, Huxian County. In addition, a buried antithetic fault(DF₃)(also a secondary branch)of the buried Taochuan-Huxian Fault(F₈)is found between the north of Zhouzhi and the north of Huxian, and it extends roughly parallel to F8 under the loose sediment. This research also reveals that in the central portion of the Weihe Basin, the northern margin fault of the Qinling Mountains, the Weihe Fault and the Taochuan-Huxian Fault, together with their branch faults, constitute a large-scale fault zone with the tectonic feature of negative flower structure, as known from the interpreted cross-sections; among them, the F₈ and DF₃ faults and their secondary strands consist of a relatively small-scale negative flower structure. By combining with relevant information such as that from a composed cross-section using geological logs of multiple boreholes, and so on, we concluded that, within the study region of this research, the fault zone with the buried F₈ fault as its principal fault was active at least in the late Pleistocene, and hence is an active fault zone. Finally, the reason is discussed in this article for the faults, mentioned above, in the Weihe Basin that show the tectonic pattern of negative flower structure, instead of that of stair-stepping or ladder structure, and one possible interpretation is proposed that the dominant motion of these active faults are not normal faulting, but sinistral strike-slip faulting. Since the Cenozoic, the subduction of the Indian plate to the Eurasian plate caused the Tibet Plateau to be pushed out to the northeast and blocked by the Ordos block. Because of obstruction in the north, the material flows eastward along Qinling Mountains in the south, resulting in the extrusion shearing effect on the Weihe Basin in the middle. In addition, recent seismic and geological studies have discovered that many active faults in Weihe Basin and its edges are obviously of sinistral strike-slip, which also proves that the movement of these active faults in the basin is not dominated by normal faulting, but sinistral strike-slipping.

The Yangbi M_S6.4 earthquake in Yunnan Province and Maduo M_S7.4 earthquake in Qinghai Province occurred in western China on May 21 and May 22, 2021, respectively, which caused huge loss of life and property. Gravity changes in the epicentral area and its surroundings before the two earthquakes can provide important reference for studying the seismogenic environment and background. The ground gravity observation is relatively sparse in western China and satellite gravity can supplement this deficiency. GRACE(Gravity Recovery and Climate Experiment)(from March 2002 to June 2017)and GRACE-FO(GRACE Follow-on)(from May 2018 to March 2021)can produce the wide-area space, quasi-real-time, long-term and near-continuous observation data, which will provide large-scale background information for the ground gravity research.

In this paper, the epicenters of the two earthquakes and their surrounding area were taken as the study area(18°~45°N, 83°~115°E). We used GRACE, GRACE-FO and GLDAS(Global Land Data Assimilation System)data to calculate long-term gravity spatial-temporal distribution in the study area with 300km fan filter. We presented the gravity rate, cumulative gravity changes, differential gravity changes in the study area for about 20 years, and the gravity time series of Maduo earthquake and Yangbi earthquake. We simulated the theoretical co-seismic gravity variation of Maduo earthquake and evaluated the possibility of detecting the co-seismic gravity signal for GRACE-FO. The research results showed that:

(1)Long-term gravity changes in the study area were mainly characterized by positive-negative-positive-negative spatial layout in four quadrants. Gravity increased in Qinghai-Tibet block and South China block, and gravity decreased in Indian block and North China block. However, the North-South seismic belt and Bayankala block were located at the low-value areas in four quadrants and their gravity changes were relatively small. This was the large-scale gravity seismogenic background in western China.

(2)The epicenters of Maduo earthquake and Yangbi earthquake were both located in the center of the four quadrants and also at the corner of the high gradient zone of satellite gravity change. And their gravity changes were very small in the last 20 years, which was consistent with the basic characteristics of the ground gravity location prediction. After a year of continuous increase in the last two years before the Maduo earthquake, the gravity in Maduo area experienced a four-month period of decrease, then it increased again. This was similar to the process of gravity change before the Tangshan earthquake.

(3)M_S≥7.0 Earthquakes in the study area since 2002, such as Wenchuan M_S8.0 earthquake, Yushu M_S7.1 earthquake, Lushan M_S7.0 earthquake, Jiuzhaigou M_S7.0 earthquake, Maduo M_S7.4 earthquake and Nepal M_S8.1 earthquake, basically occurred in the central area of the four quadrants or at the corner of the tectonic-related high gradient zone, which was consistent with the earthquake case results of earthquake prediction based on the ground gravity observations. This study provided more earthquake cases for ground gravity prediction.

(4)Based on dislocation theory simulation, the magnitude of co-seismic gravity change of Maduo earthquake in Qinghai Province reached -40~151μGal. It is difficult for GRACE-FO to detect the co-seismic gravity change of Maduo earthquake with the current accuracy. And it could be possible only when the time-variable gravity accuracy of gravity satellite was improved by 1-2 orders of magnitude. This research provided earthquake case supports for the demand demonstration of gravity satellites in China in the future.

In this study, the temporal and spatial evolution of gravity in the western region of China and its surrounding areas from March 2002 to March 2021, which coverd the epicenters of Yangbi and Maduo earthquakes, was obtained by using the satellite gravity and global hydrological data after considering the influence of periodic signals. The theoretical coseismic effects of the Maduo earthquake on the local gravity field were analyzed and the accuracy of gravity satellite to detect this seismic signal was evaluated. This study provided the important background information of large-scale gravity field for the study of Maduo earthquake in Qinghai Province and Yangbi earthquake in Yunnan Province, and also provided valuable material for the seismic demand analysis of gravity satellite in China.

Based on the absolute gravity observation data of 10 gravity datum stations of the Crustal Movement Observation Network of China(CMONOC)in Yunnan and adjacent areas during 2010-2020, the gravity datum and its dynamic variation of each gravity datum station are obtained. The gravity variation trend of 9 stations at three different time scales and time periods shows that the gravity variation increased first and then decreased, and the turning point occurred around 2014, while Kunming Station is just the opposite. The Ludian M_S6.5, Yingjiang M_S6.1 and Jinggu M_S6.6 events occurred successively in 2014 when the gravity change increased to the turning point. Thereafter, the gravity change trend decreased until the occurrence of the Yangbi M_S6.4 earthquake in 2021. The results show that the variation of the gravity field in Yunnan and adjacent area is large and fast, and the transition period of increasing to decreasing is short and the variation trend is consistent. The gravity field change mechanism may be dominated by that the Qinghai-Tibetan plateau moves to the northeast caused by the push of Indian Plate, then is blocked by the Sichuan Basin, and the crustal material under the Qinghai-Tibetan plateau moves forward to Yunnan and its adjacent regions. The repeated observations of the absolute gravity survey network covering the whole country provide abundant and reliable data for obtaining the time-variable gravity field which is related to the crustal movement. Many scholars have done a lot of research on the results of the absolute gravity dynamic changes in the Chinese mainland, but the region is mainly focused on the Qinghai-Tibetan plateau, while the results of the absolute gravity dynamic change in other regions are still rarely revealed.

(1)The absolute gravity observation data of 10 gravity datum stations in Yunnan and Panzhihua from 2010 to 2020 show that except Kunming observation station, the gravity change trend increased first and then decreased, and the turning point occurred around 2014. Kunming station is the only station in the cave among the 10 observation stations, and its gravity change may be affected by the water content of the mountain. Five gravity observation campaigns at each observation station were carried out in different months of different years. Due to the lack of hydrological data at each observation station, the seasonal gravity change was not considered. Therefore, eliminating its influence is one of the important jobs in our future study.

(2)Earthquakes of M_S6.1, M_S6.5 and Ms6.6 occurred on May 31, August 3 and October 7, 2014 in Yingjiang, Ludian and Jinggu, Yunnan Province, respectively. The epicentral distances of all the gravity datum stations to the three events are more than 100km, so the coseismic gravity changes caused by events can be ignored.

(3)The Ludian M_S6.5, Yingjiang M_S6.1 and Jinggu M_S6.6 earthquakes occurred successively during the turning point period, and the gravity changes show a decreasing trend until the occurrence of the Yangbi M_S6.4 event in 2021. The mechanism of gravity field change of Yunnan and the adjacent areas may be as follows: The Qinghai-Tibetan plateau is pushed to the northeast by the Indian plate, then blocked by the Sichuan Basin, and the crustal material under the Qinghai-Tibetan plateau moves forward to Yunnan and its adjacent regions.

(4)Yunnan and the adjacent areas are characterized by complex tectonic environment and rapid seismic energy accumulation and release, which puts forward new demands for absolute gravity measurement mode, adding absolute gravity measurement stations and shortening observation period, so as to enhance the ability of more absolute gravity measurement to serve for monitoring the seismic activity and regional geological tectonic activity in Chinese mainland.

InSAR coseismic deformation fields caused by the Maduo M_W7.3 earthquake occurring on May 22, 2021 were generated using the C-band Sentinel-1A/B SAR images with D-InSAR technology. The spatial characteristics, magnitude of coseismic deformation and segmentation of the seismogenic fault were analyzed. The surface rupture trace was depicted clearly by InSAR observations. In addition, the coseismic slip distribution inversion was carried out constrained by both ascending and descending InSAR deformation fields and relocated aftershocks to understand the characteristics of deep fault slip and geometry of the seismogenic fault. The regional stress disturbance was analyzed based on coseismic Coulomb stress change. The results show that the Maduo M_W7.3 earthquake occurred on a secondary fault within the Bayan Har block which is almost parallel to the main fault trace of the Kunlun Fault. According to field investigation, geological data and InSAR surface rupture traces, the seismogenic fault is confirmed to be the Kunlunshankou-Jiangcuo Fault. The rupture length of seismogenic fault is estimated to be~210km. The NWW direction is followed by the overall displacement field, which indicates a left-lateral strike-slip movement of seismogenic fault. The maximum displacement is about 0.9m in LOS direction observed by both ascending and descending InSAR data. The inversion result denotes that the strike of the seismogenic fault is 276°and the dip angle is 80°. The maximum slip is about 6m and the average rake is 4°. The predicted moment magnitude is M_W7.45, which is overall consistent with the result of GCMT. An obvious slip-concentrated area is located at the depth of 0~10km. The coseismic Coulomb stress change with the East Kunlun Fault as the receiver fault shows that the Maduo earthquake produced obvious stress increase near the eastern segment of the East Kunlun Fault. Thus the seismic risk increases based on the high interseismic strain rate along this segment, which should receive more attention. In addition, the coseismic Coulomb stress change with the Maduo-Gande Fault as the receiving fault indicates that the Maduo earthquake produced an obvious stress drop near the western part of the Maduo-Gande Fault, which indicates that the Maduo earthquake released the Coulomb stress of the Maduo-Gande Fault, and its seismic risk may be greatly reduced. However, there is a stress loading effect in the intersection area of the Maduo-Gande Fault and the Kunlunshankou-Jiangcuo Fault. Considering that aftershocks of Maduo earthquake will release excess energy, the greater earthquake risk may be reduced.

In the global scale, ten destructive earthquakes with magnitude larger than 7 happen on average each year. Yet the number of small earthquakes with limited or even no damage but recordable by seismographs(magnitude between 2.5 and 4.5)is over one million per year. In between, there are hundreds to thousands of earthquakes with moderate to strong magnitude(magnitude between 5.5 and 6.5)with notable destructiveness. The massive moderate to strong earthquakes are often less noticed or even overlooked, with only very few exceptions which caused human casualties and/or structure damages due to the very shallow focal depths. For medium earthquakes, the traditional seismology means can obtain the source mechanism solution of earthquake, but because of the inherent fuzziness of the source mechanism, it cannot distinguish the fault plane from the auxiliary nodal planes, because earthquakes of this magnitude usually do not produce surface rupture, and the result error is large, so it is not suitable for the study of medium and small earthquakes. It is of fundamental significance to further study the source fault of the moderate earthquakes, and more independent methods other than traditional seismology, such as satellite geodesy are needed. As one of the most applied satellite geodesy technique, interferometry of SAR(InSAR)satellite images are commonly used to obtain coseismic deformation related to earthquakes. InSAR has very high spatial sampling, though the temporal sampling is very low, which is several days to over a month depending on the satellite revisit span. The precision of coseismic deformation by InSAR can reach 2~3cm, which is good enough to obtain the surface deformation caused by a moderate earthquake. It is noted that InSAR coseismic measurements can detect 1-dimensional(1D)deformation along Line-of-Sight(LOS)direction. With multiple observing modes including descending and ascending, the InSAR deformation data is very useful for identifying surface ruptures, and for source fault plane discrimination. As a new geodetic observation technology, InSAR uses the elastic dislocation model to obtain source parameters, and the inversion results of fault parameters and slip distribution are more reliable. On September 24th, 2019, an M_W6.0 earthquake hit New Mirpur, Pakistan. The nearest known fault to the epicenter is the Main Frontal Thrust on its south side. We used the Sentinel-1A SAR imagery(TOPS-model)to reconstruct the InSAR coseismic deformation fields generated by the 2019 M_W6.0 Pakistan earthquake along the ascending and descending tracks. The ascending and descending deformation fields indicate that coseismic deformation is asymmetric by a trend of NW-SE in the south secondary fault of the Himalayan frontal thrust fault, with a maximum LOS displacement of~0.1m. The structures of ascending and descending deformation are highly consistent with each other, but the LOS displacement of southern side is obviously larger than the northern side. The continuous change of interference fringes between uplift and subsidence areas shows that there is no coherent phenomenon caused by excessively large deformation gradient or surface rupture, which indicates that the seismic fault rupture did not reach to the ground surface. Two initial fault models constrained by InSAR deformation, with a southwest-dipping and northeast-dipping fault, were utilized in the inversion. We finally determined the northeast-dipping fault as the seismogenic fault by joint inversion of ascending and descending observations, combined with tectonic setting. Our fault model suggests that an obvious slip concentrated area is located in the depth of 2~4km, with a peak slip of~0.64m and a mean rake angle of~125°. The north-dipping thrust motion with a small amount of strike-slip component dominated the faulting. The earthquake occurred in the low-dipping subduction zone between the Indian and Eurasian plates. The dip angle of the fault plane is relatively low. When the fault is ruptured, the upper wall thrust southwards and the north wall subducted northwards. Due to the compressional nappe structure, the front end of the upper wall was uplifted and the back end was stretched to become the subsidence area. Seismogenic fault is the south secondary fault of the Himalayan frontal thrust fault inferred from our coseismic fault model and rupture kinematic features. Active faults on the land have caused many large destructive earthquakes, resulting in surface faults and promoting the development of tectonic landforms. The detailed observation of coseismic surface rupture not only provides basic information for understanding the earthquake itself and estimating the earthquake recurrence period, but also helps to interpret the tectonic and geomorphic features in other areas. Since the M_W6.0 earthquake in Pakistan in 2019, no studies have been reported yet on this earthquake using InSAR technology, so the study of this earthquake provides a rare opportunity to assess the seismic risk of active thrust faults and to study the seismicity of northern Pakistan.

The continuous collision between Tian Shan and Tarim Basin causes not only the uplift of mountains, but also the earthquakes across the entire Tian Shan, particularly in the transient zone from mountains to the adjacent basins, where the critical infrastructures and residents are seriously under threat from these earthquake hazards. On 19^th January, 2020, an earthquake occurred in the Kalpintage fold thrust belt in the southwest Tian Shan foreland. We call this event the 2020 M_W6.0 Kalpintage earthquake, which is the first moderate earthquake captured by modern geodetic measurement techniques. This event therefore provides a rare opportunity to look into the local tectonics and seismic risk in southwest Tian Shan. In this study, we obtained the coseismic deformation of 2020 M_W6.0 Kalpintage earthquake from Sentinel-1A SAR and strong motion data, and then inverted its kinematic slip model. We derived the InSAR interferograms from both ascending and descending tracks. Both of them present similar deformation patterns, two deformation peaks over the Kalpintage anticline. That means: 1)The surface deformation is dominated by vertical displacement, and 2)the coseismic rupture plane is highly suspected to be the shallowly dipping decollement at the base of the sediment cover. We got the 3-D displacements of 6 strong motion stations by double integrating the strong motion acceleration signals. The result shows tiny displacement on the strong motion stations, except for the Xikeer station, which locates at the front of the Kalpintage anticline, where the InSAR interferograms are seriously incoherent. Two slip models can equally fit to the ascending and descending InSAR interferograms: One is a strike slip model with strike of N-S, the other is a thrust model with strike of E-W. This ambiguity in the slip models for the M_W6.0 Kalpintage earthquake is caused by 1)the extremely small dip angles of the causative fault, 2)the inherent shortcomings of the InSAR measurements i.e. the 1-D measurements along the line of sight, the polar orbiting direction of the SAR satellite, and 3)the serious atmospheric delay due to contrasting topography in southwest Tian Shan. We did not distinguish the two ambiguous models with InSAR data due to the weak constraints of InSAR for this event. However, the two quite different slip models show the same spatial dimension and position beneath the Kalpintage anticline, also the same seismic slip vector moving toward the Tarim Basin. We then presumed the two slip models refer to the same fault plane, the weak decollement at the base of the sediment cover, and its rupture released the compressive strain in this fold and thrust belt in the southwest Tian Shan front. The confusing problem is neither the strike slip model nor the thrust model can explain the displacement derived from strong motion. The simple error estimates show small uncertainty in the strong-motion-derived displacement, but we cannot really know the real errors without the comparison to the collocated continuous GNSS observation. Because of the discrepancy between the strong motion displacement and InSAR-derived slip model, we speculate the inelastic deformation occurred in front of the Kalpintage anticline where thick weak sediments exist. We think this earthquake ruptured the decollements in the lower sediments bounded by the adjacent anticlines, which are uplifted in this event. The M_W6.0 Kalpintage earthquake balanced the stress accommodated during the convergence of the Tian Shan and Tarim Basin. We managed to explain all of the ruptures in the southwest Tian Shan by combining the regional tectonic, geophysical data and the available earthquake catalogues with good quality and then estimated the earthquake hazards. The earthquakes, including 1902 M_W7.7 karshigar, 1996 M_W6.3 Jiashi, 1997—2003 Jiashi sequence and 2020 M_W6.0 Kalpintage earthquake, can be explained in one frame, the underthrusting of the Tarim Basin toward the southwest Tian Shan. Our calculation suggests that a M_W7.0+ event could be generated around Kalpintage anticline belt if without barriers on the decollements.

The seismic thermal anomaly has been confirmed by a large number of earthquake case studies, but the weather factor is still the limiting factor for the fine extraction of thermal anomalies. In the past, in order to eliminate the short-term noise of meteorology, the spatial domain difference value was used to replace the temperature value in the original remote sensing image, but the cloud mask will affect the spatial domain mean value, which makes the spatial domain difference value not representative. This causes thermal anomalies due to differences in cloud masks and spatial distribution of temperature. In previous RST studies, the thermal anomaly thresholds were judged to be different values of 1.6, 2, 3, and 4, and there was no stable value. When the number of background fields changes under the same threshold, the thermal anomaly area will change. Therefore, when constructing the background field, the time domain difference is used first and then the spatial domain difference is calculated to remove the weather influencing factors. By introducing skewness abnormal data monitoring algorithm, different sample numbers correspond to different thresholds in thermal abnormality judgment. In this experiment, the areas of 32.70°~42.72°N and 96.37°~106.65°E in Gansu, Qinghai and Sichuan were used as the study area. The new algorithm is used to analyze the spatial and temporal distribution of thermal anomalies in Hutubi M_S6.2 earthquake on December 8, 2016, Jiuzhaigou M_S7.0 earthquake on August 8, 2017, and Jinghe M_S6.6 earthquake on August 9, 2017. Statistical analysis is made on the statistical relationship between earthquakes of magnitude 4 and above and thermal anomalies in the study area from January 1, 2003 to August 31, 2019. Experiments and results show that: 1)By analyzing the annual mean surface temperature map in 2004, it was found that the night temperature in Qinghai Province in the study area was lower than that in other areas, and the night temperature in the southern Gansu and desert areas in the northeast of the study area was relatively high. On June 26, 2004, most of the cloudless images were in the low-temperature region of Qinghai Province, which resulted in a low average value of the spatial domain. The temperature difference between the high-temperature and cloudless regions of Qinghai Province and Gansu Province was relatively large, so the spatial domain difference caused the false thermal anomalies. The difference in time domain is calculated before calculating the difference in spatial domain, and the thermal anomaly almost disappears. Using the difference in time domain and then calculating the difference in spatial domain will eliminate the false thermal anomaly caused by the difference in temperature and spatial distribution of clouds. 2)Through experiments, it is found that the skewness in different sample numbers used in this paper corresponds to different thresholds and the position and area of thermal anomalies are closest when the RST threshold is 4.5. When the RST is at a threshold of 4.5 and the number of background fields is 12, 14, and 16, respectively, the thermal anomaly area becomes smaller as the number of background fields increases, but for the skewness of different thresholds corresponding to different sample numbers in the background when the number of fields is 12, 14, and 16, respectively, the change in thermal anomaly area is small. It is obtained that under the premise that the number of samples satisfies the abnormality judgment, different sample numbers corresponding to different thresholds will basically keep the thermal anomaly area unchanged when the number of background fields changes, and the thermal anomaly threshold judgment stability gets stronger. 3)By extracting the thermal anomalies of Hutubi M_S6.2 earthquake on December 8, 2016, Jiuzhaigou M_S7.0 earthquake on August 8, 2017, and Jinghe M_S6.6 earthquake on August 9, 2017, it was found that the thermal anomaly before the Jiuzhaigou M_S7.0 earthquake moved around the epicenter from the northwest Jungong Fault to the northeast Longxian-Baoji Fault in a clockwise direction; Before the Hutubi M_S6.2 earthquake, the whole thermal anomaly moved from south to north to the epicenter; Before the Jinghe M_S6.6 earthquake, the overall thermal anomaly moved from the vicinity of the northwest Tuster-Ballake Fault to the epicenter and was “/” shaped. 4)There were 210 earthquakes with M_S≥4.0 in the study area, and a total of 98 thermal anomalies occurred. The total number of thermal anomalies corresponding to the earthquake is 46, accounting for 46.94%; the number of earthquakes with thermal anomalies found in 210 earthquakes is 53, accounting for 25.2%, among them, 73.6%were preceded by thermal anomalies. By analyzing the relationship between thermal anomalies over the years and earthquakes, it is found that the TPR value increases with the magnitude of earthquake. Although the number of strong earthquakes in the study area is small and the TPR value of strong earthquakes in this experiment is not representative, the overall trend shows that the probability of earthquake thermal anomalies increases with the increase of the earhtquake magnitude. The greater the probability of occurrence of seismic thermal anomalies during strong earthquakes, the greater the significance is for future earthquake prediction. In this paper, we use two types of data, cloud and surface temperature, to optimize the algorithm to remove the influence of weather factors on seismic thermal anomalies. However, the influence of topography and ground features on temperature still exists, and there is currently no good way to judge and rule it out.

The Aketao M_S6.7 earthquake occurred on November 25, 2016, which was located at the intersection of Gongur extensional system and Pamir frontal thrust. This region is characterized by complex fault structure, high altitude, complex terrain conditions, sparsely populated and few observed data, so the conventional geodetic survey technology is difficult to obtain comprehensive surface deformation information, while InSAR can take advantage of its all-weather, all-day, large-area and high-density continuous monitoring of ground motion. Therefore, this study takes M_S6.7 earthquake as the research object to carry out the co-seismic deformation field extraction and fault static slip distribution inversion. Firstly, the co-seismic deformation field was obtained by using ascending and descending data of Sentinel-1A. The results indicate that the interferogram spatial decorrelation is more serious in the north side of fault, which is affected by the steep terrain. The fringes in the south side of fault were distributed as elliptical semi-petal shapes, and the fringes are smooth and clear. The northern and southern part of the fault was asymmetric: The interferogram fringes of the southern part were dense while fewer fringes were formed in the northern part, and the fringes were semi butterfly-shaped on the surface. The horizontal displacements dominated the co-seismic deformation in this event, with magnitude of 12cm in ascending and -21cm in descending. The deformation occurred mainly on the south wall of fault. Based on the right view imaging of Radar, the co-seismic deformation is consistent with the movement features of dextral strike-slip fault and the focal mechanism provided by USGS and GCMT. The cross section of aftershocks after precisely positioning showed that the dip angle of fault is larger above the depth of 15km, which is generally manifested as the shovel-like structure with the dominant tendency of southward dip. By conducting a comprehensive analysis of deformation feature and aftershocks profile, we proposed that the southwest-dipping Muji Fault is the seismogenic fault. Secondly, a large area of continuous deformation images obtained by InSAR technology contains millions of data points and there is a high correlation between them. In order to ensure the calculation efficiency and inversion feasibility in the inversion process, the quadtree sampling method was used to reduce the number of data points and the datasets were finally obtained that can be received by the inversion system on the basis of retaining the original details of the deformation field. The two tracks InSAR datasets which were down-sampled by quadtree method were used to constrain the inversion to retrieve the fault geometry parameters and slip distribution. The single-segment and two-segment static slip distribution on the fault plane based on uniform elastic half space model were constructed during inversion process. The F-test of fitting residuals based on single-segment and double-segment fault model show that the population variance of the two models was significantly different at the confidence level of 95%, and the variance of the double-fault model was smaller. Through the comprehensive analysis of predicted deformation field, residuals and F-test, it is considered that the simulated results of double fault model are better than that of the single, and the observation data can be better interpreted. The result shows that the simulated co-seismic deformation field and its corresponding observed values were consistent in morphology and magnitude, and the correlation between observed and modeled is up to 0.99. In addition, as can be seen from the spatial distribution and frequency histogram of residuals, the overall residual was not large, mainly concentrated in the range of -0.2~0.2cm with the characteristics of normal distribution. However, there were still some larger residuals on the near fault in ascending track, which may be related to the simplified model. There were two patches with significant slip distribution on each segment and the rupture basically reached the surface. The slip was mainly distributed along the downdip range of 0~20km and was about 50km along the fault strike. The rupture reached the surface and the peak slip of 0.7m was at the depth of 9km. The western segment is dominated by the right-lateral strike-slip and the eastern segment is dominated by the right-lateral strike-slip with slightly normal faulting. The seismic moment derived from inversion was 8.81×10¹⁸N·m, which is equivalent to M_W6.57. The average slip angle obtained by inversion is -175° in the west section and -160° in the east section. The synthetic analysis holds that the source characteristics of the M_S6.7 earthquake was characterized by dextral strike-slip with a slightly normal component, which was composed of two sub-seismic events. The western section was basically pure right-lateral strike-slip with a dip angle of 75°, while the eastern was characterized by dextral strike-slip with a small amount of normal component with a dip angle of 55°. The Aketao earthquake occurred on the northern Pamir salient and its tectonic deformation was mainly characterized by crustal shortening, strike-slip and internal extension of the frontal edge observed by GPS. Generally speaking, the Pamir salient was blocked by nearly east-west South Tian Shan in the process of strong northward pushing under the action of NE direction pushing of Indian plate, and “hard and hard collision” occurred between them. The eastern part of Pamir salient extruded eastward along the nearly NS trending Gongur extensional system, and at the same time rotated clockwise, which caused the nearly EW extension since the Late Quaternary. The Aketao earthquake is a tectonic event occurring at Gongur Shan extensional system, which shows that the pushing of the Indian plate in the NE direction is continuously strengthened, and also implies that the internal crustal deformation of the Pamir Plateau is still dominated by extension in EW direction, which is basically consistent with the present observation of GPS.

The hypothesis that strong earthquakes in China mainland are controlled by the movement and interaction of active-tectonic blocks was advanced by Chinese scientists, with the remarkable ability to encompass geological and geophysical observations. Application of the active-tectonic block concept can illustrate 6 active-tectonic block regions and 22 active-tectonic blocks in mainland China and its neighboring regions. Systems of active-tectonic block boundaries are characterized by a zone of decades or hundreds of strong earthquakes. One of the greatest strengths of the modern active-tectonic block hypothesis is its ability to explain the origin of virtually all the M8 and 80% M7 earthquakes on the main continent in eastern Asia. In other words, active-tectonic block boundary stands in strong causal interrelation with recurrence behaviors of strong earthquakes and thus, it is possible to predict an earthquake occurrence in principle. After nearly two decades of development and improvement, the active-tectonic block hypothesis has established its theoretical foundation for the active tectonics and earthquake prediction, and is promoting the transition from probabilistic prediction to physical prediction of strong earthquakes. The active-tectonic block concept was tested by application to a well-documented, high-frequent earthquake area, and was found to be an effective way of describing and interpreting the focal mechanism and seismogenic environment, but there are still many problems existing in the active-tectonic block hypothesis, which confronts with rigorous challenges. Future progress will continue to be heavily dependent on the high-precision synthetic seismogram, especially of critical poorly documented settings. It is well known that strong earthquakes occur anywhere in the interactions among the active-tectonic block boundaries where there is sufficient stored elastic strain energy driving fault propagation, and then releasing the stored energy. Therefore, future studies will focus on the mechanism and forecast of the strong earthquake activity in the active-tectonic block boundary zone, with fault activity within the active-tectonic block boundary zone, quantifying current crustal strain status, upper crust and deep lithosphere coupling relation, strong earthquake-generating process and its precursory variation mechanism in seismic geophysical model as the main research contents, which are the key issues regarding deepening the theory of active-tectonic block and developing continental tectonics and dynamics in the modern earth science.

It has been reported that there is thermal anomaly within a certain time and space preceding an earthquake, and previous research has indicated potential associations between the thermal anomaly and earthquake faults, but it is still controversial whether physical processes associated with seismic faults can produce observable heat.Based on rock experiments, some scholars believe that the convective and stress-induced heat associated with fault stress changes may be the cause of those anomalies. Then, did the thermal anomaly before the Wenchuan earthquake induced by the fault stress change?It remains to be tested by numerical simulations on the distribution and intensity of thermal anomalies. For example, is the area of thermal anomaly caused by the fault stress changes before the earthquake the same as the observation?Is the intensity the same?To clarify the above questions, a two-dimensional thermo-hydro-mechanical(THM)finite element model was conducted in this study to simulate the spatial and temporal variations of thermal anomalies caused by the underground fluid convection and rock stress change due to the tectonic stress release on fault before earthquake. Results showed that the simulated thermal anomalies could be consistent with the observed in magnitude and spatio-temporal distribution. Before the Wenchuan earthquake, deformation-related thermal anomalies occurred mainly in the fault zone and its adjacent hanging wall, which are usually abnormal temperature rise, and occasionally abnormal cooling, occurring in the fault zone after the peak temperature rise. In the fault zone, the thermal anomaly is usually greater than the order of 1K of the equivalent air temperature and is controlled by the combined effect of fluid convection and stress change. The temperature increases first and then decreases before the earthquake. In the hanging wall, it's weaker than that of the fault zone, mainly depending on the convection of the fluid. The temperature gradually increases before the earthquake and is dramatically affected by the permeability. Usually, only when the permeability is larger than 10^-13m², can the air temperature rise higher than 1K occur. The results of this study support the view that fluid convection and stress change caused by fault slip before the earthquake can produce observable air temperature anomalies.

The development of high-rate GNSS seismology and seismic observation methods has provided technical support for acquiring the near-field real-time displacement time series during earthquake. But in practice, the limited number of GNSS continuous stations hardly meets the requirement of near-field quasi-real-time coseismic displacement observation, while the macroseismographs could be an important complement. Compared with high-rate GNSS, macroseismograph has better sensitivity, higher resolution(100~200Hz)and larger dynamic range, and the most importantly, lower cost. However, baseline drift exists in strong-motion data, which limits its widespread use. This paper aims to prove the feasibility and reliability of strong motion data in acquiring seismic displacement sequences, as a supplement to high-rate GNSS.
In this study, we have analyzed the strong-motion data of Wenchuan M_S8.0 earthquake in Longmenshan fault zone, based on the automatic scheme for empirical baseline correction proposed by Wang et al., which fits the uncorrected displacement by polynomial to obtain the fitting parameters, and then the baseline correction is completed in the velocity sequence. Through correction processing and quadratic integration, the static coseismic displacement field and displacement time series are obtained. Comparison of the displacement time series from the strong motions with the result of high-rate GPS shows a good coincidence. We have worked out the coseismic displacement field in the large area of Wenchuan earthquake using GPS data and strong motion data. The coseismic displacement fields calculated from GPS and strong motions are consistent with each other in terms of magnitude, direction and distribution patterns. High-precision coseismic deformation can provide better data constraint for fault slip inversion. To verify the influence of strong-motion data on slip distribution in Wenchuan earthquake, we used strong motion, GPS and InSAR data to estimate the stress drop, moment magnitude and coseismic slip model, and our results agreed with those of the previous studies. In addition, the inversion results of different data are different and complementary to some extent. The use of strong-motion data supplements the slip of the fault in the 180km segment and the 270~300km segment, thus making the inversion results of fault slip more comprehensive.
From this result, we can draw the following conclusions:1)Based on the robust baseline correction method, the use of strong motion data, as an important complement to high-rate GNSS, can obtain reliable surface displacement after the earthquake. 2)The strong motion data provide an effective method to study the coseismic displacement sequence, the surface rupture process and quick seismogenic parameters acquisition. 3)The combination of multiple data can significantly improve the data coverage and give play to the advantages of different data. Therefore, it is suggested to combine multiple data(GPS, strong motion, InSAR, etc.)for joint inversion to improve the stability of fault slip model.

Slip rate is one of the most important parameters in quantitative research of active faults. It is an average rate of fault dislocation during a particular period, which can reflect the strain energy accumulation rate of a fault. Thus it is often directly used in the evaluation of seismic hazard. Tectonic activities significantly influence regional geomorphic characteristics. Therefore, river evolution characteristics can be used to study tectonic activities characteristics, which is a relatively reliable method to determine slip rate of fault. Based on the study of the river geomorphology evolution process model and considering the influence of topographic and geomorphic factors, this paper established the river terrace dislocation model and put forward that the accurate measurement of the displacement caused by the fault should focus on the erosion of the terrace caused by river migration under the influence of topography. Through the analysis of the different cases in detail, it was found that the evolution of rivers is often affected by the topography, and rivers tend to migrate to the lower side of the terrain and erode the terraces on this side. However, terraces on the higher side of the terrain can usually be preserved, and the displacement caused by faulting can be accumulated relatively completely. Though it is reliable to calculate the slip rate of faults through the terrace dislocation on this side, a detailed analysis should be carried out in the field in order to select the appropriate terraces to measure the displacement under the comprehensive effects of topography, landform and other factors, if the terraces on both sides of the river are preserved. In order to obtain the results more objectively, we used Monte Carlo method to estimate the fault displacement and displacement error range. We used the linear equation to fit the position of terrace scarps and faults, and then calculate the terrace displacement. After 100, 000 times of simulation, the fault displacement and its error range could be obtained with 95%confidence interval. We selected the Gaoyan River in the eastern Altyn Tagh Fault as the research object, and used the unmanned air vehicle aerial photography technology to obtain the high-resolution DEM of this area. Based on the terrace evolution model proposed in this paper, we analyzed the terrace evolution with the detailed interpretation of the topography and landform of the DEM, and inferred that the right bank of the river was higher than the left bank, which led to the continuous erosion of the river to the left bank, while the terraces on the right bank were preserved. In addition, four stages of fault displacements and their error ranges were obtained by Monte Carlo method. By integrating the dating results of previous researches in this area, we got the fault slip rate of(1.80±0.51)mm/a. After comparing this result with the slip rates of each section of Altyn Tagh Fault studied by predecessors, it was found that the slip rate obtained in this paper is in line with the variation trend of the slip rate summarized by predecessors, namely, the slip rate gradually decreases from west to east, from 10~12mm/a in the middle section to about 2mm/a at the end.

On November 18, 2017, a M_S6.9 earthquake struck Mainling County, Tibet, with a depth of 10km. The earthquake occurred at the eastern Himalaya syntaxis. The Namche Barwan moved northward relative to the Himalayan terrane and was subducted deeply beneath the Lhasa terrane, forming the eastern syntaxis after the collision of the Indian plate and Asian plates. Firstly, this paper uses the far and near field broadband seismic waveform for joint inversion (CAPJoint method)of the earthquake focal mechanism. Two groups of nodal planes are obtained after 1000 times Bootstrap test. The strike, dip and rake of the best solution are calculated to be 302°, 76° and 84° (the nodal plane Ⅰ)and 138°, 27° and 104° (the nodal plane Ⅱ), respectively. This event was captured by interferometric synthetic aperture radar (InSAR)measurements from the Sentinel-1A radar satellite, which provide the opportunity to determine the fault plane, as well as the co-seismic slip distribution, and assess the seismic hazards. The overall trend of the deformation field revealed by InSAR is consistent with the GPS displacement field released by the Gan Wei-Jun's team. Geodesy (InSAR and GPS)observation of the earthquake deformation field shows the northeastern side of the epicenter uplifting and the southwestern side sinking. According to geodetic measurements and the thrust characteristics of fault deformation field, we speculate that the nodal plane Ⅰ is the true rupture plane. Secondly, based on the focal mechanism, we use InSAR data as the constraint to invert for the fine slip distribution on the fault plane. Our best model suggests that the seismogenic fault is a NW-SE striking thrust fault with a high angle. Combined with the slip distribution and aftershocks, we suggest that the earthquake is a high-angle thrust event, which is caused by the NE-dipping thrust beneath the Namche Barwa syntaxis subducted deeply beneath the Lhasa terrane.

In this paper, we processed and analyzed the Sentinel-1A data by "two-pass" method and acquired the surface deformation fields of Menyuan earthquake. The results show the deformation occurred mainly in the south wall of fault, where uplift deformation is dominant. The uplift deformation is significantly larger than the subsidence and the maximum uplift of ascending and descending in the LOS is 6cm, 8cm respectively. Meanwhile, based on the Okada model, we use the ascending and descending passes data as constraints to invert jointly the fault distribution and source parameters through constructing fault model of different dip directions. The optimum fault parameters are:The dip is 43°, the strike is 128°with the mean rake of 85°. The maximum slip is about 0.27m. The inverted seismic moment M₀ is 1.13×10¹⁸N·m, and the moment magnitude M_W is 5.9. The SW-dipping Minyue-Damaying Fault is possibly the seismogenic fault, based on the comprehensive analysis of the focal mechanisms, aftershocks relocation results and the regional tectonic background. The focus property is dominated by thrust movement with a small amount of dextral strike-slip component. The earthquake is the result of local stress adjustment nearby the Lenglongling Fault under the background of northeastward push and growth of Tibet Plateau.

The May 12, 2008 M_S7.9 Wenchuan earthquake is ranked as one of the most devastating natural disasters ever occurred in modern Chinese history. The Longmenshan Fault(LMSF) zone is the seismogenic source structure, which consists of three sub-parallel faults, i.e., the Guanxian-Jiangyou Fault(GJF) in the frontal, the Yingxiu-Beichuan Fault(YBF) in the central fault and the Wenchuan-Maowen Fault(WMF) in the back of the LMSF. In this study, geological survey and seismic profiles are used to constrain the faults geometry and medium parameters. Three visco-elastic finite element models of the LMSF with different main faults are established. From the phase of interseismic stress accumulation to coseismic stress release and postseismic adjustment, the Wenchuan earthquake is simulated using Continuous-Discrete Element Method(CDEM). Modeling results show that before the 2008 Wenchuan earthquake, the GJF becomes unstable due to the interaction between its unique fault geometry and the tectonic stress loading. In the fault geometry model, the GJF is the most gently dipped fault among the three faults, which in return makes it having the smallest normal stress and the greatest shear stress. The continuous shear stress loading finally meets the fault failure criteria and the Wenchuan earthquake starts to initiate on the GJF at the depth of 15~20km. The earthquake rupture then propagated to the YBF. At the same time, due to the GJF and YBF rupture, the interseismic stress accumulation has been greatly reduced, causing the WMF failed to rupture. Although the stress accumulation in the WMF has been reduced significantly after the earthquake, yet it has not been released completely, which means that the WMF likely has with high seismic risk after the 2008 Wenchuan earthquake. We also find that the stress perturbation caused by gently dipping segment of the fault can promote the passive rupture in the steeply dipping segment, making the upper limit of dip angles larger than traditional assumption.

In tectonically active regions, geomorphic features such as fluvial terraces can be interpreted as the consequence of tectonic and climatic forcing. However, deciphering and distinguishing tectonic impacts and climate changes remain a challenge. In this study, we examine the terraces along the Hongshuiba river and Maying river, which flow across the Fudongmiao-Hongyazi fault in the northern margin of the Qilian Mountains. Our purpose is to analyze the relative roles of tectonics and climate in shaping orogenic topography in this area. 8~9 levels of river terraces were identified through field observations, interpretation of satellite images and using DEMs. According to relative heights and ages of T₅ of the Hongshuiba river and T₆ of the Maying river, the incision rates are calculated to be (10.2±2.0)mm/a and (12.2±2.8)mm/a, respectively. Furthermore, the thrust rate along the Fodongmiao-hongyazi fault was determined based on offset terraces and OSL dating, which are ten times less than river incision rates approximately. Comparing the uplift rate and incision rate in the northern margin of the Qilian Mountains and adjacent areas, we inferred that climate change is the most plausible controlling factor in the evolution of the river terraces, while tectonics plays a minor role in this process.

According to the structure of the Himalayan orogenic belt, a low-angle antilistric thrust-slip fault model is used to simulate the ramp on the rupture portion of the Main Himalayan Fault. Based on descending Alos -2 and Sentinal -1 data, we invert for the coseismic slip models of the Gorkha earthquake and its largest aftershock, Kodari earthquake. In contrast to the inversion using Alos -2 or Sentinal -1 separately, the joint inversion of both data sets has stronger constraint for the deep slip and can obtain more details in Gorkha earthquake. The rupture depth obtained by joint inversion can be as deep as 24km underground, cutting across the locking line to the transition of locked and the creeping zone. The largest slip is as large as 4.5m appearing 17km underground and the dip angle is between 3°and 10°. Gorkha and Kodari earthquakes have the similar focal mechanisms, both of which are mainly thrusting, and yet some right-lateral slip component in Gorkha earthquake. The inversion results reveal that slip models of the Nepal mainshock and its largest aftershock are complementary in space and the Kodari earthquake occurs in the gaps of slip in Gorkha earthquake. The epicenter of the Kodari earthquake is just right in the transitive zone of the positive and negative Coulomb stress change, where the Coulomb stress change can reach 0.4MPa. We thus argue that Kodari earthquake has been triggered by the Gorkha earthquake.

The 2008 Gaize M_W6.4 earthquake,occurring on the tensional active fault zone located between Lhasa terrane and Qiangtang terrane in the interior of Tibet is a typical normal-faulting event.In this paper,we resolve the three-dimensional coseismic displacement fields of the earthquakes using a least-square iterative approximation solution with a priori knowledge,according to the theoretical basis that InSAR measurements are extremely insensitive to N-S component.Results show that the boundary dividing the two sides of the main-shock fault is very clear in the vertical movement,and two remarkable subsidence centers can be observed on the hanging wall,while amplitude of the west one (-48.9cm) is larger than the east (-41.4cm),but the maximum uplift on the footwall is only 5cm.In addition to some northward movement with amplitude less than 5cm around the aftershock fault,the north-south deformation field suggests an overall southward movement.The three-dimensional results indicate that the induced surface movement is predominantly vertical and mostly occurred on the upper side,while there are obvious east-west separation and eastward rotation in the horizontal plane.The full vectors are consistent with simulated deformation field with the RMSE less than 6cm,so the research demonstrates the feasibility of the method to recover precise three-dimensional deformation field.On the whole,the three-dimensional deformation field coincides with the tensile fracture characteristics of Gaize earthquakes,and the tectonic stress background of coeval east-west extension and north-south shortening.

Coseismic displacement plays a role in earthquake surface rupture, which not only reflects the magnitude scale but also has effect on estimates of fault slip rate and earthquake recurrence intervals. A great historical earthquake occurred in Huaxian County on the 23^rd January 1556, however, there was lack of surface rupture records and precise coseismic vertical displacements. It's known that the 1556 Huaxian earthquake was caused by Huashan front fault and Weinan plateau front fault, which are large normal faults in the east part of the southern boundary faults in Weihe Basin controlling the development of the basin in Quaternary. Here, we made a study on three drilling sites in order to unveil the coseismic vertical displacements.
It is for the first time to get the accurate coseismic vertical displacements, which is 6m at Lijiapo site of Huashan front fault, 7m at Caiguocun site, and 6m at Guadicun site of Weinan plateau front fault. These coseismic displacements measured based on same layers of drilling profiles both at footwall and hanging wall are different from the results measured by former geomorphological fault scarps. It's estimated that some scarps are related with the nature reformation and the human beings' activities, for example, fluviation or terracing field, instead of earthquake acticity, which leads to some misjudgment on earthquake displacements. Moreover, the vertical displacements from the measurement of geomorphological scarps alone do not always agree with the virtual ones. Hence, we assume that the inconsistency between the results from drilling profiles and geomorphological scarps in this case demonstrates that the fault scarp surface may have been demolished and rebuilt by erosion or human activities.

Isostatic gravity anomaly is considered a sign of the isostatic state of the crust, and studies show that the isostatic state of crust is closely connected with the structural features and seismicity in many areas. In order to investigate the isostatic state of crust and to understand its relation to structural features and seismicity in North China Craton, a new isostatic residual gravity map of North China Craton has been computed using recently released earth gravitational model and digital terrain models. Free-air gravity anomalies of North China Craton have been prepared using the gravity data set of Earth Gravitational Model 2008(EGM2008). EGM2008 data set is believed to be reliable and studies show that EGM2008 free-air gravity anomalies have a general accuracy of 10.5mGal(1mGal=10^-3cm/s²)in China. The topographic-isostatic corrections were computed based on an Airy-Heiskanen model of local compensation using a strict algorithm based on digital elevation model(DEM), the average crust thickness of the study area was derived from CRUST2.0, and the topographic and bathymetric data sets were derived from digital elevation model ASTER GDEM 2009(1arc second resolution)and ETOPO1(1arc minute resolution)respectively. Topographic-isostatic corrections were then added to the free-air gravity anomalies to determine the isostatic gravity anomalies of North China Craton with a gridding resolution of 5arc minutes. According to the results of calculations, distribution of isostatic gravity anomalies and its relations to structural features and seismicity of North China Craton were discussed. The results indicate that the spatial distribution of isostatic gravity anomalies is remarkably uneven in North China Craton, and isostatic gravity anomalies are very different between different fault blocks. Isostatic gravity anomalies of North China Craton are mainly controlled by neo-tectonic movements, and are significantly influenced both by lateral variations in crust density and deep structures. The close relation between isostatic gravity anomalies and neo-tectonic movements may imply that there are crustal features that are not compensated regionally and isostatic disequilibrium in North China Craton. The results also indicate that there are some connections between the distributions of isostatic gravity anomalies and seismicity in North China Craton, earthquakes tend to occur around areas with remarkable high or low isostatic gravity anomalies and at transition zones between positive and negative gravity anomalies, and we suggest that special attention should be paid to areas with similar isostatic gravity anomaly characteristics when performing seismic hazard analysis.

In water area,shallow groundwater area and plains with thick Quaternary sediments,drilling is necessary for active fault survey.Due to complex facies change of Quaternary terrestrial formation,there is no mature scheme for the key problems such as borehole arrangement,decision and correlation of isochron surfaces and deciphering of episodic activity of fault.A test of drillhole exploration was implemented across the northern segment of Nankou-Sunhe Fault buried under Beijing plain.Based on the data of shallow seismic investigation,we drilled a row of boreholes.A combined borehole section was built by sequence stratigraphy,lithology and facies analysis,magnetic susceptibility and absolute chronology,which can define the location,geometry and accumulated displacements in several time spans.The result shows that the fault has an episodic movement since 60ka BP.The active stages of the fault are 60ka to 47ka BP,36ka to 28ka BP,and 16ka to present,respectively.Other intervals are relatively stable.The average vertical slip rate is 0.35mm/a from 60 ka to 37ka BP,0mm/a from 37ka to 32ka BP,0.78mm/a from 32ka to 12ka BP,and 0.35mm/a since 12ka BP.Stratigraphic cyclicity is the main issue of sequence stratigraphy.The cyclicity is of multilevel character.Controlled by climate fluctuations,tectonic movements and matter source factors,each sequence has a specialty itself as effective index to the correlation of borehole strata.Correlation of borehole strata from big to small sequence in turn can effectively reduce the blindness and the uncertainty.Hiatus of sediment strata of low base level such as low-water-level system territory and transgression territory on uplifted side indicates existence of fault scarp and active stage of fault slip.Along with the rise and later fall of base level,together with the attenuation of fault activity,high-water-level system territory and regression system territory may deposit on the uplifted side.Contrarily,homo sequence on both sides indicates weak activity of fault.Magnetic susceptibility reflects well the size difference of terrestrial formation and its vertical fluctuation corresponds well with sequence cyclicity.United application of sequence stratigraphy,lithology and facies analysis,magnetic susceptibility and age dating can achieve high-precision correlation of strata across boreholes.Since the late Quaternary shows distinct climate fluctuation of millenary scale,the substitute index of climate has a potential in high-precision correlation of strata.The borehole correlation of thick stratigraphic succession with a distance of 20～40m is easy in alluvial plains away from the affected zone of fault action.Whereas within rupture zone of fault,the correlation of strata thinner than 1m with a distance no more than 5m is usually difficult if there are no evident horizon markers,although such correlation is necessary for deciphering single surface rupture event.Some fault-scarp-derived colluvial wedges with a width less than the distance of neighbour holes may be missed.Therefore once the fault zone is limited within a bound less than 20m,hole distance less than 2～3m is necessary for probing single event.For the limit of borehole distance and the precision of stratigraphic subdivision,the correlation of borehole strata by sequence stratigraphy cannot reach sub-meter precision,and therefore can only decipher active and quiet stages of fault,but not single surface rupture event.An active stage may include several single surface rupture events.

The key to WebGIS technology is to issue the vector geography graph through the internet and provide the information service for the general browser end user. Considering the complexity, multiplicity and magnanimous space taking of the earthquake disaster evaluation information, the WebGIS based regional fast earthquake disaster evaluation system adopts a three layered network architecture, including the backstage database, the application server (map server) and the Web server, and the client composition. Different from the traditional evaluation method as taking the building as the statistical unit, this fast evaluation system takes the city administrative area, the township or the town as the working unit. Using foundation database and other census data, we can determine the earthquake influence on a given target area according to the earthquake background analysis or the estimation of effect of a given earthquake in the future. By means of the relational model of population data and the earthquake losses, as well as combining the analysis result of earthquake resistance and seismic vulnerability, the possible damage to the target area can be predicted, and accordingly the economic losses and personnel casualty can be estimated.With the platform of ESRI Corporation's ArcIMS 4.1, and mainly the development of technology of VB+ArcObject server module, this system has realized the system core computation, the vector graph superimposition, the dynamic updating of the vector graph attribute data and the distribution graph, as well as the map issuing and the dynamic data management in the server end. The client side is designed with the use of ASP dynamic homepage and the databank administration techniques. The system consists of the function modules of foundation data management and maintenance, fast earthquake damage evaluation, reporting management and system assistance management, and etc. This system makes full use of the existing achievements in fast earthquake disaster evaluation as well as the census data of the Fujian Province, therefore possesses such merits as low investment, regular data updating and being easily accessible as well.

The paper deals with new information about the Nanshancun-Chakou active fault.The total length of the zone is about 35 km.It is divided into two segments, respectively 5km and 15km.The style of the active fault is normal-dip-slip accompanied by sinistral stike-slip.The active fault had many times of activity during Holocene.There was ground surface fracture zone along the active fault in 1830 earthquake.The northern side of the fault fell down.The vertical displacements on the ground along the fault are mostly 2-4m.The recurrent interval of the latest two times of destroyed earthquakes along the fault was about 3 500a according to the research.The paper will show the evidence to explain the serious destruction of the ancient temples along the fault in the 1830 earthquake from the the records carved in stones of the temple.

In this paper,we introduce a method which can be used to determine the gradient of vertical deformation rates and discuss the relation between the gradients of deformation rates and tectonic and seismic activities.Using the digital results from the map of present-day vertical deformation rates in China,we draw a contour diagram of the horizontal gradients of vertical deformation rates in China.A comparison between the zones with high gradient of deformation rates and the locations of strong earthquakes is made and the possible high-risk areas of strong earthquakes in the coming twenty years are also pointed out.