Abstract
Competing hypotheses have been proposed to explain the seismic hazard (i.e. whether earthquakes with M >= 7 occur) of the southern Longmen Shan (LMS). This region did not rupture during the 2008 M-w 7.9 Wenchuan earthquake, but later generated the 2013 M-w 6.6 Lushan earthquake. Currently, the maximum possible earthquake magnitude, its average recurrence interval, and the seismogenic structure of the southern LMS, remain poorly documented. This study aims to re-evaluate seismogenic structures and seismic hazard along the southern LMS. We first describe the sub-surface structural geometry, as well as the total slip and Quaternary activity of the Range Front blind thrust (RFBT), using high-resolution seismic reflection profiles, borehole data, and intensity-derived macroscopic epicenters. This thrust, which generated the 1970 M-s 6.2 Dayi and 2013 M-w 6.6 Lushan earthquakes, extends for > 250 km along the LMS range front. Integrating new evidence of active faulting and folding and previous quantitative horizontal shortening rate results, we estimate that the Quaternary slip rate of the RFBT is nearly 1 mm/yr, with a minimum total slip of 5 km since 8-5 Ma. Furthermore, we calculate the accumulation rate of seismic moment, 8.04 (+/- 2.09) x 10(17) N.m/yr, for the main active thrusts on the southern LMS, to compare with the moment release rate for earthquakes in the region. When we combine this with the Gutenberg-Richter distribution obtained from historical and instrumental catalogs, we estimate that the potential maximum moment magnitude of an earthquake in the southern LMS is 7.7. Finally, we conclude that the entire southern LMS is capable of generating much larger earthquakes (M-w 7.3-7.7) than seen recently, with an average frequency of once every 1000-1400 years. Therefore, our findings confirm that there is potential for large earthquakes in the southern LMS, especially on the RFBT, which extends beneath the densely populated Chengdu Plain.