On the Role of Multiple Interactions in Remote Aftershock Triggering

Observed seismicity rate increase following 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 the mainshock in that region seems to be insignificant, and the dynamic stress changes can only enhance the seismicity during the passage of the seismic waves, but not at later times. In quest for a viable triggering mechanism for delayed aftershocks, I have developed a new simple method for quantifying the degree to which the triggering effect of each aftershock is locally more important than the triggering effect of the mainshock. Application of this method to the Landers and the Hector Mine remote aftershock sequences shows that many of Landers and Hector Mine delayed aftershocks in remote sites are not directly triggered by their mainshock, but are instead aftershocks of previous aftershocks. Finally, I have examined the role of multiple stress transfers in quasi-static earthquake simulations [Ziv, 2003], and found that multiple static stress transfers from early aftershocks can induce aftershocks in sites where the stress perturbation due to the mainshock alone is negligible. I suggest that delayed aftershocks, i.e., aftershocks occurring after the passage of the seismic waves, are not directly triggered by the mainshock, but are instead aftershocks of previous aftershocks.