Detailed structures of the subducted Philippine Sea Plate beneath northeast Taiwan: A new type of double seismic zone

Abstract

We studied the detailed structure of the subducted Philippine Sea plate beneath northeast Taiwan where oblique subduction, regional collision, and back arc opening are all actively occurring. Simultaneous inversion for velocity structure and earthquake hypocenters are performed using the vast, high‐quality data recorded by the Taiwan Seismic Network. We further supplement the inversion results with earthquake source parameters determined from inversion of teleseismic P and SH waveforms, a critical step to define the position of plate interface and the state of strain within the subducted slab. The most interesting feature is that relocated hypocenters tend to occur along a two‐layered structure. The upper layer is located immediately below the plate interface and extends down to 70–80 km at a dip of 40°–50°. Below approximately 100 km, the dip increases dramatically to 70°–80°. The lower layer commences at 45–50 km and stays approximately parallel to the upper layer with a separation of 15±5 km in between down to 70–80 km. Below that the separation decreases and the two layers seem to gradually merge into one Wadati‐Benioff Zone. We propose to term the classic double seismic zones observed beneath Japan and Kuril as “type I” and that we observed as “type II,” respectively. A global survey indicates that type II double seismic zones are also observed in New Zealand near the southernmost North Island, Cascadia, just north of the Mendocino triple junction, and the Cook Inlet area of Alaska. All of them are located near the termini of subducted slabs in a tectonic setting of oblique subduction. We interpret the seismogenesis of type II double seismic zones as reflecting the lateral compressive stress between the subducted plate and the adjacent lithosphere (originating from oblique subduction) and the downdip extension (from slab pulling force). The upper seismic layer represents seismicity occurring in the upper crust of a subducted plate and/or along the plate interface, whereas the lower layer is associated with events in the uppermost mantle.