A shallow aftershock sequence in the north-eastern end of the Wenchuan earthquake aftershock zone

Abstract

Previous studies show that mature faults are filled with fault gouge in the shallow part and thus cannot accumulate enough strain energy for earthquakes. Therefore most earthquakes are deeper than 5 km, except those events occurring on new faults or in intact rocks. From field observation, Wenchuan earthquake is found to rupture the free surface about 200 km, but the rupture may extend underground much further from teleseismic body waves inversion and aftershocks distribution. In the northeastern end of the rupture zone, deep rupture may induce stress increase near the free surface, and trigger shallow earthquakes. An Ms 5.7 aftershock occurred at Qingchuan, northeast end of Wenchuan earthquake fault on July 24, 2008, featuring thrust mechanism with a 3 km source centroid depth. The shallow focal depth is confirmed with the sPL phase recorded at station L0205. As Rayleigh wave is well only developed for source depth less than 1/5 of epicentral distance, the observed large amplitude of Rg at a distance of 15 km implied depth of 3 km or less. Dozens of aftershocks’ sPL waveforms are also analyzed to confirm the source depths less than 3 km. On the other hand, no surface ruptures are found by geological survey or InSAR studies. It is strongly suggested that these aftershock sequences initiate fresh rupture in intact rocks triggered by stress increase from the deep co-seismic rupture of the Wenchuan mainshock.