On the interpretation of subevents in teleseismic waveforms: The 1994 Bolivia deep earthquake revisited

Abstract

We evaluate the methodologies that aim at resolving the space‐time slip distribution during large earthquakes. In one common approach, the source process of an event is assumed to be composed of a series of subevents. Critical points in teleseismic body waves, such as peaks and coherent arrivals, are viewed as discrete points in the spatiotemporal history of an earthquake. We show that these features can be interpreted as the instantaneous centroid locations of the moment release history. We demonstrate this result with an analysis of the Mw=8.2 Bolivia 1994 deep event. We reconstruct the slip distribution in two contrasting ways. First, broadband P waves are interpreted as being composed of subevents. Five peaks in the moment release rate are located relative to the onset of the rupture using the azimuthal variations of onset‐to‐peak times. Second, we invert broadband P waves for the slip distribution on a subhorizontal fault. We use a nonlinear tomographic imaging technique called simulated annealing and an L1 misfit norm. Rupture velocities appear to be <2 km s−1, and largest slip values are found to the east and north of the onset in a 40 by 50 km region. The instantaneous centroids of the moment release history correspond closely to the locations found using the subevent analysis. However, especially near the middle and the end of the rupture, the instantaneous centroid locations are in regions of little moment release. We discuss how the subevent analysis and the instantaneous centroid approach lead to dissimilar interpretations of rupture processes, especially for events with length to width ratios near unity. Another result of this study is that the rupture of the 1994 Bolivia event appears to end 50 km to the northwest of the onset, implying that the slip area is larger that previously thought. Comparing the slip distribution with the aftershock distribution suggests that more than one physical mechanism is necessary to account for the rupture process of this great deep event.