四国の中央構造線およびその周辺地域についての地震学的考察  [in Japanese] A Seismological Examination of the Median Tectonic Line and Its Surrounding Area in Shikoku, Southwest Japan  [in Japanese]

四国の中央構造線周辺地域に発生する地震の震源を再検討した上で, その震央ならびに深さ分布を求め, 構造線との関係について考察している。深さが5 kmより浅い地震の多くは構造線の南側にのみ観測されている。震源の深さ分布のずれからも, これは構造線を境にして南側(三波川変成帯)の上昇を示唆する。深さ8〜18 kmの地震では燧灘における活動が目立ち地形との関連が印象的である。下部地殻には地震が観測されないので, 23 kmより深い地震はすべて最上部マントル内のものであるが, この北傾斜で分布する地震は, 四国西部では構造線の制約を受けず構造線を越えて中国地方へ連続した分布を示す。しかし中東部では構造線の制約を受けて北側での活動は極めて低く, しかもその分布は中国地方に想定されるモホ面直下に水平に分布する。これらの地震を発生させる主圧力の方向は, 地殻内地震ではほぼ東西, マントル地震では南北である。

We have observed in Shikoku that earthquakes occur in the upper crust, mostly in the granitic layer (so called 6 km/s layer) and the upper most mantle, and that the former and latter earthquakes are generated by forces in the E-W and N-S directions, respectively. This paper geophysically examines the Median Tectonic Line (MTL), mainly based on the focal distribution of micro earthquakes. Crustal earthquakes shal lower than 5 km in focal depth scarcely occur in the northern side of MTL. This feature is also seen, somewhat obscure, for the focal distribution in a depth range of 5-8 km. Crustal earthquakes with depths of 8-23 km (not observed in the lower crust deeper than 23 km) do not show such a feature except in the eastern part of Shikoku. A vertical discrepancy of a few kilometers is observed between focal depth distributions of these crustal earthquakes in the northern and southern sides of MTL. The discrepancy is supposed to be produced by uplifting of the southern side (Sanbagawa Metamorphic Belt) against the northern one. Foci of earthquakes in the uppermost mantle distribute northwards with a dip angle of about 10°. The distribution continuing from the south terminates at MTL in the central and eastern parts with some exceptional foci in the northern side of MTL. Those exceptional foci do not locate at the extension of the above mentioned dipping distribution, but the earthquakes occur just beneath the horizontal Moho discontinuity at 35 km depth existing in the district north of MTL. In the western part, the dipping focal distribution of mantle earthquakes goes across MTL without any restraint. It is clear from these results that MTL has an effect on the occurrence of earthquakes. However it is not always in the same way throughout the regions along MTL. It is considerably probable that MTL extends to the Moho depth with a northward dip near vertical.