On the Role of Multiple Interactions in Remote Aftershock Triggering: The Landers and the Hector Mine Case Studies

Abstract

The observed seismicity rate increase after large earthquakes in sites that are located several source lengths away from the mainshock centroid poses a major problem. This is because the static stress change induced by a mainshock in that region seems to be insignificant, and the dynamic stress changes can only en- hance the seismicity during the passage of the seismic waves but not at later times. In quest for a physically viable triggering mechanism for delayed aftershocks in remote sites, we examine earthquake activities in remote sites that were triggered by two California earthquakes, the magnitude 7.1 Hector Mine earthquake and the mag- nitude 7.3 Landers earthquake. We introduce a new method for quantifying the de- gree to which the triggering effect of each aftershock is locally more important than the triggering effect of the mainshock. We apply this method to the Landers and the Hector Mine remote aftershock sequences. We show that multiple stress transfers from early aftershocks to later aftershocks played an important role in the enhance- ment of both the Landers and the Hector Mine aftershock activities in remote sites. We present a time-space diagram of the Hector Mine remote aftershock sequence in the Imperial Valley, which shows that this sequence is made up of several subse- quences and that the onset of activity migrated southward.