Late-Quaternary fault activity of the Longmen Shan fault zone -Evidence from paleoseismic trenching

The fault activity of the Longmen Shan fault zone has been focused since the Wenchuan earthquake occurred on May 12, 2018. Paleoseismic trenching has been suggested to be the most effective way to reveal Late-Quaternary fault rupture behavior. This paper integrates about 10-year paleoseismic studies on the Longmen Shan fault to further reveal its Late-Quaternary rupture behavior and recurrence characteristics. Herein we suggest that rupture behavior of the Longmen Shan fault shows segmentation characteristics. Fault activity of the middle segment of the Yingxiu-Beichuan and Jiangyou-Guanxian fault (including the Xiaoyudong fault) has been the most intense part since Holocene. Specifically, the segment ruptured three times since about 6000 years with a similar coseismic offset, the penultimate and third oldest event occurred at 3300-2300 cal BP and 5920-5730 cal BP, which indicates an average recurrence interval of about 3000 years. The northern segment of the Yingxiu-Beichuan fault had independent rupture behavior, the penultimate event on the segment ruptured at about 665-1030 AD that is associated with historical record 942 AD earthquake. The third oldest event occurred at 8240-7785 BC. The southern segment of the Jiangyou-Guanxian fault (Dachuan-Shaungshi fault) shows a weaker seismic potential than that of the middle and northern segments. The present paleoseismic data except for the 2018 Wenchuan earthquake that are associated with the Yingxiu-Beichuan fault suggest that no evidence has supported a cascading rupture behavior of the Yingxiu-Beichuan fault, which indicates that the 2018 Wenchuan earthquake might be an unusual great seismic event. Moreover, deformation evidence revealed from trenching on the Qingchuan fault might not be related to fault offset, but more possibly be related to responding deformation due to coseismic right-laterally offset of the Yingxiu-Beichuan fault.