Across-Slab Propagation and Low Stress Drops of Deep Earthquakes in the Kuril Subduction Zone

Abstract

Deep earthquakes occur down to 700 km depth where pressure is up to two orders of magnitude greater than in the crust. Rupture characteristics and propagation mechanisms under those extreme conditions are still poorly constrained. We invert seismic waveforms for the spatial dimensions, duration and stress drop of deep-focus earthquakes (Mw 6.7–7.7) in the Kuril subduction zone. We find stress drops of ∼1–10 MPa and rigidity-corrected spatial dimensions and durations similar to crustal earthquakes. Radiated efficiency >1 is observed, suggesting that undershooting is prevalent in deep earthquakes, consistent with laboratory derived weakening mechanisms. Comparisons with subduction models suggest across-slab propagation within regions with temperatures T < ∼ 1,000°C, similar to shallow events. Hence, despite different triggering mechanisms, the same physics seems to control the rupture propagation of both shallow and deep earthquakes.