1994 年 103 巻 5 号 p. 528-536
We have studied recent crustal activities in and around the Izu Peninsula on the basis of crustal movements detected by geodetic surveys. Among them the 1986 eruption of Izu-Oshima and crustal uplift in the eastern Izu Peninsula since 1978 are characterized by magma intrusion.
Trilateration and leveling revealed a graben formation associated with the 1986 eruption of Izu-Oshima. Subsidence of average 30 cm relative to the Okata tide station, the northern tip of the island, was observed in the southeastern and northwestern parts of the island, while uplift20 cm was seen in the northeastern and southwestern parts. Furthermore the uplifted parts moved apart and the subsided parts were displaced toward the central cone, Mt. Mihara. We interpret these movements by a vertical tensile fracture model and obtain an opening of 2.7 m of 12 km × 10 km fracture running through the island from northwest to southeast. This model suggests a large scale magma intrusion at the eruption.
An anomalous uplift in the northeastern Izu Peninsula has been observed since 1976 accompanied by intensive seismic swarms. Since the occurrence of the Izu-Oshima earthquake on January 14, 1978, swarm activities have been concentrated east off the city of Ito. However the center of uplift was located south of the city of Ito. On the other hand, large crustal extension was observed across the source area of swarm. Submarine eruption occurred in the middle of the source area in 1989. We can interpret these events by a vertical tensile fracture model trending NW-SE as well. W e estimate an opening of 1 m of 20 km × 15 km fracture in the source area of swarm to explain the observed uplift and extension during 1978-1988. The top of the modeled fracture is estimated to be 3 km deep. We consider a 5 km × 5 km fracture situated closer to the land opened by 75 cm associated with the 1989 eruption. Its top margin is as shallow as 1 km for the 1989 eruption. Associated with the 1993 activity, the upward shift of the top margin of tensile fracture from 3 km to 2 km or less was suggested on the basis of temporal variations in side of the network near the source area. These change of depth of top margin of the fracture may mean the upward movement of magma. Therefore we would be able to evaluate the possibility of eruption by means of observation of crustal movements.