VELOCITY STRUCTURE BENEATH THE HIDAKA MOUNTAINS IN HOKKAIDO, JAPAN

Abstract

One- and three-dimensional velocity structures down to 80 km beneath the Hidaka Mountains in Southern Hokkaido, Japan, were derived by a simultaneous inversion of the P-arrival time data from local earthquakes. Arrival time data for the derivation were obtained from the observation of earthquakes recorded at 27 seismic stations. In order to obtain an appropriate starting P-wave velocity structure and hypocentral parameters to be used in three-dimensional inversions, one-dimensional inversions were first carried out. The three-dimensional inversions were made with a simplified algorithm for the calculation of travel times and their partial derivatives with respect to the hypocentral parameters.One of the most significant features of the three-dimensional velocity structure obtained is the existence of an inclined low velocity zone. This zone has P-wave velocities that are as much as 10% slower than the velocities in the surrounding regions. This low velocity zone is 20-25 km thick. It is located beneath the area from the western to the central portion of the Hidaka Mountains, extends in depth from 10 to 65 km, and possesses a large dip angle of 60°. A remarkably high velocity zone is also found to stretch in a strikingly northwest-southeast direction just beneath the crest of the Hidaka Mountains. A portion of this high velocity zone is found to extend to the south-west so as to overlie the low velocity zone. The thickness of the high velocity zone is estimated to be about 10 km.Comparison of the obtained three-dimensional velocity structure and the seismicity shows that the low velocity zone is seismicly active, while the high velocity zone is relatively inactive.The small value of P-wave velocity in the inclined low velocity zone, a value which was estimated to be only 7 km/s at a depth of 60 km, allows us to infer that the material in the low velocity zone originates from the crust. This inference leads to the possibility that the inclined low velocity zone is subducted crust resulting from collision between the Kurile and Northeastern Japan arcs. The three-dimensional velocity structure also suggests the possibility that the descending Pacific plate has been deformed so as to be confined to the region below the inclined low velocity zone.