P and S Wave Travel Time Inversions for Subducting Slab Under the Island Arcs of the Northwest Pacific

Abstract

We have observed slablike high P and S velocity anomalies around the Wadati-Benioff zone under island arcs of the northwest Pacific through travel time tomographic inversions. Nineteen years of International Seismological Centre travel time residuals for events and stations in this large region are used. Analyses of resolution and noise show that the images are generally well resolved. The images illustrate that slab anomalies are continuous along strike in most parts of the upper mantle of the region and become contorted and generally broadened with depth. Near the bottom of the upper mantle, fingering of the slabs, including segmenting and spreading, is indicated. The fast anomalies associated with the Japan, Izu-Bonin, and Mariana subduction zones tend to flatten to subhorizontal at depth, while downward spreading may occur under parts of the Mariana and Kurile arcs. The fast anomalies below 700 km are not in the shape of a single coherent sheet. The principal compressional axes of focal mechanisms in the region consistently follow the downdip direction of the high-velocity slab, even when it bends to subhorizontal at depth. The depth at which compression begins to dominate the downdip stress regime in the slab apparently depends on bending of the slab and its dip. Slab fingering and intense deep seismicity probably are the consequence of the slab encountering a barrier of some form around the “670-km” discontinuity.