(1)Earthquakes near the plate boundary from Sagami Bay to off the southeast Coast of the Boso Peninsula


The Philippine Sea Plate is subducting under the Kanto region from the Sagami Trough, which extends from Sagami Bay to an area off the southeast of the Boso Peninsula (Fig.5-6).

There are two types of earthquakes that occur near the plate boundary from Sagami Bay to this area southeast of the Boso Peninsula: first, interplate earthquakes occurring due to the slipping movement at the boundary of the subducting Philippine Sea Plate and the plate on land; and second, earthquakes that occur within the subducting Philippine Sea Plate.

Earthquakes related to the subducting Philippine Sea Plate occur even in somewhat deeper areas on land in the Kanto region. There is an explanation of these earthquakes in 5-1 (3).

1) Interplate earthquakes caused by the subducting Philippine Sea Plate

Known examples of this type of earthquake with M 8 or so include the 1923 Kanto Earthquake (M 7.9, also known as the Great Kanto Earthquake) and the 1703 Genroku Earthquake (M 7.9-8.2). These earthquakes occur in locations close to land, and part of the source region of these earthquakes may actually lie inland. These large earthquakes generate extremely strong ground motion, primarily in the southern Kanto region. They have caused significant damage, collapsing houses and causing fires. This type of earthquake occurs in relatively shallow locations underneath the sea floor. As a result, fault movement causes crustal deformation (upheaval and subsidence) on the sea floor and generates large tsunami, that can seriously damage coastal areas. Also, the source region of these earthquakes is near land, so it is possible that the tsunami will strike land immediately after the earthquake.

2) Earthquakes within the subducting Philippine Sea Plate

The Philippine Sea Plate is subducting from the Sagami Trough toward the Kanto region, but destructive earthquakes are not known to occur within plates in relatively shallow locations. An explanation of earthquakes that occur in relatively deeper locations that are extensions of the landmass is found in 5-1 (3).

The Izu Peninsula located on the Philippine Sea Plate has similar characteristics to the land plate. Therefore, it can not be subducting under the plate and appears to collide with the Japanese archipelago. Thus, the force is rather complex in such areas as the Izu Peninsula and western Kanagawa Prefecture, and there is substantial seismic activity. (For details on earthquakes in western Kanagawa Prefecture, refer to 5-1 (3) and 5-3 (7); for details on the earthquakes in the Izu Peninsula, refer to 6-1 (2) and 6-2 (6)).