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Although the Median Tectonic Line (MTL) fault system in Shikoku, southwest Japan, is one of the most active faults during the late Quaternary in Japan, the present-day seismicity along the MTL is not so remarkable. However microseismic activities have recently been reported in some regions along the MTL in Shikoku. It has been pointed out that these activities are those at segment boundaries of the MTL fault system. We carried out a temporal microseismic observation in one of these regions, the region around borders between Ehime, Kagawa and Tokushima prefectures, in order to determine the earthquake hypocenters precisely so that we can discuss the hypocenter distribution in context of the tectonics of the MTL. The seismometer network consisted of six observation points and had its area extent of about 10 km by 10 km. Surface trace of the Ikeda fault goes through the network with its strike of N75E. About 10 km to the west of the network, there is a segment boundary of the MTL fault system which divides the Ikeda fault and the Ishiduchi fault. Hypocenters of 82 earthquakes are located during November 1990-August 1991. The seismic activity is high beneath the network, and no hypocenter is located near the segment boundary of the Ikeda and Ishiduchi faults. Most events located in the southern side of the MTL are in a depth range of 5-8km, and the distribution is almost flat or slightly inclines toward south from the MTL. The distribution pattern of the hypocenters in the northern side of the MTL is widely different from that in the southern side. The hypocenter depth increases with being away northward from the surface trace of the MTL. Although the number of events located is too few to conclude that the distribution is planar, the dip angle of about 50 is measured from a cross-sectional view on a vertical plane perpendicular to the strike of the MTL. The gap in the hypocenter distribution just beneath the MTL dipping almost vertically to the north, which has been reported by KIMURA and OKANo (1992) as an evidence that the MTL is near vertical, is not found in the present study.

Hypocenter Distribution of Microearthquakes around a Central Portion of the Median Tectonic Line in Shikoku, Southwest Japan