Interaction between adjacent left-lateral strike-slip faults and thrust faults: the 1976 Songpan earthquake sequence

Abstract

Based on the published focal mechanisms we have built the fault model of the main shocks of the 1976 Songpan earthquake sequence and calculated the coseismic Coulomb stress changes in the region. The results show that most of the aftershocks had occurred in the region where the Coulomb stresses had been increased, indicating a triggering relationship between the main shocks and the aftershocks. We also show that the first main shock (M s = 7.2), which is a left-lateral slip event, had increased the Coulomb stresses by 5×105 Pa at the second main shock (a thrust event with M s = 6.7). Therefore, we conclude that the first main shock had triggered the second main shock. The third main shock is also a left-lateral event, however, the triggering relationship between the third main shock and the previous two events is less obvious. General model calculations show that there is a good triggering relationship between adjacent left-lateral slip fault and thrust fault, but triggering between parallel slip faults is rather weak.