Horizontal Crustal Strain in the Izu Peninsula-Mt. Fuji Region Derived from GEONET Data and its Tectonic Implication

Abstract

In order to understand the active tectonics in the Izu collision zone, the Pacific coast of central Japan, we analyzed the daily coordinate data from GEONET (GPS Earth Observation NETwork), which is a nationwide dense network of continuous GPS observation operated by Geographical Survey Institute (GSI), in the Kanto-Tokai districts surrounding the collision zone for the period from January 1997 to January 2003. In the collision zone the Izu Peninsula, the northern cusp of the Izu-Bonin volcanic arc, is colliding against Honshu as the northernmost part of the Philippine Sea plate, and an active volcanic chain is crossing the collision boundary. For six successive one-year intervals we obtained distributions of principal axes of horizontal crustal strain rate, dilatation rate, and maximum shear strain rate in the Kanto-Tokai districts based on velocity vectors at every 0.1 degrees which were interpolated from observed velocity vectors. We examined the distributions of dilatation rate in particular, and found that all remarkable seismic and volcanic activities in the study area were detected in the distributions, proving the reliability of analyzed results. The notable feature in the distribution of dilatation rate is the existence of positive areas around Mt. Fuji, the most remarkable volcano to the north-northwest of the Izu Peninsula. One possibility for the cause of the positive dilatation is an inflation of magma chamber beneath Mt. Fuji. The other possibility is the regional tectonic effect due to the collision process. The crustal strain in the northern part of the Izu Peninsula is rather small, whereas that in the southern part is considerably large, which seems to reflect the present stage of the Izu collision process.