Holocene paleoearthquake history on the Qingchuan fault in the northeastern segment of the Longmenshan Thrust Zone and its implications

Abstract

Although much work has been performed for faults with high slip-rates, little attention has been paid to low slip-rate faults, such as the Longmenshan Thrust Zone (LTZ), on which the Wenchuan earthquake occurred. The LTZ is a long and matured fault that evolved during the Mesozoic as a structural boundary, but its Quaternary activity had been considered insignificant. The Wenchuan earthquake on the central segment of the LTZ and the following Lushan earthquake on the southwestern segment illustrate the necessity of assessing the regional seismic potential around the northeastern extension of the LTZ. However, little is known about its activity and paleoearthquake history. To solve these problems, we conducted paleoseismological trench excavations at two sites on the Qingchuan fault to establish its paleoearthquake history. Only one (and the latest) event that occurred in the Holocene is identified. Based on the radiocarbon dating, the faulting event is constrained to occur between 4115 and 3820B.C., and a long recurrence interval of greater than ~6000years is thus estimated. Judging from the matured fault structure of the Qingchuan fault, the latest event was likely to have ruptured the full length of the fault. According to the empirical scaling laws between magnitude and rupture length, the magnitude of the event is estimated to be Mw 7.6–7.9. On the basis of the slip rate from GPS measurements and the elapsed time since the last event, it is estimated that a seismic moment equivalent to Mw ~7.5 has been accumulated on the Qingchuan fault. Considering the increased Coulomb failure stress and the shortened time of earthquake recurrence triggered by the Wenchuan earthquake, it is suggested a high seismic risk along the Qingchuan fault and its neighboring area. Furthermore, the slow strain buildup, unadapted geometry, and matured fault structure of the LTZ may be the reason why it produces rare but large intraplate earthquakes.