On July 12, 2020, a M5.1 earthquake occurred in the Guye District of Tangshan City. This earthquake is notable as the only moderate seismic event exceeding magnitude 5 in the Tangshan area over the past two decades. However, the exact seismogenic fault responsible for this earthquake remains undetermined, complicating efforts to assess future seismic risks in the region. Post-earthquake damage assessments revealed that the macroseismic damage was distributed along two primary fault zones: a long northwest(NW)trending band and a short northeast(NE)trending band. The most significant damage occurred at the intersection of these two bands. Based on the regional geological structure and stratigraphy, field surveys identified the NE-trending Tangshan-Guye fault as a Holocene-active fault, while the NW-trending Mozhouyu fault was classified as a Quaternary fault within the area of greatest damage. Analysis of Sentinel-1A InSAR time-series data revealed differential deformation along the Mozhouyu fault. Relocation results of earthquakes greater than magnitude 1.0 over the past decade in the Tangshan region showed seismic activity distributed in two primary bands. One band aligns with the NE-trending Tangshan-Guye fault, with concentrated activity at its intersection with the Mozhouyu fault. Following the M5.1 earthquake, multiple authorities determined that the focal mechanism indicated a strike-slip earthquake, with two conjugate planes oriented in the NE and NW directions. This finding is consistent with the alignments of the Tangshan-Guye and Mozhouyu faults. Through comprehensive analysis, including post-earthquake field surveys, regional deformation data, and the relocation of smaller seismic events, it was concluded that the surface damage from the Tangshan Guye earthquake followed both NE and NW orientations. Of the two intersecting faults in the damaged area, the Mozhouyu fault is a middle Pleistocene fault, while the Tangshan-Guye fault is the most significant Holocene-active fault in the region. The characteristics of these conjugate faults align with both the source parameters and relocated seismic sequences of the Tangshan Guye earthquake. The right-lateral strike-slip motion along the Tangshan fault zone, combined with regional NE—NEE-directed compressive stress, likely caused the Tangshan-Guye fault to be blocked by the Qinglongshan complex anticline during its eastward expansion. Subsurface data further indicate that the Qinglongshan complex anticline marks a boundary of regional physical property differences. Therefore, it is concluded that the Tangshan-Guye fault and the Mozhouyu fault were the conjugate seismogenic faults responsible for the M5.1 earthquake on July 12, 2020.

The Tangshan Guye earthquake is a typical moderate-intensity strike-slip event in the North China Plain. An analysis of 705 focal mechanism solutions from 2002 to 2020 indicates that most earthquakes in the region are predominantly strike-slip in nature. Historical strong earthquakes in the North China Plain also exhibit high-angle strike-slip faults as their primary seismogenic structures, a conclusion supported by extensive seismological research. A substantial body of seismic studies suggests that the failure of the North China Craton during the early Cenozoic was driven by crustal extension, resulting in the formation of listric(shovel-shaped)normal faults. However, these faults are no longer the main seismogenic structures for present-day earthquakes. Since the late Pleistocene, tectonic activity in the North China Plain has been characterized by the development of new, steeply dipping strike-slip faults, which cut through the older listric normal faults. These steep dip strike-slip faults have become the primary seismogenic structures responsible for regional seismicity. Future seismic hazard assessments in the North China Plain should focus on the activity of these steep dip faults, as they are more likely to generate significant earthquakes. This shift in tectonic stress is attributed to a combination of factors, including the eastward expansion of the Tibetan Plateau, the rigid deformation of the Ordos Block, and the westward subduction of the Pacific and Philippine plates. Since the late Pleistocene, these forces have redefined the tectonic landscape of the region, increasing the likelihood of strike-slip faulting.

Small earthquakes have been recorded in Yibin area, Sichuan Province since 1970, the frequency and intensity of seismicity have shown an increasing trend in recent ten years, and the earthquakes are distributed mainly in Changning, Gongxian and Junlian areas. Based on the seismic data from January 2008 to May 2015 recorded by Sichuan and Yunnan regional networks and Yibin local network, seismicity analysis, precise location and velocity structure inversion for earthquakes in Yibin area are carried out, the three-dimensional spatial distribution of seismic activity and the velocity structure at different depths in this region are investigated, trying to analyze the seismic activity law and seismogenic mechanism in Yibin area.
The earthquake relocation result shows that the spatial cluster distribution of earthquakes is more obvious in Yinbin area, the earthquakes are concentrated in Changning-Gongxian and Gongxian-Junlian regions. The seismic activity presents two dominant directions of NW and NE in Changning-Gongxian region, and shows asymmetric conjugate distribution, the long axes of NW-trending and NE-trending seismic concentration area are about 30km and 12km respectively, and the short axes are about 5km. There is a seismic sparse segment near Gongxian, the frequency and intensity of seismicity in the southeast side are obviously higher than that in the northwest side, and the earthquakes with larger magnitude are relatively deep, the focal depth is gradually shallower with the distance away from Gongxian. Seismic activity is sparse in the west and dense in the east in Gongxian-Junlian region, the predominant direction of earthquakes in the seismic dense area of the eastern segment is NE. Seismic activity extends in opposite direction in the easternmost part of the two earthquake concentrated area.
The P-wave velocity structure at different depths in the study area is obtained using joint inversion method of source and velocity structure. In view of the predominant focal depth in this region, this paper mainly analyzes the velocity structure of the upper crust within 10km. Within this study area, the P-wave velocity of earthquake concentration areas is relatively high within 10km of the predominant focal depth, especially in the northwest of Gongxian and eastern Junlian area, the P-wave velocity on the southeast of Gongxian increases gradually with depth, especially at 6km depth. These high-velocity zones are generally related to brittle and hard rocks, where the stress is often concentrated.
Comparing earthquake distribution and velocity structure, seismic activity in this area mainly occurs in high-low velocity transition areas, the inhomogeneity of velocity structure may be one of the factors controlling earthquake distribution. The transition zone of high and low velocity anomalies is not only the place where stress concentrates, but also the place where the medium is relatively fragile, such environment has the medium condition of accumulating a large amount of strain energy and is prone to fracture and release stress.

The attenuation characteristics and site response are calculated respectively for each individual tectonic unit in Sichuan (Sichuan Basin,west Sichuan plateau and Panzhihua-Xichang area),using digital waveform data recorded by regional seismic networks and relevant seismic phase data collected from China Seismograph Network.The frequency dependent Q(f) is obtained by the iterative grid-search technique described by Atkinson and Mereu based on trilinear geometrical spreading model.The source spectra are determined by the model of Brune and the site responses of seismic stations are derived by Moya's method using genetic algorithms.Comparison to conventional M_L estimates shows that the network local magnitude bias is quite significant at low and intermediate magnitudes.The bias at the j^th station for the i^th event is defined as ΔM_ij=M_ij-M_i, where ΔM_ij is the station magnitude and M_i the network-average value.For comparison,we mapped the spatial distribution of biases by digital seismograms recorded from 10535 earthquakes of magnitude 2.5≤M_L≤4.9 that occurred in Sichuan from January 1,2009 to June 30,2015.Based on the above data,the attenuation characteristics,site response and their effects on magnitude determination in Sichuan are analyzed.Our results demonstrate that the associated model for regional quality factor for frequencies can be expressed as Q₁(f)=450.6f^{0.513 4} for Sichuan Basin,Q₂(f)=136.6f^{0.581 3} for west Sichuan Plateau and Q₃(f)=101.9f^{0.666 3} for Panzhihua-Xichang area.Site response results indicate that different stations show different amplifications.Maps of biases appear to be different,but with similar dominant spatial distribution.For stations in Sichuan Basin,their greater magnitudes are functions of low attenuation in structure and amplification effects of both seismic stations and basin effects.For stations in west Sichuan Plateau,the possible causes of these lower magnitudes are severe dependence upon source region due to extreme lateral variations in either structure or path effect attenuation.For stations in Panzhihua-Xichang area,broken medium caused by strong tectonic activity or large earthquakes and heat flow up-welling along active faults may be the main reasons of low magnitude values when earthquakes occur in western Sichuan and eastern Tibetan region.And the greater magnitudes for earthquakes along the Longmen Mountains appear to be well correlated with edge effect of sedimentary basin on strong ground motion.In our study,stations magnitude biases appear to be extremely correlated with tectonic structures and different regions when seismic rays passing through,magnitudes are affected significantly by lateral variations in attenuation characters rather than site responses.