The Upper-Mantle P-Wave Velocity Structure beneath the Northern Japan Arc, as Inferred from J-Array Data

Abstract

The upper-mantle P-wave velocity structure beneath the northern Japan arc is investigated based on the data recorded in the J-Array. The data set includes 260 seismograms and 1, 817 travel time picks including 274 later arrivals from 33 events in the Kuril-Kamchatka region. Seismograms are slant-stacked and iteratively downward continued to obtain the upper-mantle structure, which well explains the observed data. The travel times and relative amplitudes of later arrivals are used to give additional fine constraints on the structure around upper-mantle discontinuities. The most prominent feature of the estimated model is the 410 km discontinuity, which is deeper (420 km) and has a larger velocity jump (around 8.0%) than those in previous studies for neighboring regions. This velocity jump and a transition width less than 10 km are consistent with the calculated or experimented properties of the equilibrium phase change of olivine. The depth of 660 km and 5.6% velocity jump at the 660 km discontinuity are less reliable because corresponding later arrivals are not picked. The 240 km width of the transition zone is not reconciled with the model of cold and olivine-rich mantle beneath subduction zones. The model has additional minor inflections at depths near 540 and 780 km. Another major characteristic of the model is the lack of a low-velocity zone. In the uppermost mantle, the velocity increases linearly at depths between 35 and 85 km and is almost constant between 85 and 130 km.