南部フォッサマグナ・フィリッピン海プレート北縁の地震活動の特徴

Abstract

Recently, an idea was proposed that the eastern Japan Sea deformation zone represents a nascent convergent zone between the North American and Eurasian plates. This supposed convergent plate boundary extends southwards through the Fossa Magna, a colliding sector, to the Suruga Bay forming a triple junction near Mt. Fuji or the Kofu Basin where the three convergent boundaries among Eurasian, North American and Philippine Sea plates meet. In order to evaluate this idea, the seismic activity including focal mechanisms of large and small earthquakes in the South Fossa Magna and the northern margin of Philippine Sea plate are examined.The subducting Philippine Sea plate, descending northwestwards along the Suruga and Sagami troughs was well defined by using the hypocenters and focal mechanisms of high resolution and accuracy. However, the plate boundary passing through the inland region, the South Fossa Magna, between the Sagami and the Suruga troughs was not defined due to poor seismic activity. On the basis of available seismic data, the newly proposed convergent plate boundary passing through the South Fossa Magna could not be found.