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

Abstract

The 2013 M8.3 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 ã 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 rupture mode can be explained by lateral variations of rise time controlled by thermal thinning of the slab near its northern end.

Tomography of the subducting Pacific slab and the 2015 Bonin deepest earthquake (Mw 7.9)

“…Meng et al 52. suggested that the 2013 Okhotsk deep earthquake (Mw 8.3, 610 km depth) was affected by thermal thinning of the Pacific slab because it occurred near the northern end of the slab as revealed by seismic tomography53.…”

Multiple branching rupture of the 2009 Tonga‐Samoa earthquake

“…Frequency‐dependent rupture behavior has been observed for large megathrust earthquakes and continental earthquakes [e.g., Lay et al , ; Yao et al , ; Kiser and Ishii , ; Meng et al , ; Koper et al , ; Wang and Mori , , ; Fan and Shearer , ; Grandin et al , ], and variations have been seen in both along‐strike and dip directions [e.g., Kiser and Ishii , ; Yao et al , ; Meng et al , ; Melgar et al , ]. The observed frequency‐dependent variations could be expressions of depth‐varying frictional properties [e.g., Yao et al , ], multiple segmentation [e.g., Yin and Yao , ; Denolle et al , ], different rupture properties over the fault plane [e.g., Kiser and Ishii , ], or different mechanisms [e.g., Meng et al , ]. The frequency dependence of the Tonga‐Samoa earthquake is different from these cases in the sense that the frequency‐dependent rupture behavior is associated with different faults.…”

Global and along‐strike variations of source duration and scaling for intermediate‐depth and deep‐focus earthquakes

“…Our results suggest that deep earthquakes source behavior is not strongly influenced by the temperature of the slab, in contrast to observations of b values [ Houston , ]. If, for example, shear instability [ Prieto et al ., ; Meng et al ., ; Chen et al ., ] is the sole mechanism to explain intermediate‐depth and deep‐focus earthquakes, a correlation with thermal properties or plate age of the slab is expected [ Tibi et al ., ]. However, for similar plate ages, both short and long events are observed, suggesting a combination of rupture properties and mechanisms [ Zhan et al ., ].…”

“…For example, seismic observations of the recent M w 8.3 Sea of Okhotsk and its aftershocks have been used to interpret the mechanisms of deep earthquakes, with contradictory conclusions. A thermal control due to horizontal rupture beyond the metastable olivine wedge has been proposed [ Meng et al ., ], although relatively high rupture velocities are observed [ Ye et al ., ]. A cascade‐triggering mechanism, consistent with thermal shear instability along vertical faults, has also been proposed [ Wei et al ., ], and even supershear rupture has been reported for an M ~6 aftershock [ Zhan et al ., ].…”