A New Approach to Study the Upper-Mantle Seismic Discontinuities Based on Triplication Data: Application to the Kuril Subduction Zone Using Hi-net Array

Abstract:

Constraining seismic properties of the 410- and 660-km discontinuities is crucial for understanding the mantle composition and dynamics. One approach to study the transition zone is to use the “triplicated” arrivals of seismic data. When a wavefield encounters an abrupt increase in wave speed with depth, three different phases, direct, reflected, and transmitted phases, result, and they can arrive at the same distance, producing the triplicated arrivals. Properties of the triplication pattern provide constraints on the depth and velocity jump of the discontinuities.

One of the challenging aspects of using the triplication data, however, is to identify the three individual phases, since they arrive close in time and the waveforms often overlap. In order to separate the phases, we apply Radon transform to the data. Based on the transformed data, the caustics are identified, and the depth and the velocity jump of the discontinuities are obtained.

This method is applied to study the Kuril subduction zone, beneath a region northeast of Japan. We take advantage of the High-Sensitivity Seismograph Network in Japan that consists of more than 700 stations, to capture the triplication pattern with dense sampling in distance. Given the distribution of the stations, deep and intermediate earthquakes in the Sea of Okhotsk region that are located within the triplication distance range are considered. We constrain the percentage velocity jump and depth of the 410- and 660-km discontinuities. The discontinuity depth estimates show significant deviations from the global average, but are, in general, comparable to the SS precursors and tomographic studies of the region.