Seismic anisotropy evidence for dehydration embrittlement triggering intermediate-depth earthquakes

Jian Wang,Zhenxing Yao,Dapeng Zhao

doi:10.1038/s41598-017-02563-w

Abstract

It has been proposed that dehydration embrittlement of hydrous materials can trigger intermediate-depth earthquakes and form a double seismic zone in a subducting slab. Seismic anisotropy may provide a possible insight into intermediate-depth intraslab seismicity, because anisotropic properties of minerals change with varying water distribution, temperature and pressure. Here we present a high-resolution model of P-wave radial anisotropy tomography of the Japan subduction zone down to ~400 km depth, which is obtained using a large number of arrival-time data of local earthquakes and teleseismic events. Our results reveal a close correlation between the pattern of intermediate-depth seismicity and anisotropic structures. The seismicity occurs in portions of the Pacific and Philippine Sea slabs where positive radial anisotropy (i.e., horizontal velocity being faster than vertical one) dominates due to dehydration, whereas the inferred anhydrous parts of the slabs are found to be aseismic where negative radial anisotropy (i.e., vertical velocity being faster than horizontal one) dominates. Our anisotropic results suggest that intermediate-depth earthquakes in Japan could be triggered by dehydration embrittlement of hydrous minerals in the subducting slabs.

Highlights

Double seismic zones, where intraslab earthquakes occur in two distinct dipping planes at depths of ~50– ~250 km, are observed widely in subduction zones regardless of the oceanic plate age, convergence rate, or stress orientation[1,2,3]
The maximum ray-azimuthal gap angle[36] (MRAGA), which denotes the azimuthal coverage of ray paths around a grid node (Supplementary Fig. S3), is set to be 45° during inversions
Compared with our previous study[27], which shows a good resolution at depths

Summary

Introduction

Double seismic zones, where intraslab earthquakes occur in two distinct dipping planes at depths of ~50– ~250 km, are observed widely in subduction zones regardless of the oceanic plate age, convergence rate, or stress orientation[1,2,3]. The hypothesis of dehydration embrittlement is considered to be the leading mechanism, which suggests that earthquakes in double seismic zones are linked to brittle failure associated with dehydration reactions of hydrous minerals in the slab crust and uppermost mantle[9,10,11,12]. This popular hypothesis has been tested mainly by thermal-petrologic models and laboratory deformation experiments for hydrous minerals in subducting slabs[11, 13]. The mechanism causing seismic anisotropy in the deep upper mantle is still in debate, anisotropic structures may exist at least down to the mantle transition zone[29] and the lower mantle[30]

Methods

Results

Discussion

Conclusion