Teleseismic evidence for lateral heterogeneities in the northeastern Japan arc

We have measured and analyzed 4211 P wave first-arrival times from 100 teleseismic events recorded by 67 high-sensitivity seismic stations in the northeastern Japan arc. The relative travel time residuals show regional variations of about 1.6 s. The mean relative residuals averaged for all 100 events produce a spatial pattern of early arrivals at stations near the Pacific coast and delayed arrivals in the central part of the island arc and along the Japan Sea coast. Large delayed arrivals are observed at stations in volcanic areas. This pattern is generally consistent with the heat flow distribution and the major structural features that have been determined by local and regional earthquake studies. Large early arrivals appear in northern Kanto and southern Tohoku for the rays from the teleseismic events in the southwestern direction. Using a simple and plausible model of plate subduction, we have shown that this anomalous pattern can be explained by the change of the geometry of the subducting Pacific plate beneath Kanto. The observed teleseismic residual distributions can be explained with a 3-D velocity model which includes the high-velocity subducting Pacific plate and low-velocity anomalies in the crust and mantle wedge beneath active volcanoes, suggesting that the 3-D model represents well the velocity structure in this region. These teleseismic observations provide a new piece of information on the significant lateral heterogeneities of seismic velocity structure in the northeastern Japan arc.