The variations in source parameters of deep similar earthquakes

Abstract:

The generation mechanism of deep earthquakes in subducting slabs remains enigmatic. We analyze five groups of co-located similar earthquakes (depth > 200 km) in the Pacific Plate beneath central Japan, whose waveforms are nearly identical at five or more stations over a 40 s time window that includes both of the P and S arrivals. Our result of hypocenter relocations using waveform-derived differential-time data suggests that deep earthquakes with the waveform similarities occur on complementary ruptures of neighboring patches on a fault, rather than by repeatedly rupturing the same fault patch. The estimated stress drops of similar earthquakes are almost constant within each group but differ by an order of magnitude even between groups separated by ~20 km, while the scaled energy and radiation efficiency are significantly different even for each group. The wide range of source parameters estimated for similar earthquakes in a common slab environment suggests that those parameters are considerably affected by small-scale heterogeneities along the fault. We interpret the differences in source parameters as being caused by differences in relative thermal shear instability during seismic ruptures. A twofold process of rupture initiated by fluid-related embrittlement and rupture propagation assisted by thermal instability is probably a fundamental mechanism of intraslab earthquakes.