Abstract
On 30 May 2015 an isolated deep earthquake (~670 km, Mw 7.9) occurred to the west of the Bonin Islands. To clarify its causal mechanism and its relationship to the subducting Pacific slab, we determined a detailed P-wave tomography of the deep earthquake source zone using a large number of arrival-time data. Our results show that this large deep event occurred within the subducting Pacific slab which is penetrating into the lower mantle. In the Izu-Bonin region, the Pacific slab is split at ~28° north latitude, i.e., slightly north of the 2015 deep event hypocenter. In the north the slab becomes stagnant in the mantle transition zone, whereas in the south the slab is directly penetrating into the lower mantle. This deep earthquake was caused by joint effects of several factors, including the Pacific slab’s fast deep subduction, slab tearing, slab thermal variation, stress changes and phase transformations in the slab, and complex interactions between the slab and the ambient mantle.
Highlights
To date, several studies have investigated this unusual deep earthquake but obtained controversial results on its source location relative to the subducting Pacific slab
Map views of the Vp tomography are shown in Supplementary Figure S3
The low-V zones represent the source areas of arc magmatism and volcanism caused by a combination of fluids from the slab dehydration and corner flow in the mantle wedge driven by the plate subduction e.g. refs 11–14
Summary
Several studies have investigated this unusual deep earthquake but obtained controversial results on its source location relative to the subducting Pacific slab. Porritt and Yoshioka[4] used teleseismic receiver functions to study the mantle structure down to a depth of ~800 km beneath a seismic station above the 2015 Bonin deep event. Their results indicate that the base of the MTZ is at a depth of 690 km, i.e., ~10–20 km below the hypocenter of the 2015 deep event, and there are significant P-to-S wave conversions in and below the MTZ. 5. In this work we apply a multiscale tomographic method[5,6] to abundant P-wave arrival-time data recorded by world-wide seismic stations (see Supplementary Figure S1) including those from the dense seismic networks in Japan and East China (Fig. 2) to determine a high-resolution mantle tomography beneath the Izu-Bonin region. The target Bonin region (Fig. 2 and Supplementary Figure S2) is well sampled by the up-going and down-going rays of both the direct P waves and later phases[6,9]
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