Abstract
We present a new method to obtain in-situ seismic wave velocities in the source region of shallow earthquakes. Travel times of initial P- and S-waves of earthquakes occurring in a small area are analyzed. First, assuming a certain P velocity model, hypocenters of earthquakes are determined using first P arrival times only at near stations. Second, the velocity in the source region is calculated by the regression analysis of travel times of earthquakes observed at each distant station. Third, we compare it with the assumed velocity there. The above two steps are repeated until they agree with each other. We have applied this method to subcrustal earthquakes with depths of 23-38 km in two small regions in the Tokai district, and P and S velocities of 6.9-7.0 and 4.0 km/s were obtained, respectively. Since these values are quite similar to those in the main layer of the oceanic crust, we can conclude that subcrustal earthquakes in these regions occurred within the subducted oceanic crust of the Philippine Sea plate. Moreover, apparent P and S velocities of 7.2-7.3 and 4.1-4.2 km/s were also obtained. Thus we can also conclude that no mantle substance exists above the subducted oceanic crust there.
