A source extent analysis of the Lancang earthquake ( M s=7.6) of 1988

Source extent parameters of the 1988 Lancang earthquake (M _s=7.6) were estimated by computing the second-central moment of displacement pulses of far field long period SH waves. We inverted the source duration T, the fault length L and the directivity parameter D by the least squares, and obtained that T=11.77 s, D=15.05 km·s, and L=70.94 km. We also find that this event is a symmetrically bilateral rupture and fault segments in two opposite rupture directions have the same value of 35 km in length. Combining analysis of aftershock distribution, the results would imply that there is an area in the joint part of the rupture area of two main shocks, which is not broken yet during the main shocks. Due to the dislocation accompanied during the main shocks, the strain would be rearranged. The joint part has enough strength to accumulate enough strain energy to excite a larger aftershock.