1998 年 50 巻 4 号 p. 461-470
The source region of the 1946 Nankai earthquake is examined based on the recent studies of microearthquakes, focal mechanisms and the crustal structure in Shikoku, southwest Japan as well as the aftershock distribution just after the main shock. Although it is generally known that an aftershock distribution is nearly equal to the fault zone and the source area of tsunami, such an agreement has not been recognized in the case of the 1946 Nankai earthquake. In the present study, we point out the misinterpretation about the aftershock area and the focal plane in the previous works, and give a new interpretation indicating good agreement among the aftershock distribution, source area of tsunami, crustal movements, disaster area and fault region. In the previous works, the defect of the observation network in the west of Shikoku and the east of Kyushu was not taken into consideration on the estimation of the aftershock area. Furthermore, the focal plane was not properly estimated owing to misunderstanding of the crustal movements and the source area of tsunami. Since the focal plane exists in the focal layer in the uppermost mantle, we consider that the aftershocks occurred only in this layer and the crustal earthquakes were induced by stress release due to the occurrence of the main shock. Our microearthquake observations show that this layer has a thickness of about 5km from the west of Shikoku to the western part of the Kii peninsula and from the Median Tectonic Line in the north to the Nankai Trough in the south.