CRUSTAL STRUCTURE IN THE PROFILE ACROSS THE SOUTHERN PART OF HOKKAIDO, JAPAN, AS DERIVED FROM EXPLOSION SEISMIC OBSERVATIONS

Abstract

The first arrival data from the explosions of 1968 in the sea off Cape Erimo and of 1969 in the sea off the Shakotan Peninsula and at the Teine quarry, have been analyzed to derive the crustal structure in the profile between Cape Erimo and the Shakotan Peninsula in the southern part of Hokkaido, Japan. In these explosions, the fixed-array moving shot point technique was applied and the records were obtained from observing stations on land with magnetic-tape-recording systems.
Most of the records from the shots off Cape Erimo were not used for the derivation of the crustal structure. The quality of records from the shots off the Shakotan Peninsula was excellent at the stations northwest of the Ishikari-Yufutsu plain, but it was fairly poor at the other stations. The time-term method was applied only to data of the highest possible quality, so that the crustal structure was obtained only in the northwestern half of the profile. Under the explosion site in the Sea of Japan, the crust, consisting of granitic and basaltic layers with nearly the same thickness, is about 17km thick at a point about 50km distant from the shoreline. The crustal thickness increases abruptly toward land and reaches about 30km at a point 40km distant from the shoreline, where the basaltic layer is about twice the thickness of the granitic layer. The velocities obtained in each layer in the crust and in the upper mantle are fairly low, but the gravity anomalies calculated by assuming the densities expected from these velocities agree well with the observed gravity anomalies.