ON THE INTERPRETATION OF SUBEVENTS IN TELESEISMIC WAVE-FORMS - THE 1994 BOLIVIA DEEP EARTHQUAKE REVISITED

We evaluate the methodologies that aim at resolving the space-time sli p distribution during large earthquakes. In one common approach, the s ource process of an event is assumed to be composed of a series of sub events. Critical points in teleseismic body waves, such as peaks and c oherent arrivals, are viewed as discrete points in the spatiotemporal history of an earthquake. We show that these features can be interpret ed as the instantaneous centroid locations of the moment release histo ry. We demonstrate this result with an analysis of the M-w = 8.2 Boliv ia 1994 deep event. We reconstruct the slip distribution in two contra sting ways. First, broadband P waves are interpreted as being composed of subevents. Five peaks in the moment release rate are located relat ive to the onset of the rupture using the azimuthal variations of onse t-to-peak times. Second, we invert broadband P waves for the slip dist ribution on a subhorizontal fault. We use a nonlinear tomographic imag ing technique called simulated annealing and an L1 misfit norm. Ruptur e velocities appear to be < 2 km s(-1) and largest slip values are fou nd to the east and north of the onset in a 40 by 50 km region. The ins tantaneous centroids of the moment release history correspond closely to the locations found using the subevent analysis. However, especiall y near the middle and the end of the rupture, the instantaneous centro id locations are in regions of little moment release. We discuss how t he subevent analysis and the instantaneous centroid approach lead to d issimilar interpretations of rupture processes, especially for events with length to width ratios near unity. Another result of this study i s that the rupture of the 1994 Bolivia event appears to end 50 km to t he northwest of the onset, implying that the slip area is larger that previously thought. Comparing the slip distribution with the aftershoc k distribution suggests that more than one physical mechanism is neces sary to account for the rupture process of this great deep event.