Seismic wave conversion near the upper boundary of the Pacific plate beneath the Kanto district, Japan

A detailed structure of the Pacific plate beneath the Kanto district, Japan, was studied by the use of conspicuous SP converted waves that are generated near the upper boundary of the Pacific plate. These SP waves are observed for earthquakes occurring in the lower plane of the double seismic zone of the subducted Pacific plate. The location of the conversion interface is estimated by inverting the observed TSP-P times. 293 seismic traces recorded at 42 stations for 49 earthquakes are used for analysis. The conversion interface is located above the upper seismic plane of the double seismic zone and is equivalent to the upper boundary of the Pacific plate within the estimated uncertainty. In general, the conversion interface coincides well with the zone of low-angle thrust-fault-type earthquakes in the entire area studied. Comparison with the result of 3-D tomographic inversion reveals that the conversion interface is probably located in the low-velocity layer on the top of the Pacific plate, which is interpreted as the oceanic crust of the Pacific plate.