2002 Volume 54 Issue 4 Pages 507-520
In August 1999, we made a marine-land cooperative seismic experiment in the forearc region of the northeastern Japan to clarify seismic structure in the source region of the 1978 Miyagi-oki earthquake. At land seismic stations, we observed prominent later arrivals interpreted as the wide angle reflections from deeper part of the crust of the overriding NE Japan arc and also from the subducting Pacific plate. To locate these reflectors precisely, we inverted first arrival time data obtained at both land and ocean bottom stations for P wave velocity (Vp) distribution. Using the thus estimated Vp structure model, we mapped the seismic reflectors corresponding to the observed reflection arrivals. Among these reflectors, the Moho of the NE Japan arc was found to shallow towards the east, from 34km beneath the coastal line to about 20km at 70km off the shoreline. The estimated Moho depth beneath the coast is larger by about 5km than those in the previously presented crustal models and there is a strong intra-crustal reflector at Moho depths in these previously presented models. We also found two landward dipping parallel reflectors in the upper mantle, at a depth range of about 30-60km. The location of the shallow one coincides with the focal depth distribution of the interplate seismicity in this area, suggesting that this is the plate boundary, the rupture plane of the 1978 Miyagi-oki interplate earthquake. The deeper one, located less than 10 km deeper than the plate boundary, should be the Moho of the subducting oceanic crust.
