Abstract
Recent studies on the velocity structure beneath the Kanto-Tokai District, Honshu, Japan, pointed out the large lateral heterogeneity of velocity structure. This heterogeneity is large enough to suspect that hypocenters determined by a laterally homogeneous velocity model are systematically deviated from true locations.
Computation of travel times for a laterally inhomogeneous velocity model by three-dimensional (3-D) ray tracing revealed that travel time residuals of -1.0 to 1.2 s are produced if the lateral inhomogeneity is neglected. Residual distribution varied according to hypocenter locations. This indicates that a laterally homogeneous velocity model with station corrections is not sufficient to obtain accurate travel times in this district.
According to a numerical experiment, the resultant deviation in hypocenter location is up to +-5 km and +-8 km for epicenter and focal depth, respectively. For obtaining reliable hypocenter distribution, about 10, 000 hypocenters of earthquakes which were observed by the stations of the National Research Center for Disaster Prevention for the period from 1983 to 1985 were relocated by using the 3-D velocity model. Shape of earthquake clusters became clearer after the relocation. For the computation of hypocenters, we used an approximate method of 3-D ray tracing, which gives a sufficient approximation of the exact ray tracing.
