The isolated ∼680 km deep 30 May 2015 MW 7.9 Ogasawara (Bonin) Islands earthquake

Abstract

The isolated ~680 km deep 30 May 2015 MW 7.9 Ogasawara (Bonin) Islands earthquake Deep-focus earthquakes, located in very high-pressure conditions 300 to 700 km below the Earth's surface within sinking slabs of relatively cold oceanic lithosphere, are mysterious phenomena. The largest recorded deep-focus earthquake (M W 7.9) in the Izu-Bonin slab struck on 30 May 2015 beneath the Ogasawara (Bonin) Islands, isolated from prior seismicity by over 100 km in depth, and followed by only a few small aftershocks. Globally, this is the deepest (680 km centroid depth) event with M W ≥ 7.8 in the seismological record. Seismicity indicates along-strike contortion of the Izu-Bonin slab, with horizontal flattening near a depth of 550 km in the Izu region and rapid steepening to near-vertical toward the south above the location of the 2015 event. This event was exceptionally well-recorded by seismic stations around the world, allowing detailed constraints to be placed on the source process. Analyses of a large global data set of P, SH and pP seismic phases using short-period back-projection, subevent directivity, and broadband finite-fault inversion indicate that the mainshock ruptured a shallowly-dipping fault plane with patchy slip that spread over a distance of ∼40 km with a multi-stage expansion rate (∼5+ km/s down-dip initially, ∼3 km/s up-dip later). During the 17 s total rupture duration the radiated energy was ∼3.3 × 10 16 J and the stress drop was ∼38 MPa. The radiation efficiency is moderate (0.34), intermediate to that of the 1994 Bolivia and 2013 Sea of Okhotsk M W 8.3 deep earthquakes, indicating that source processes of very large deep earthquakes sample a wide range of behavior from dissipative, more viscous failure to very brittle failure. The isolated occurrence of the event, much deeper than the apparently thermally-bounded distribution of Bonin-slab seismicity above 600 km depth, suggests that localized stress concentration associated with the pronounced deformation of the Izu-Bonin slab and proximity to the 660-km phase transition likely played a dominant role in generating this major earthquake.

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

“…on their point that the 2015 Bonin deep event occurred within the slab and close to the lower slab boundary, but we do not prefer their model which shows a flat slab in the MTZ at the 2015 Bonin hypocenter. The slab pileup model4 is similar to the first model of Ye et al 2. and is even more complex, which is quite different from our model (Fig.…”

“…Note that the hypocenter is just the initial point of the earthquake rupture. Waveform inversions show that the average source depth of this great deep event is ~680 km and its overall source dimension is ~40 km in a shallowly-dipping fault plane2.…”

“…Globally, it is the deepest event with M ≥ 7.8 in the seismological record2. It is an isolated event locating over 100 km deeper than the Wadati-Benioff zone seismicity recorded so far (Fig.…”

Sequence of deep-focus earthquakes beneath the Bonin Islands identified by the NIED nationwide dense seismic networks Hi-net and F-net

“…The detected sequence of M-6-class deep-focus earthquakes occurred after a large (Mw 7.9) deep-focus earthquake that occurred on May 30, 2015, at a depth of 680 km beneath the same region (e.g., Takemura et al 2016;Ye et al 2016). Although the hypocenter depth of the Mw 7.9 earthquake was approximately 150 km deeper than those of typical deep-focus earthquakes (400-520 km), the seismicity of typical earthquakes increased after this earthquake (bottom subfigure of Fig.…”

Unusual Strong Ground Motion Across Japan From the 680 km Deep 30 May 2015 Ogasawara Islands Earthquake

“…The source‐slip model for the 2015 Ogasawara Islands earthquake employed in the present simulation is based on the results of the source‐rupture inversions of Ye et al () exploiting waveform analysis of teleseismic data. The source is approximated by two rectangular fault planes with sizes of 25 km by 25 km and 9 km by 9 km; the separation between the source segments is 32 km to the west and 8 km in depth.…”