1930年および1976年前後の伊豆東方線の活動

Abstract

The crustal upheaval amounting to 10cm for latest two years in the Izu peninsula, Japan, was detected by the Geographical Survey Institute in March, 1976. This upheaval developed afterward laterally to south-western direction from Ito to Shimofunabara through Hiyekawapass. The upheaval was associated with small and microearthquake activities whose epicenters are restricted within the upheaved region. In order to interpret this event, the author assumes slow fault movement at depth along the downward extension of thrust fault on the East-off-Izu tectonic line. The propagating fault movement at different depth and horizontal position can provide a model for migrating crustal movement.Preceding the 1976 crustal upheaval, the crustal upheaval of 1cm in 1971 at Oiso and of 2cm in 1972 at Manazuru occurred. Furthermore, the observed crustal upheaval associated with the 1974 Izu-hanto-oki earthquake can not be interpreted by the calculated crustal movement assuming the right-lateral strike slip faulting in Minami-izu and is rather suggestive for creep dislocation at depth. Therefore, we can presume a sequence of propagating crustal upheaval Oiso-Manazuru-Ito-Minami-izu.On the other hand, the 1923 Kanto earthquake was related mainly to the right lateral faulting along the Sagami fault and also to the faulting along northern part of the East-off-Izu tectonic line. After the 1923 event, there occurred the Ito earthquake swarm and the Kita-izu earthquake in 1930.The sequence of the Ito earthquake swarm and the crustal activities in 1930 are very similar to the one of the 1976 crustal upheaval and the microearthquake activities. For both crustal activities, large or moderate earthquake were associated during the course of rapid accerelating upheaval and small earthquakes were followed during subsequent crustal subsidences.Based on these facts, we can conclude that creep dislocation propagation were recurred around 1930 and 1976. These aseismic slip at depth along the East-off-Izu tectonic line will release some part of crustal strains that are stored on the northern boundary of the Philippine Sea Plate.