The 2013 Okhotsk deep-focus earthquake: Rupture beyond themetastable olivinewedge and thermally controlled rise time near the edge of a slab

The 2013M8.3 Okhotsk earthquake involves two primary mechanisms of deep-focus earthquake rupture, mineral phase transformation of olivine to spinel and thermal shear instability. Backprojection imaging of broadband seismograms recorded by the North American and European networks indicates bilateral rupture toward NE and SSE. The rupture paths of the NE segment and other regional M7 earthquakes are confined in narrow regions along the slab contours, consistent with the phase transformation mechanism. However, the SSE rupture propagates a long distance across the slab and aftershocks are distributed across a ~60 km wide zone, beyond the plausible thickness of the metastable olivine wedge, favoring thermal shear weakening. While the NE rupture is only visible at high frequencies, the SSE rupture is consistently observed across a broad-frequency range. This frequency-dependent rupturemode can be explained by lateral variations of rise time controlled by thermal thinning of the slab near its northern end.