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 generat ed near the upper boundary of the Pacific plate. These SP waves are observe d for earthquakes occurring in the lower plane of the double seismic zone o f the subducted Pacific plate. The location of the conversion interface is estimated by inverting the observed TSP-P times. 293 seismic traces recorde d at 42 stations for 49 earthquakes are used for analysis. The conversion interface is located above the upper seismic plane of the do uble seismic zone and is equivalent to the upper boundary of the Pacific pl ate 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 co nversion 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 Pac ific plate.