A SEISMIC WAVE VELOCITY ALONG THE SANBAGAWA METAMORPHIC BELT IN SHIKOKU AREA, SOUTHWESTERN JAPAN

Abstract

Explosion seismic observations were made in the Sanbagawa high-pressure metamorphic terrane, Shikoku area, Japan. The profile was nearly in the direction parallel to the trend of the metamorphic belt. In this direction a considerably high seismic wave velocity for the upper crust had been expected due to a large amount of basic rock on the surface and predominant schistosity. The observed velocity for the upper crust, however, was found to be around 6.1km/s, a value that is commonly observed in the continental area. The crossover-distance between a travel-time curve for the granitic layer and that for the basaltic layer or the upper mantle was greater than 150km. This fact may be interpreted, combined with the gravity data, as indicating that the thicknesses of the granitic and the basaltic layers under the terrane do not differ significantly from those of the normal continental structure.