Remote triggering of high magnitude earthquakes along plate boundaries

It has been shown that large magnitude earthquakes can remotely trigger other large magnitude earthquakes within three days. Such triggering of high magnitude earthquakes is potentially indicative of fault systems at the end of their seismic cycles. Here a method is developed to examine local earthquake history to determine how susceptible a given area is to remote triggering of high magnitude earthquakes. The method is applied to all plate boundaries. Only 14% of global tectonic boundaries are not susceptible to remote triggering, while 86% show susceptibility to varying degrees. The most highly susceptible locations begin triggering at lower magnitudes, dependent on the type of plate boundary. Varying patterns in susceptibility to remote triggering are observed around individual plates. Finite element modeling of the Cocos Plate reveals normal modes which appear consistent with its spatial patterns of high susceptibility. Many of the natural frequencies of the Cocos Plate are closely associated with the frequencies of free oscillations of the earth and could be induced by large earthquakes. Analysis of the stress tensors generated by the normal modes supports a delayed triggering mechanism involving one-sided negative (compressive) stress normal to the plane of the fault.

Seismic and aseismic slip on the central Peru megathrust

Slip on a subduction megathrust can be seismic or aseismic, with the two modes of slip complementing each other in time and space to accommodate the long-term plate motions. Although slip is almost purely aseismic at depths greater than about 40 km, heterogeneous surface strain suggests that both modes of slip occur at shallower depths, with aseismic slip resulting from steady or transient creep in the interseismic and postseismic periods. Thus, active faults seem to comprise areas that slip mostly during earthquakes, and areas that mostly slip aseismically. The size, location and frequency of earthquakes that a megathrust can generate thus depend on where and when aseismic creep is taking place, and what fraction of the long-term slip rate it accounts for. Here we address this issue by focusing on the central Peru megathrust. We show that the Pisco earthquake, with moment magnitude M(w) = 8.0, ruptured two asperities within a patch that had remained locked in the interseismic period, and triggered aseismic frictional afterslip on two adjacent patches. The most prominent patch of afterslip coincides with the subducting Nazca ridge, an area also characterized by low interseismic coupling, which seems to have repeatedly acted as a barrier to seismic rupture propagation in the past. The seismogenic portion of the megathrust thus appears to be composed of interfingering rate-weakening and rate-strengthening patches. The rate-strengthening patches contribute to a high proportion of aseismic slip, and determine the extent and frequency of large interplate earthquakes. Aseismic slip accounts for as much as 50-70% of the slip budget on the seismogenic portion of the megathrust in central Peru, and the return period of earthquakes with M(w) = 8.0 in the Pisco area is estimated to be 250 years.