5 Characteristics of seismic activity in the Kanto region


The destructive earthquakes of the Kanto region are well known, including as they do the 1923 Kanto Earthquake (M 7.9, also known as the Great Kanto Earthquake) that occurred near Sagami Bay (Sagami Wan). This earthquake resulted in more than 140,000 dead or missing. Many other destructive earthquakes have occurred in this region in the sea and on land, causing damage from ground motion or the tsunami. The oldest known of these is the 818 earthquake in Kanto Shokoku (Provinces) (larger than M 7.5). Others include the 1703 Genroku Earthquake (M 7.9-8.2), the 1855 Ansei Edo (former name of Tokyo) Earthquake (M 6.9), the 1894 Meiji Tokyo Earthquake (M 7.0), and the 1931 West Saitama Earthquake (Nishi-Saitama Earthquake) (M 6.9). Most of these earthquakes caused destruction primarily in the southern Kanto region, but destructive earthquakes have also occurred in the northern Kanto region, including the 1949 Imaichi Earthquakes (M 6.2 and M 6.4). In addition, earthquakes in surrounding areas have indirectly caused damage in the Kanto region, such as those occurring near the Izu Peninsula (Izu Hanto) or off the coast of Tokai District. Also, tsunami generated by earthquakes overseas has caused damage, such as the 1960 Chile Earthquake Tsunami. The primary destructive earthquakes known to have taken place in the Kanto region are shown in Fig.5-1, Fig.5-2, and Fig.5-3.

Seismic activity in the Kanto region is roughly divided into three types. First are earthquakes that occur near the Pacific Plate boundary which inclines toward land from the Japan Trench in the open sea east of the Kanto region. Second are earthquakes that occur near the Philippine Sea Plate boundary which inclines toward land from the Sagami Trough in the area from Sagami Bay to the open sea off the coast of the Boso Peninsula (Boso Hanto). Third are earthquakes that occur in shallow locations on land. As we will explain later, the plate boundaries underneath the Kanto region have a complex structure. Earthquakes that occur near plate boundaries are often of the same type that occur in deep locations on land. Therefore, the Kanto region is one of those in Japan with an extreme amount of seismic activity. Both earthquakes that occur in shallow land locations due to active faults and earthquakes that occur due to subducting plates can occur directly beneath urban areas (Fig.5-9).

The Pacific Plate is approaching the Kanto region from the east-southeast at a rate of about 8 cm a year. This plate is subducting in the direction of the Kanto region from the Japan Trench. The seismic activity in this area accompanying the subduction of the plate has been observed to a depth of about 200 km. This activity continues into the area beneath the Chubu and Kinki regions (Fig.5-5, Fig.6-4). Meanwhile, the Philippine Sea Plate is approaching the Kanto region from the southeast at a rate of about 3 to 4 cm a year. It is subducting toward the northwest as if to wedge between the land plate extending from the Sagami Trough and the Pacific Plate. The seismic activity accompanying the subduction of the Philippine Sea Plate can be observed to a depth of about 100 km in the northern Kanto region (Fig.5-6). The subduction of these two plates creates a complex structure, in which the plates abut beneath the southern Kanto region. Also, the Izu Peninsula located on the Philippine Sea Plate is colliding with the Japanese archipelago. Hence, the boundary between the Philippine Sea Plate and the plate on land is just 10-30 km below the surface of the earth in the southern Kanto region. As a result, the southern Kanto region, like the Tokai region, has the potential to become the source region of great interplate earthquakes extending to relatively shallow areas directly beneath urban districts (Fig.5-9).

The topography of the Kanto region is characterized by the broad Kanto Plain (Kanto Heiya) surrounded by mountainous and hilly areas, including the Kanto Mountains (Kanto Santi). There are many active faults in the southern part of the Kanto region, particularly in the southern Boso Peninsula, the southern tip of the Miura Peninsula (Miura Hanto), and western Kanagawa Prefecture. Several active faults are also known to exist in the central part of the Kanto Plain. Many of these have been covered by sediment in relatively recent times, so their topography in many cases is not yet clear. Fig.5-7 provides bird's eye views of the topography and distribution of the active faults in the Kanto region from the south and the southeast. The southern part of the Kanto region is known to have more crustal movement than the northern part. This movement is thought to be related primarily to the subducting Philippine Sea Plate (Fig.5-8).

Recent seismic activities in the Kanto region and surrounding area include frequent earthquake swarms in areas from the Izu Islands to the Izu Peninsila, and many others such as the 1978 Earthquake near the Coast of Izu Oshima (Izu Oshima Kinkai Earthquake) (M 7.0), the 1980 Earthquake off the East Coast of Izu Peninsula (Izu-hanto Toho-Oki Earthauake) (M 6.7), and the 1990 Earthquake near the Coast of Izu Oshima (Izu Oshima Kinkai Earthquake) (M 6.5). Seismic activity in the area surrounding the Boso Peninsula has included the 1987 Earthquake off the East Coast of Chiba Prefecture (Chiba-ken Toho-Oki Eartquake) (M 6.7) and the 1996 earthquake off the Coast of Choshi (Tyosi) (M6.4). Brisk seismic activity regularly occurs once every few years in southwest Ibaraki Prefecture. This occurs at somewhat deeper locations of roughly 50 km, and generates earthquakes of M 5-6. Other activity includes the 1988 Earthquake in the eastern Tokyo Metropolitan District (M 6.0, a depth of 96 km), and the 1992 Earthquake in the southern Tokyo Bay near the Uraga Channel (M 5.9, a depth of 92 km). No large earthquakes of M 7 or greater have occurred in the past several decades near the Sagami Trough, from the interior of the Kanto region and Sagami Bay to an area off the southeast of the Boso Peninsula. Fig.5-4 shows recent seismic activity on land in locations deeper than 30 km.