(2)Earthquakes occurring near the plate boundary off the east Coast of the Kanto region


The Pacific Plate is subducting toward the Kanto region from the Japan Trench and the Izu-Ogasawara Trench, which extend north to south along an offshore area east of the Kanto region (Fig.5-5).

There are two types of earthquakes that occur near the plate boundary off the open sea east of the Kanto region. First are interplate earthquakes occurring from the slipping movement at the boundary of the subducting Pacific Plate and the plate on land. Second are earthquakes that occur within the subducting Pacific Plate.

Earthquakes related to the subducting Pacific 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 Pacific Plate

Known cases of interplate earthquakes caused by the subducting Pacific Plate from the open sea east of the Kanto region to an area in the open sea off Fukushima Prefecture since the Meiji period (1868) include the 1909 (M 7.5) and 1916 (M 7.0) earthquakes in the open sea southeast of the Boso Peninsula and the 1938 Earthquake off the East Coast of Fukushima Prefecture (Fukushima-ken Toho-Oki Earthquake) (M 7.5). Many of these earthquakes occur in relatively shallow locations under the sea floor. Most cause crustal deformation (upheaval and subsidence) on the sea floor due to fault movement, and are accompanied by tsunami. While great earthquakes of M 8 or so sometimes occur in the Japan Trench in the open sea off the coast of Sanriku, no earthquakes of this magnitude have occurred in the open sea off the coast of Fukushima and Ibaraki Prefectures or further south since the Meiji period (1868), though both earthquakes were caused by the subducting Pacific Plate. An examination of historical accounts, however, shows that extensive damage was caused by a tsunami from Fukushima Prefecture to the coast of the Boso Peninsula and Hachijojima Island (Hatizyo Jima ) due to an M 8 earthquake in 1677 that is thought to have occurred in the open sea east of the Boso Peninsula. Details of the source region of this earthquake are unknown, however. This is the only destructive earthquake known from historical data that occurred in the open sea east of the Kanto region. Large M 7 earthquakes are known to occur further south along the Izu-Ogasawara Trench, including, for example, the 1972 Earthquake off the East Coast of Hachijojima Island (Hachijojima Toho-Oki Earthquake) (M 7.2). Great M 8 or so earthquakes are unknown to have occurred here.

2) Earthquakes within the subducting Pacific Plate

One destructive earthquake of this type was the 1953 Earthquake off the Coast of Boso (Boso-Oki Earthquake) (M 7.4). This earthquake is thought to have occurred on a normal fault in a shallow location near a spot where three ocean trenches meet (the Japan Trench, the Izu-Ogasawara Trench, and the Sagami Trough) and was accompanied by a tsunami. Few earthquakes of this type are known, and the mechanism for the old earthquakes classified as interplate earthquakes in the preceding paragraph 1) is not well known. Therefore, it is possible that some of them could be classified as earthquakes occurring within a subducting plate.