Paleoseismic events on the Shuangshi-Dachuan fault of the seismic gap, Longmen Shan thrust belt, eastern margin of the Tibetan Plateau

Abstract

Over a five year period, the Longmen Shan thrust belt was the source of the 2008 Ms 8.0 Wenchuan earthquake and the 2013 Ms 7.0 Lushan earthquake, leaving a seismic gap between the seismogenic structures of these two earthquakes. In this study, we report on a trenching and dating program to assess the rupture behavior of the Shuangshi-Dachuan fault, as well as analyzing its geological significance, and discussing the seismic potential of the seismic gap. One paleoseismic event (5382–385 yr B.C.) and another possible event (1013–1560 yr A.D.) were identified in our study. Based on geological physiognomy (fault geometry and paleoseismic events, etc.) and geophysical marks etc, the Shuangshi-Dachuan fault in the seismic gap can be divided into two segments (a southern segment 1 and a northern segment 2). Based on six trenches across the Shuangshi-Dachuan fault in the seismic gap, using the event window method, at least four paleoseismic events are identified (120–435 yr B.P., 720–905 yr B.P., 2475–2717 yr B.P., 2335–3830 yr B.P. (closest to 3830 yr B.P.)). The change in characteristics such as strike, main stress, maximum dislocation of single paleoseismic events, and strike fault activity trace, from segment 1 to segment 2, reflect the character of the Longmen Shan thrust belt transition zone. Vertical displacements of single earthquakes from the six trenches in the seismic gap are similar and small (0.2–0.4 m), the reason for which may be that the seismic gap is a weak part in the upper crust of ductile rocks with low stress. The seismic capacity of the seismic gap is likely Ms < 7.0 and the probability that large earthquakes will occur in the near future is low.