地震活動に関係した地殻変動(4) ―中部地方北西部の地震活動と尾小屋で観測された地殻変動―

Abstract

Continuous observations of crustal movements have been made with tiltmcters at the Ogoyaobservation station since 1948.The analysis of the observation for a long period suggests that the mean rate of groundtilts at Ogoya changed in the middle of 1959 coincidentally with the alternation of shallow earthquake sequence in the Chtibu district. The earthquakes of preceding sequence occurred in thezone of the Neodani-Fukui faulting system (A-region) and had the mechanism of quadrant type.On the other hand, the earthquakes of the following sequence had that of cone type or quadranttype with null vector inclined to the earth's surface and their generating area spread over theHida-Kitamino province (B-region) from the A-region.The most remarkable changes in the mean rate of the ground tilts appear in the direction ofN25ｰW. This direction is almost same to that of compressional axis of shallow earthquakes andmsnimum principal axis calculated from the results of the retriangulation in the northwesternpart of the Chtsbu district. Further, these seismo-tectonical axes seem to coincide with the com-pressional axis presumed from the geological and morphological evidences in the district.Besides, it is conceivable that the peculiar mode of ground tiltings related to the individualearthquakes appear principally in this direction and varies according to the alternation ofearthquake sequences.