This study determined the source parameters of the 2012 Tangshan MS4.8 earthquake and the 2020 Guye MS5.1 earthquake. It discusses the tectonic settings of these two earthquakes and their causal relationship with the 1976 Tangshan MS7.8 earthquake, aiming to provide constraints on the deep seismic environment in the Tangshan seismic zone. First, based on observations from the Beijing-Tianjin-Hebei Digital Seismic Network, the CAP method was employed to invert the source mechanisms and moment focal depths for the Tangshan MS4.8 and Guyu MS5.1 earthquakes. Results indicate that the fault planes I for the Tangshan MS4.8 and Guyu MS5.1 earthquakes are 230°/71°/-166° and 240°/74°/-174°, respectively, while fault planes II are 135°/77°/-20° and 148°/84°/-16°, respectively. Both earthquakes are strike-slip type. Subsequently, sPL depth phase determination was applied to measure the focal moment depths of these two earthquakes, both found to be 13 km deep—closely matching the focal depth of the Tangshan MS7.8 earthquake. Subsequently, dual-difference positioning was applied to refine the locations of both earthquake sequences. Results revealed that the epicenters of both sequences exhibited a northeastward distribution with nearly vertical depth profiles. Furthermore, the majority of aftershocks for both sequences occurred above the mainshock, indicating a rupture process characterized by progression from greater to shallower depths. Based on these findings, both the Tangshan MS4.8 and Guyi MS5.1 earthquakes are attributed to northeast-trending, steeply dipping strike-slip faults. Finally, integrating source parameter measurements with deep seismic reflection profiles from the source region, insights into the deep seismic environment within the Tangshan Fault Zone were obtained: (1) A crustal-scale strike-slip fault, the Tangshan Deep Crustal Fault, exists beneath the Tangshan Fault Zone. This fault interacts with multiple overlying scarp-like normal faults to form a flower-like strike-slip fault structure, which is presumed to be the seismic structure responsible for the Tangshan MS4.8, Guyu MS5.1, and Tangshan MS7.8 earthquakes. (2) The hypocentral depths of all three earthquakes were near 13 km. Based on seismic source zone tomography and deep seismic reflection profiles, this depth coincides with the boundary between the lowvelocity anomaly within the crust and the top of the Tangshan Deep Crustal Fault. This reflects a transition in crustal deformation behavior from brittle to brittle-ductile. Consequently, the 13-km depth is likely the location of the brittleductile transition boundary within the study area’s crust. (3) At the 13-km depth, the contrast in physical properties between the two sides of the brittle-ductile transition zone facilitates stress locking on the more rigid side. This creates a favorable tectonic setting for the generation of moderate-to-strong earthquakes, potentially explaining why such earthquakes in the study area predominantly occur near the 13-km depth.

Seismological study of the deep seismogenic environment in the Tangshan Fracture Zone

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发表日期：

2025年10月17日

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He Li,
Xi-yi Wang,
Yi-bing Dong,
Jian-guo Wang,
Hui Zhang,
Li-xia Wang,
Bo-yang Wu

摘要

This study determined the source parameters of the 2012 Tangshan M_S4.8 earthquake and the 2020 Guye M_S5.1 earthquake. It discusses the tectonic settings of these two earthquakes and their causal relationship with the 1976 Tangshan M_S7.8 earthquake, aiming to provide constraints on the deep seismic environment in the Tangshan seismic zone. First, based on observations from the Beijing-Tianjin-Hebei Digital Seismic Network, the CAP method was employed to invert the source mechanisms and moment focal depths for the Tangshan M_S4.8 and Guyu M_S5.1 earthquakes. Results indicate that the fault planes I for the Tangshan M_S4.8 and Guyu M_S5.1 earthquakes are 230°/71°/-166° and 240°/74°/-174°, respectively, while fault planes II are 135°/77°/-20° and 148°/84°/-16°, respectively. Both earthquakes are strike-slip type. Subsequently, sPL depth phase determination was applied to measure the focal moment depths of these two earthquakes, both found to be 13 km deep—closely matching the focal depth of the Tangshan M_S7.8 earthquake. Subsequently, dual-difference positioning was applied to refine the locations of both earthquake sequences. Results revealed that the epicenters of both sequences exhibited a northeastward distribution with nearly vertical depth profiles. Furthermore, the majority of aftershocks for both sequences occurred above the mainshock, indicating a rupture process characterized by progression from greater to shallower depths. Based on these findings, both the Tangshan M_S4.8 and Guyi M_S5.1 earthquakes are attributed to northeast-trending, steeply dipping strike-slip faults. Finally, integrating source parameter measurements with deep seismic reflection profiles from the source region, insights into the deep seismic environment within the Tangshan Fault Zone were obtained: (1) A crustal-scale strike-slip fault, the Tangshan Deep Crustal Fault, exists beneath the Tangshan Fault Zone. This fault interacts with multiple overlying scarp-like normal faults to form a flower-like strike-slip fault structure, which is presumed to be the seismic structure responsible for the Tangshan M_S4.8, Guyu M_S5.1, and Tangshan M_S7.8 earthquakes. (2) The hypocentral depths of all three earthquakes were near 13 km. Based on seismic source zone tomography and deep seismic reflection profiles, this depth coincides with the boundary between the lowvelocity anomaly within the crust and the top of the Tangshan Deep Crustal Fault. This reflects a transition in crustal deformation behavior from brittle to brittle-ductile. Consequently, the 13-km depth is likely the location of the brittleductile transition boundary within the study area’s crust. (3) At the 13-km depth, the contrast in physical properties between the two sides of the brittle-ductile transition zone facilitates stress locking on the more rigid side. This creates a favorable tectonic setting for the generation of moderate-to-strong earthquakes, potentially explaining why such earthquakes in the study area predominantly occur near the 13-km depth.