THE UPPER-MANTLE P-WAVE VELOCITY STRUCTURE BENEATH THE NORTHERN JAPANARC, AS INFERRED FROM J-ARRAY DATA

The upper-mantle P-wave velocity structure beneath the northern Japan are is investigated based on the data recorded in the J-Array. The dat a set includes 260 seismograms and 1,817 travel time picks including 2 74 later arrivals from 33 events in the Kuril-Kamchatka region. Seismo grams are slant-stacked and iteratively downward continued to obtain t he upper-mantle structure, which well explains the observed data. The travel times and relative amplitudes of later arrivals are used to giv e additional fine constraints on the structure around upper-mantle dis continuities. The most prominent feature of the estimated model is the 410 km discontinuity, which is deeper (420 km) and has a larger veloc ity 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 equi librium phase change of olivine. The depth of 660 km and 5.6% velocity jump at the 660 km discontinuity are less reliable because correspond ing 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 a t depths near 540 and 780 km. Another major characteristic of the mode l is the lack of a low-velocity zone. fn the uppermost mantle, the vel ocity increases linearly at depths between 35 and 85 km and is almost constant between 85 and 130 km.