(1)Earthquakes near the plate boundary off the coast of Pacific Ocean


The Philippine Sea Plate is subducting toward the Chubu region from the Suruga Trough and the Nankai Trough, which lie in the Pacific Ocean off Suruga Bay (Suruga Wan) and the Chubu region (Fig.6-3). The Pacific Plate is subducting toward the Chubu region from the Japan Trench off the east coast of the Kanto region (Fig.6-4).

The earthquakes that occur near the plate boundaries off the coast of Pacific Ocean are classified as either interplate earthquakes caused by the subduction of the Philippine Sea Plate, or earthquakes that occur within the Philippine Sea Plate.

1) Interplate earthquakes caused by the subduction of the Philippine Sea Plate

Great earthquakes caused by the subduction of the Philippine Sea Plate have occurred along the Suruga Trough and the Nankai Trough.

This type of earthquake has caused damage from seismic ground motion over wide areas, including the Chubu region, and has caused the tsunami disasters on the Pacific Ocean coast from the Boso and Izu Peninsulas to Kyushu. Extensive damage has been caused in the Chubu region from these earthquakes, whose source region lie off the coast of the Kii Peninsula (Kii Hanto) and the Tokai region. Great earthquakes can occur whose source region ranges over a wide area from the Suruga Bay to western Shikoku, as with the 1707 Hoei Earthquake (M 8.4). This type of earthquakes cause severe damage over a wide area in the Chubu region. Such earthquakes have recurred in the past from the Suruga Trough to the Nankai Trough, and records of many those earthquakes exist in historical accounts. See 8-1 (1) for a detailed explanation of recurrence of this type of earthquake.

The forthcoming Tokai Earthquake, which has caused much concern, is an earthquake of this type. The hypothesized source region of this earthquake would be limited to the area along the western side of the Suruga Trough. In the Tokai region, crustal movement such as the subsidence of the west coast of Suruga Bay, has been confirmed to occur with the subduction of the Philippine Sea Plate (Fig.6-7).

2) Earthquakes within the subducting Philippine Sea Plate

Earthquakes occur within the Pacific Plate and the Philippine Sea Plate that are subducting under the Chubu region. These occur in deep locations, and are not thought to cause significant damage. The 1952 Yoshino Earthquake (M 6.8) occurred within the subducting Philippine Sea Plate at a depth of about 60 km in central Nara Prefecture. In the Chubu region, it caused slight damage in Aichi, Gifu (Gihu), and Ishikawa Prefectures. In addition, the March 1997 earthquake in eastern Aichi Prefecture (M 5.8) also occurred within the subducting Philippine Sea Plate, and a seismic intensity of 5 upper in JMA scale was observed in Toyohashi.