Upper mantle velocity structure in the western Pacific rim estimated from short-period recordings at Matsushiro Seismic Array System

摘要

We investigate lateral variation of the upper mantle P-wave velocity structure in the western Pacific rim using travel times of P phases from regional shallow events. Onsets of direct and later arrivals are manually picked up from short-period waveforms at Matsushiro Seismic Array System in central Japan by consulting the correlation among the waveforms at seven array stations. For the path along Kurile and Alaska seismic zones, observed first arrivals of shallow events (d = 5 - 50 km) are well explained by a standard travel time table for Japan. Observations provide no strong evidence of a distinct low-velocity zone in the uppermost mantle, and triplicated branches are adequately explained by a slight modification of model 12 by Sugiyama and Nakanishi (1989, 1999). Observed travel times of direct P phases from events along the eastern and western Philippine Sea are slightly larger at around 15° and slightly smaller at around 25°, and the upper mantle is slower and the transition zone and uppermost lower mantle faster in the eastern and western Philippine Sea than in the northwestern Pacific. Such differences could reflect a high temperature in the upper mantle, and accumulation of subducted material at the base of transition zone as a result of the past subduction.