2016 SSA Abstract

Zhang, H., S. van der Lee, C. Bina, and Z. Ge, Rupture mechanism of the May 24, 2013 Mw8.3 Sea of Okhotsk deep-focus earthquake, Seismological Research Letters, 87:2B, SSA Meeting Issue, 543, 2016.


Deep-focus earthquakes occur at depths greater than 300 km. Pressure and temperature of the earth's interior typically increase with depth, precluding brittle failure as a possible mechanical process for deep-focus earthquakes. Candidate rupture mechanisms for deep-focus earthquakes include phase transformations (Green and Burnley, 1989; Kirby et al., 1991; Green et al., 2016), thermal shear instabilities (Kanamori, 1998; Wiens, 2001), and dehydration embrittlement (Jung et al., 2004).

The 2013 Mw8.3 Sea of Okhotsk earthquake, the largest deep-focus earthquake since records began, provides a new perspective in understanding the rupture mechanism of deep-focus earthquakes. Back-projected teleseismic P wave trains from USArray (TA) and a virtual array comprising several European seismic networks (EU), filtered into two complementary frequency bands, yield multi-segment rupture patterns that depend on the frequency band. The moment release in each segment, or subevent, was estimated by inverting teleseismic P waveforms from azimuthally well distributed global network stations (GSN). Combining results from both methods, both arrays, and both frequency bands shows that the deep-focus earthquake's asymmetrical, bilateral rupture occurred on a sub-horizontal, curved fault plane and can be separated into three principal segments.

The first segment ruptured downdip along the sub-horizontal fault at a velocity of 4.3 km/s and radiates high-frequency seismic energy. The third segment ruptured updip along the subhorizontal fault at a velocity of 3.6 km/s and radiates low-frequency seismic content. The second segment of the rupture transits from downdip to up-dip at a velocity of 4.6 km/s and mainly radiates high-frequency component seismic energy. The slab strike-perpendicular components of the mostly slab strike-parallel rupture for each of three segments are 23 km, 19 km, and 31 km wide, respectively. Each of the rupture distances is wider than the estimated thickness of the metastable olivine wedge.

The estimated widths of the ruptured segments outside of the metastable olivine wedge require an explanation beyond transformational faulting. Likewise, the large seismic efficiency of 0.6 determined by Ye et al. (2013) and fast rupture velocity preclude a thermal shear instability as the rupture mechanism. The simplest explanation of the earthquake would be that the dehydration embrittlement mechanism, which operated at intermediate depths, is still operating at these great depths. If the deep Okhotsk earthquake indeed represents dehydration, the entirety of the subducting Pacific lithosphere cannot be completely dry at these great depths. A more speculative possibility, worthy of future investigation in a laboratory, is that some other form of transformational faulting, perhaps involving transformation of metastable pyroxene to metastable akimotoite, may occur and accommodate the earthquake.

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