Upper-mantle seismic structure beneath a region northeast of Japan based on Hi-net array triplication data

Understanding upper mantle discontinuities and properties of the mantle transition zone gives important insights into mantle composition and dynamics. One approach to study the transition zone is to use triplicated arrivals of seismic data that allow one to infer depths and velocity jumps of the discontinuities. However, this approach has been unpopular compared to other techniques, such as that using underside reflections, converted phases, or seismic tomography, because the triplication pattern is difficult to capture using sparsely distributed stations.

We take advantage of dense network of stations and examine triplication data from earthquakes at different depths to unravel the complex wave arrivals and determine the wave speed structure. Deep earthquakes (depth>410 km) do not generate triplication associated with the 410-km discontinuity, and are thus first investigated to constrain the 660-km discontinuity. Using this knowledge of the wave speed structure around 660-km discontinuity, shallower events are analyzed to resolve the seismic structure for the 410-km discontinuity. Therefore, constraints on each discontinuity can be obtained from separate sets of data without interference from the seismic structure around the other discontinuity.

We apply this technique to data recorded by the Japanese High-Sensitivity Seismograph Network with more than 700 densely spaced stations. Given the distribution of the stations, earthquakes in the Sea of Okhotsk region are chosen to investigate the seismic structure of the 410- and 660-km discontinuities beneath a region northeast of Japan. Since we focus on the regional one-dimensional seismic structure, the deep and shallow events at similar locations ensure that the data are sampling the same structure. Our results are consistent with previous studies on this region using SS precursors and tomographic imaging. They show significant deviations from global average in depths of the discontinuities due to subducting Pacific Plate, and imply local temperature effects on the depths of mineral phase changes.