Crustal Structure of Shikoku, Southwestern Japan as Derived from Seismic Observations of the Iejima and the Torigatayama Explosions

Abstract

Explosion seismic observations were made for the investigation on the crustal structure of Shikoku along a profile in NE-SW direction crossing the Median Tectonic Line (MTL), one of the most pronounced tectonic lines in Japan. Seismic waves generated by the Iejima explosion of 3500kg in charge amount and the first Torigatayama explosion of 2400kg were observed at sixteen temporary stations in January, 1977 and those by the second Torigatayama explosion of 4100kg, at thirteen temporary stations in February, 1978. A detailed shallow velocity structure in relation to the MTL was obtained from clear refracted waves through the granitic layer. The first layer shows an abrupt change of its thickness at the MTL. The layer is about 4km thick to the northeast of the MTL, but it might almost disappear in the immediate southwest of the MTL. Apart from the MTL towards southwest it tends to thicken. The deeper structure up to the upper mantle was obtained by making use of clear later arrivals such as reflected and refracted waves besides a small number of first arrivals at long distances. The granitic layer with a velocity of 6.1km/s has an average thickness of about 20km and is underlained by the basaltic layer with a velocity of 6.7km/s. The velocity of the upper mantle is 7.8km/s and the depth of the Moho discontinuity is 30-35km.