The magnitude forecasting of strong earthquakes is of great scientific significance, and also plays an important role in the actual work of earthquake prevention and disaster reduction. Based on relevant research progress at home and abroad, this paper focuses on the identification of asperities and cascading ruptures in magnitude forecasting of strong earthquakes. The occurrence of earthquakes usually relates to the destruction of asperities. For the identification of asperities, comprehensive analysis can be carried out mainly from the aspects of the media nature in the seismogenic zone, small-medium seismicity, fault motion state, stress state, friction attribute, etc. The occurrence of cascading ruptures will increase the magnitude of earthquakes and the risk of earthquake disasters. The factors controlling its probability mainly include the geometric properties of faults, such as fault surface curvature, geometry change, step distance, as well as, the physical properties of fault and surrounding media, such as the friction properties of fault surfaces, changes in the width of seismogenic layer in adjacent fault segments, stress distribution of fault surfaces and shallow low-velocity fault zones. The research of relevant models and parameters depends on the development of intensive observation, fault exploration and other basic work, the application of numerical simulation, inversion and other technologies, and the integration of multiple disciplines. It is hoped that this paper can provide a reference for the forecasting of strong earthquake magnitude in key fault sections of the Chinese Mainland.

The Sichuan-Yunnan region is subjected to frequent earthquakes due to the collision of the Indian and Eurasian plates, rendering it a ideal experimental site for seismic research. This paper presents a rapid method for constructing a finite element mesh model with complex faults based on a geometric mesh model of the region, using the example of the geometry model of major faults in fault system of Sichuan-Yunnan region. The following research has been conducted: ① The Advancing Front Technique (AFT) has been improved to generate a three-dimensional triangle mesh based on the geometric model, and automatically identify areas that require local refinement and conduct mesh refinement. ② An algorithm for identifying the intersection lines of intersecting faults has been proposed, and it can recognize the intersection lines between different surfaces (including fault surfaces and study area's boundaries) based on the spatial geometric relationship between non-parametric surfaces. ③ The rapid method for constructing a finite element mesh model with complex faults based on a geometric mesh model of the region has been improved. First, the original fault model is manipulated using expansion and connection operations to enable it to represent the true shape of the fault geometrically. Subsequently, the boundary of the study area and the fault mesh are combine. The intersection lines between different fault planes or between fault planes and the study area's boundary are then identified. Then, using the improved advancing front technique, the fault surface is remesh with the intersection lines as constraints to repair the mesh topology and improve the mesh quality. Finally, a tetrahedral calculation mesh model containing faults is automatically generated using the newly generated triangular mesh of the fault surface as constraints. ④ The above method has been applied to the China seismic experimental site to construct a three-dimensional calculation mesh model containing the main fault system which lays the foundation for using finite element numerical simulations to study the seismic dynamics in the study area.

Based on the magnetotelluric profile data, combining with geological profiles, medium-depth seismic reflection exploration profiles, modern seismic catalogue, ALOS-dem data, satellite images and other data, the Shandong section of the Yishu fault zone is selected as the research area. The 3D model of the Yishu fault zone in the depth of 20 km in Shandong is established by using the three-dimensional modeling software Blender, and the modeling process of “satellite image—surface elevation model—shallow fault line—underground structure of the fault revealed by magnetotelluric exploration—underground fault plane—underground seismic ball—3D model”. The following conclusions are obtained: ① The Yishu fault zone shows a 3D structure of two grabens plus one horst from the surface to the underground as a whole; ② The main faults show the characteristics of segmental activity, with obvious differences in occurrence at different segments and complex fault contact relations; ③ Modern earthquakes are mainly distributed along the two faults of the eastern graben in the Yishu fault, with the characteristics of zonal and cluster distributions. The frequency of seismic activity in the strong activity section is obviously higher than that in the weak activity section. The establishment of the three-dimensional model of the Yishu fault zone provides a basis for discussing the influence relationship between earthquakes and active faults, studying the seismogenic mechanism of the Yishu fault zone and reproducing earthquake scenarios.

Obtaining shallow underground structures in densely populated areas at a low cost and with reliability, is of great importance for both earthquake safety assessments and underground space development. Distributed Acoustic Sensing (DAS) is an emerging observation technology that has developed rapidly in recent years and has different sensing mechanism from traditional seismometers. It can achieve high-density and long-distance vibration measurements using an ordinary communication optical cable and an interrogation unit. We used a 460 m buried optical fiber recorded for 12 hours of background noise in Fangshan, Beijing, to obtain the Rayleigh wave phase velocity dispersion curve and invert the underground shallow S-wave velocity profile. A low-velocity soil layer with a thickness of about 15 m and little undulation, which was similar to the results obtained using the traditional ambient noise H/V spectral ratio method, was observed, however, with more detailed structural information. DAS high-resolution near-surface velocity structure detection is feasible, if existed long-distance buried optical cable resources can be utilized, it will provide a new means for low-cost, high-resolution imaging of the near-surface.

The Mengshan piedmont fault is a relatively active NW trending fault in the western Shandong block. In order to study its Quaternary activity characteristics, the latest activity and deep structure of the Mengshan piedmont fault have been preliminarily studied by using field observation, trench profile analysis, geochronology measurement and other methods in combination with gravity anomaly characteristics, which is of great significance for evaluating the seismic risk of the Mengshan piedmont fault. The results show that: ① Since the Quaternary, the Mengshan piedmont fault is mainly characterized by sinistral strike-slip and normal fault movement. The fault, bounded by Yujiazhuang Village, is divided into east and west sections. The western section has not been active since the Quaternary, while the eastern section has been active mainly in the late Pleistocene. ② The Mengshan Piedmont fault cuts deep to the middle and lower crust, which is a deep major fault. ③ There may have been two paleoseismic events in the Mengshan Piedmont fault since the late Pleistocene, (44.1±3.4)~(24.3±1.2) ka and (22.7±1.8)~(6.2±0.5) ka, respectively. ④ There have been many M≈5 earthquakes the fault zone, which has the potential risk of triggering moderate and strong earthquakes. It is suggested to strengthen the in-depth study of the fault in the future.

The determination of stress field and fault geometry is an important basis for earthquake dynamics. In order to determine the stress field and fault geometry in Mengyuan region, where occurred an M_S6.9 earthquake on Jan 8, 2022, by using focal mechanism, firstly, focal mechanisms were collected from 1927 to 2022 in Menyuan region of Qinghai Province. For multiple focal mechanisms of the same earthquake, the central focal mechanism solution was as the focal mechanism of the earthquake, and used as the basic data for solving the stress field and fault geometry. Then cluster analysis was carried out with the focal mechanism node data, to obtain the possible fault plane shape of the seismic sequence, and solve the tectonic stress field in the study area. Finally, the slip angle was estimated by projecting the tectonic stress field onto the fault plane, and the relationship between the stress field and the seismogenic fault was simulated. The results showed that the overall strike and dip angle of the Mengyuan earthquake sequence are 103.19° and 72.44°, respectively, which were consistent with the azimuth of the Lenglongling fault from field geological survey. The azimuth and plunge of the compressional, tensional axes of the tectonic stress field in the study region were 242.37°, 0.93°, 334.79°, and 68.98°, respectively. It could be explained that the NE spreading in northeastern margin of the Tibet block was blocked by the stable Alxa Block, resulting in a thrust and strike-slip fault system in the study region. By projecting the stress field onto the fault plane of the earthquake sequence, the slip angle of the fault was estimated as 50.68°, the relative shear stress and normal stress were 0.822 and -0.077, respectively, which indicated that the Menyuan area had a large shear stress and a weak compression state on the steeper NWW—SEE fault under the NE-trending compressive stress of the Tibet and Alxa Massifs.

According to historical records, in 1038, an M71/2 earthquake occurred in Xinzhou and Dingxiang County. Previous studies have shown that the seismogenic structure of this earthquake is the northern piedmont of Xizhoushan Fault, the epicenter of which is located near Chafangkou, Dongcun town, Dingxiang County. The leveling of the Chafangkou segment of the northern piedmont fault of Xizhoushan fault began in 1985, and the two plates of the fault moved at a total of 9.09 mm to 2021, with an average annual creep slip of 0.25 mm. The trend of creep deformation shows that there is creep movement in the segment of Chafangkou every year, but the overall creep rate is low. The content of clay in the seismic fault sumple of Chafangkou is 23% by using the method of X-ray diffraction, the clay content of fault breccia near the fault is 8%, and the total clay mineral content decreases gradually from old fault mud to fault breccia. The study shows that the clay content is the key factor to determine whether the fault is creeping or not. The natural fault of the northern piedmont of Xizhoushan fault is simulated in the confining pressure environment of 5 km underground (about 130 MPa), and high temperature and high pressure experiments were carried out at a constant disturbing pressure of 100 MPa. The results show that the natural faults in the northern piedmont of Xizhoushan fault exhibit steady creep slip at the temperature of 25℃, 50℃, 100℃ and 150℃ and the disturbing velocity of 0.122 μm/s and 1.22 μm/s。

In this paper, the 2022 Qingzhou M_L4.1 earthquake was taken as the research target. According to the abnormal characteristics of regional gravity field changes in the preparation and occurrence of large earthquakes summarized by previous scholars, the spatial scale and order of magnitude of changes near the epicenter were analyzed by means of calcalating gravity changes in two adjacent periods and cumulative gravity changes. The variation of the single point of the measuring points in the area with significant variation of regional gravity field, including Gaobingxi, Jinan Station, Wangfen and Miaozi, as well as the variation of gravity difference between the north and south of the Gaobingxi site, was also analyzed. According to the surrounding disturbance of the two measuring points in the Gaobingxi site, including the precipitation, land subsidence and terrain correction, the reasons for the change of gravity difference between the two measuring points were analyzed. It is suggested that the gravity field changes before and after the Qingzhou M_L4.1 earthquake, and the change is related to the underground material migration caused by the earthquake preparation and occurrence.

In this paper, we studied seismicity and relocated earthquakes with M_L≥0.5 occurring in the Jinping reservoir and its surrounding area before and after the impoundment. Then we inversed the mechanism solutions of 71 M_L≥3.0 earthquakes occurred the stress field before and after impoundment. The initial response time of the earthquake Jinping Level Ⅰ Reservoir in the was relatively long, which should be related to the lack of coverage of the flooding area near the earthquake swarm. After expanding to Hulugou, seismicity increases sharply, the changes of seismicity in the following stages have good relationship with the impoundment process. After the impoundment, a large number of earthquakes concentrated near Hulugou in Tibetan Autonomous County of Muli, with focal depths ranging from 6 to 14 km, which became shallower compared to the focal depths before the impoundment. Except for the results of focal mechanism solutions in the Hulugou, P-axis and T-axis distribution, regional stress field except for Hulugou are consistent with previous research results. The focal mechanisms of the earthquake near Hulugou are mostly strike type, with the P-axis mainly in the NS and NNW-SSE directions, and the T-axis mainly in the EW and NEE-SWW directions. The maximum and minimum principal stress axes of the regional stress field are nearly horizontal, and the intermediate stress axis is nearly vertical, the R-value is 0.5, different from other regions. We infer that their mechanisms should be influenced by pore pressure diffusion and infiltration weakening.

Before an earthquake, the gravimeter may record high-frequency disturbance signals related to the source. On February 18, 2020, an M4.1 earthquake occurred in Changqing, Shandong Province. The observation data of PET gravimeter at Tai'an station, about 52 km away from the epicenter, showed energy enhancement and the vertical displacement of DF ground pulsation increased (0.13×10^-6 m) since 5 days before the earthquake. In order to clarify the relationship between this phenomenon and the Changqing earthquake, time-frequency analysis in the 0.1~0.5 Hz frequency band is carried out to the gravity data in February 2020 and the vertical displacement of the DF pulsation signal is calculated. The results are synchronized with the global model of seismic background noise energy radiation (ASSM); After excluding the influence of typhoon, comparatively analyzing of the gravity data of Jiaxiang station and the vertical component acceleration data of JCZ-1 seismometer at Tai'an station, obtaining weather information and wave height data, the pulsation signal excited by the signal source from the wave is determined. The mean square deviation of the DF vertical displacement of the gravimeter in Tai'an station is used as the prediction index to test the R-value. The result shows that this phenomenon is not related to the 2020 Changqing M4.1 earthquake. The analysis methods and processes of gravity data change, as well as the methods and ideas for determining the signal source that causes data change in this paper, provide a reference for the anomaly identification and determination of gravimeter observation data, which can be applied to seismic analysis and prediction in related situations.

In order to further test and improve the judgment basis for identifying foreshocks through the characteristics of “narrow frequency band and low-frequency migration”, this paper compares and analyzes the FFT spectra of the Maduo M_S4.2 earthquake in Qinghai on December 24, 2020, and the Yushu M_S4.7 earthquake in Yushu on April 14, 2010. based on the waveform data recorded by digital seismic networks in Qinghai, Gansu, and other regions. The results show that the frequency band of Maduo M_S4.2 earthquake spectrum is wide, the dominant frequency band is 0~5.0 Hz. The frequency band of the Yushu M_S4.7 earthquake narrowed and shifted to the low-frequency end, the dominant frequency band of 0~1.5 Hz. The comparative analysis shows that the “frequency band narrows and shifts to low frequency” phenomenon is effective for direct foreshock identification. In the future, it is necessary to focus on the possibility of strong earthquakes occurring near the source area in the short term (1~3 months) after the frequency spectrum shift of small and medium earthquakes occurring.

We study the prediction efficiency of Pattern Informatics method (PI) in different areas of the North-South Seismic Belt in this paper. The earthquake catalogue since 1970 is taken from the China Earthquake Networks Center. Both the change interval and forecast interval are fixed as 5 years and the grid size is taken as 1°×1°. We have the retrospective forecasting for 6 earthquakes above M_S6.0 in the study region since 2017. The forecasting efficiency of PI is tested by ROC (Receiver Operating Characteristic) method. The results show that: ① The prediction efficiency of the south, middle and north sections of the North-South Seismic Belt shows a trend of decreasing gradually, but the change is small. This indicates that the prediction performance of PI method is affected by the intensity of regional seismicity to a certain extent. ② The prediction efficiency of piecewise calculation for the North-South Seismic Belt is better than that of the whole calculation for the North-South Seismic Belt. In terms of prediction efficiency, the north and middle sections of the North-South Seismic Belt are the best, the south, middle and north sections are the second, the middle and south sections are relatively low, and the North-South Seismic Belt as a whole is the lowest. The analysis shows that the calculation process of PI method involves the normalization of seismicity, so PI prediction performance is better for areas with similar seismicity. Hot spots generated by PI in regions with different seismicity are mainly generated in areas with high seismicity and the prediction efficiency is low. Therefore, regional division of the North-South Seismic Belt by seismicity can improve the prediction efficiency. ③ There are persistent high probability “hot spots” in Yongde-Lushui area in southwest Yunnan and near the southern section of Longmenshan Fault. These areas should be concerned for earthquakes with M_S≥6 in the next five years.

By retrospectively collating seismic belt anomalies in Ningxia since 1970, we analyzed the temporal and spatial characteristics of seismic belt anomalies, as well as, the anomaly threshold of internal and external seismic frequency ratio, obtained the parametric fitting equations of major earthquake and seismic belt, summarized and the prediction indexes of seismic belt in Ningxia, which provides some support and basis for the short-term earthquake tracking, discussion and evaluation technical schemes in Ningxia. The results showed that ① There were 12 significant seismic belts with M_L≥2.5 in Ningxia and its adjacent areas since 1970, among which, 8 earthquakes with M≥5 occurred at the end of the strip within one year after the end of the belts, and the dominant seismic reflection period was half a year, which could pass the R-value scoring test of prediction efficiency, and had certain medium-term prediction significance. ② There was a good linear positive correlation between the onset time, magnitude of the main earthquake, and the length, duration, total energy of the belt. The reference mean value could be given by the regression equation. The end of the belt was a favorable place for the occurrence of moderate strong earthquakes in the future. ③ In Ningxia, the average frequency ratio of the inner and outer belt was generally low. During the formation of the seismic reflection belts, there was a significant high value more than 0.75 but less than 2 times the mean square error, and the abnormal high value of the falsely reported belts was not significant.

In order to obtain reliable the source parameters quickly, such as focal mechanism solutions for mid-strong earthquakes, an interactive inversion software based on CAP method is designed and implemented. The software has a graphical interface to read the earthquake waveform from regional seismic network. Stations used for inversion are selected by user. Then the software can be used to automatically process the observed waveforms, calculate the theoretical Green's function and invert the focal mechanisms of the earthquakes. It is proved that the software is convenient, reliable and robust by testing multiple earthquakes with different tectonic environments, different magnitudes and different types. The software is easy to quickly obtain focal mechanism solutions of the intermediate or strong earthquake. It is significance to provide the solutions data for emergency response, and it is beneficial to the study on seismic source properties.