Seismogenic Structure of the 5 September 2022 Sichuan Luding MS6.8 Earthquake Sequence

On September 5, 2022, an M(s)6. 8 earthquake hit the Moxi Town of Luding County in Sichuan Province, China, caused 97 deaths and 20 people missing. The intensity of its meizoseismic area was IX. To investigate the seismogenic structure of this M(s)6. 8 earthquake and its tectonic deformation characteristics, we relocated the Luding earthquake sequence and calculated the focal mechanism solutions of the M(s)6. 8 mainshock and its M-s>3. 0 aftershocks using the CAP waveform inversion method, which is based on the seismic phase and waveform data recorded on and before October 22, 2022. The spatial distribution of the relocated Luding earthquake sequence reveals a north-northwest (NNW) trending aftershock zone, with its long axis about 55 km length of dense aftershocks along Moxi segment, the southern segment of Xianshuihe fault zone. The northern and southern ends of the aftershock zone were near the north of Xindianzi in Luding County and south of Caoke in Shimian County, respectively. We observed an obvious zonal clustering feature of the aftershock activity, and there are sparse aftershock areas on the south side of Nanmenguan and near Wandong, respectively. The aftershock zone is therefore divided into three sections, as northern, middle, and southern ones. The middle section is relatively wider, but it becomes narrower both in the southern and northern sections. The M(s)6. 8 mainshock and the Luding M(s)5. 0 aftershock on October 22 occurred in the middle section, while the Shimian M(s)4. 5 aftershock on September 7 was located in the southern section, and no M-s >= 4. 0 aftershocks occurred in the northern section. The moment magnitude of the mainshock is M-W 6. 44 from CAP inversion. The strike, dip and rake angles of the nodal planes I and II of the focal mechanism solution for the mainshock are 75 degrees /90 degrees /155 degrees and 165 degrees /65 degrees /0 degrees, respectively. The centroid depth of the mainshock is at 3. 0 km, and the depth for seven n >3. 0 aftershocks ranges from 2. 5 to 7. 5 km, which means most of the accumulated strain energy was released in shallow upper crust during the Luding mainshock and its M-s >3. 0 aftershocks. The areal strains (As) exhibit the pure strike-slip faulting style for the mainshock and the aftershocks in the southern and northern sections, while the aftershocks in the middle section are of extensional (i. e., normal faulting). Based on the spatial distribution of aftershocks, the focal mechanism solutions of the entire sequence and the structural trending in the aftershock zone, we conclude that the major seismogenic fault of the Luding M(s)6. 8 earthquake is the NNW-trending left-lateral strike-slip Moxi fault on the southern segment of Xianshuihe fault zone. The variations of the trend, dip angles and dip -orientations along the Moxi segment indicate the geometrical complexity of the major seismogenic fault. Nonetheless, the strain rosettes show the consistency in tectonic deformation along the seismogenic fault north of Caoke Town, Shimian County: all of them display a pure NEtrending extensional and NW -trending compressional strike-slip faulting. The three M-s>3. 0 normal -faulting aftershocks in the middle section and the four M-s >= 3. 4 normal -faulting and oblique-normal -faulting events in the 2016 Luding earthquake swarm imply the possible existence of a series of SSE (NNW) -striking normal faults, some of which might contribute to the occurrence of aftershocks there. The strain rosettes of the seven events reveal that the seismogenic faults in the western part of the middle section exhibit dominantly or purely NE-trending extensional deformation. It is possible that a sequence of micro-ruptures (similar to 2 km) with normal faulting sense are trending SSE (NNW) on the west side of the mainshock. The geometry of the two nodal planes from the focal mechanism solution indicates that the high-altitude Gongga Mountain was in a collapsing state during the interseismic and coseismic cycles.