Velocity structure of the crust beneath the northeastern part of Honshu, Japan, as derived from local earthquake data.

The P-wave velocity in the crust beneath the Northeastern part of Honshu, Japan, is estimated by using the arrival-time data of initial P waves from local earthquakes. To determine simultaneously hypocenter parameters, station corrections, and the parameters of the velocity model, the least-squares method developed by Crosson is applied to two different data sets. Firstly for the data set consisting of all of events well beneath the land, the estimated velocities are 6.0-6.1km/s for the upper 12km of the crust, and 6.3-6.4km/s for the intermediate crust between 12-and 20-km depths. The velocity of the lower crust remains unsolved since it depends on the Moho depth assigned. If the Pn velocity is assumed to be 7.53 km/s as previously obtained for the Kesennuma-Oga profile, however, some restrictions are put on the velocity of the lower crust; it would be 6.6km/s or greater if the Moho depth is greater than 33km. A low-velocity zone would be present in the lower crust if the Moho is positioned at a depth of 30km or less. Secondly for the data set consisting partly of events located within a narrow zone near the Aseismic front, the inverted velocities for the upper and intermediate crusts are similar to those for the first set. However, the inverted velocity of the lowermost half space is 7.8km/s, a value appreciably greater than 7.53km/s. This result indicates that the velocity for the very upper mantle just beneath the Aseismic front is at least greater than 7.8km/s. Although the change of Pn velocity from 7.7 to 8.1km/s may take place at the Aseismic front, a low Pn velocity of 7.53km/s seems to take place a few tens of kilometers to the west of it.