关键词: 青白江M_S5.1地震, 地震反射剖面, 地震重定位, InSAR同震形变, 龙泉山断裂带

Abstract:

On February 3, 2020, an earthquake with a magnitude M_S5.1 occurred in Qingbaijiang District, Chengdu City, Sichuan Province. The epicenter is located in the north segment of the Longquan Shan fault zone in the western Sichuan Basin. This fault zone locates in the forebulge of the foreland thrust belt of the Longmen Shan fault zone in the southeast margin of Tibetan plateau and is the east boundary of the western Sichuan foreland basin at the same time, so it has special tectonic significance. There are two branch faults in the north segment of Longquan Shan fault zone, which are distributed on the east and west sides, respectively, and the epicenter distance is almost similar to the two faults. At present, the seismogenic fault, earthquake genesis and dynamic source of the earthquake are not clear. As this earthquake is a moderate earthquake event, it is usually very uncertain to interpret it with structural geological or seismological data alone. Therefore, this study attempts to carry out a comprehensive study on the Qingbaijiang M_S5.1 earthquake by performing cross fusion of multi-disciplinary data, adopting the multi-constraint method from geophysics, seismology and geodesy, and combining with structural geology and fault related fold theory. We collected three seismic reflection profiles located in the north segment of the fault zone to reveal the basic structural characteristics underground. The detachment layer in the middle-lower Triassic Jialingjiang-Leikoupo formation is developed at the depth of 4~6km below the anticline, and two obvious opposite thrust faults are developed on the two wings of the anticline, which are breakthrough fault-propagation fold deformation. The east branch thrust fault gradually rises from the detachment layer of Leikoupo formation to the surface, and the west branch thrust fault is exposed on the surface and connected with the detachment layer downward. The waveform data recorded by 14 fixed stations within 150km from the epicenter of Sichuan seismic network are studied and collected. The focal depth, focal mechanism and moment magnitude of the earthquake are obtained by using CAP waveform inversion method. The focal depth is 5km, indicating that the earthquake is related to shallow fault activity, the focal mechanism is 18°/32°/100° for nodal plane I and 186°/59°/84° for nodal plane Ⅱ, the moment magnitude is 4.64. Using the travel time data of P and S seismic phases, the Qingbaijiang earthquake sequence is relocated by HypoSAT location method and double difference location method. It is concluded that the epicenter position of the main earthquake is 30.73°N and 104.48°E. From February 4 to June 26, 2020, a total of 61 aftershock events were relocated, with magnitude 0≤M_L≤3.0 and depth ranging from near surface to 15km. The 61 aftershocks spread about 5km in the NW-SE direction and have conjugate distribution in NW and NE directions, which may be related to the small thrust fault developed on the east branch of Longquan Shan Fault. Aftershocks have a good linear distribution in NE direction, which is closer to the east branch of the north segment of Longquan Shan fault zone, and the distribution direction is also consistent with the fault strike. On the seismic reflection profile, the aftershock projection is densely distributed along the east branch fault. The occurrence of the east branch fault is consistent with the focal mechanism nodal plane I, which is a low angle thrust fault dipping to NW. The InSAR coseismic deformation field near the epicenter is extracted by using the Sentinel data of orbit 55 and orbit 62 collected from ESA, including 8 single view complex images of orbit 55 and orbit 62, respectively. The surface deformation caused by this earthquake is in the middle of two thrust faults, and the maximum coseismic deformation can reach 4cm. The deformation caused by the earthquake is uplifting in the northwest and depressing in the southeast of the epicenter. The largest depression is located between the epicenter and the east branch fault. The thrust activity of the east branch fault is more in line with the above surface deformation characteristics. In this study, the seismotectonics of the 2020 Qingbaijiang M_S5.1 earthquake is analyzed in detail using multi-disciplinary and multi-constraint method. The east branch fault in the north segment of the fault zone is determined as the seismogenic fault, and the possible seismic dynamic background is discussed. This result provides a scientific basis for fault activity analysis and seismic risk assessment in Longquan Shan area and has a great significance for further exploring the expansion and growth of Longmen Shan in the southeast margin of Tibetan plateau toward Sichuan Basin.

Key words: Qingbaijiang M_S5.1 earthquake, seismic reflection profile, seismic relocation, InSAR coseismic deformation field, Longquanshan fault zone