Depth Variations in Large Megathrust Earthquake Rupture Processes

Abstract:

Large earthquakes on plate boundary megathrust faults exhibit substantial variations in rupture characteristics determined from seismic wave observations. There is significant scatter in stress drop, moment-scaled energy release, earthquake source spectrum, and rupture duration. We analyze 113 earthquakes with Mw ≥ 7.0 identified as being on megathrust faults on the basis of their locations and focal mechanisms, analyzing far-field P and S wave observations supplemented by very long-period seismic wave inversions (W-phase, global centroid moment tensor), examining the source scaling behavior, spatial patterns, and depth-dependence of the source parameters. Depth-dependent characteristics have previously been recognized in the occurrence of shallow tsunami earthquakes with depleted short-period source spectra and unusually long source durations, and in enhanced coherent short-period energy release from the deeper portions of megathrusts that fail in huge ruptures. Analysis of the large number of intermediate size events demonstrates that there is little depth-dependence of stress drop or total radiated energy release for megathrust events that are not tsunami earthquakes, but there is systematic increase in relative short-period energy content in the source spectra for deeper events. This is manifested in reduced high-frequency fall-off slope of the source spectra as depth increases. In some cases this is possibly related to compound nature of the ruptures with varying scale-lengths of heterogeneity, but in some cases it is very smooth behavior. Isolating this as a source effect is challenging, as systematic reduction of average attenuation factor with increasing depth can account for some of the systematic pattern. There are some correlations with estimated average megathrust temperature, with higher spectral decay rate correlating with higher slab surface temperature at 30 km depth. We consider the implications of the new source parameter data set for mechanisms operating in the megathrust environment.