Anomalous Movements in Oshima Volcano Associated with the Off Ito Submarine Eruption Revealed from GPS Measurements.

Abstract

A geodetic network consisting of four stations with 50-200km baseline lengths was constructed in 1988 in the Kanto-Tokai districts of central Japan, and has been surveyed repeatedly using the Global Positioning System (GPS) in order to observe the crustal movements. In this paper we report the results of the GPS measurements from February 1989 through July 1990. The short-term repeatability of baseline vectors is a few parts in 107 for horizontal components and better than 106 for vertical components. The short-term repeatability will be improved by using precise satellite orbit model and more advanced tropospheric model than the present one. The results of GPS observation show us that a point near Mihara Volcano in Izu-Oshima, Japan, subsided abruptly in July 1989 about 10cm during a half month and recovered to the former level in November 1989. This anomalous change is likely to have a connection to the Off Ito earthquake swarm and volcanic eruption activities in July 1989. It is a future problem to confirm this connection and to clarify that mechanism.