Diversity of deep earthquakes with waveform similarity

Abstract

The generation mechanism of deep earthquakes in subducting slabs remains enigmatic. We analyze deep earthquakes at depths of 200-500 km in the Pacific Plate beneath central Japan, then identify five groups of 11 similar earthquakes whose waveforms are similar at five or more stations over a 40 s time window that includes the P and S arrivals. Hypocenter relocations using waveform-derived differential-time data suggest that similar deep earthquakes occur on complementary ruptures of neighboring patches on a fault, rather than by repeatedly rupturing the same fault patch. The stress drops of similar earthquakes are almost constant within each group but differ by an order of magnitude (~10 to 100 MPa) even between groups separated by ~20 km, while the other parameters are significantly different even for each group such as the apparent stress (~0.1 to 10 MPa) and radiation efficiency (~0.01 to 1). The wide range of source parameters estimated for similar earthquakes in a common slab environment suggests that source parameters can be significantly affected by small-scale heterogeneities along the fault. Thus, the predominant-rupture process of deep earthquakes can be diverse even at different patches on the same fault plane or at the identical depths in a common slab. A dual process which includes seismic rupture assisted by both fluid-related embrittlement and thermal shear instability may explain the wide range of source parameters estimated in this study.