Seismological evidence for the existence of anisotropic zone in the metastable wedge inside the subducting Izu‐Bonin Slab

Abstract

The mechanism of deep earthquakes has been a puzzle since their discovery almost 70 years ago. Experimental observations at high‐pressure have reported faulting caused by spinel‐filled anti‐cracks in metastable olivine and suggesting that the anticrack mechanism is responsible for deep earthquakes. We compared the shear‐wave splitting of earthquakes from the upper and lower planes of the double seismic zone to detect anisotropy caused by the faultings inside the metastable wedge. We detected the anisotropic zone inside the metastable wedge at the depth of about 340–390 km, which corresponds to the depth range of a second peak on the histogram of earthquakes. The observed polarization direction, east‐west, is almost parallel to the direction of subduction, which can be explained by a view that the shear‐wave splitting is caused by the en echelon arrays of spinel‐filled anti‐cracks and anticrack‐associated faultings.