8 Characteristics of seismic activity in the Chugoku-Shikoku region


The destructive earthquakes in the Chugoku-Shikoku region include the great M 8 level earthquakes that have recurred in the Pacific Ocean, and the large M 7 level earthquakes that have occurred on land. Of these, the great earthquakes that occur in the Pacific Ocean cause strong seismic ground motion over a wide area and large tsunami, which cause extensive damage. Since the Meiji period beginning in 1868, there has been the 1946 Nankai Earthquake (M 8.0) that occurred primarily in the Shikoku area and caused more than 1,000 fatalities. Destructive earthquakes that have occurred in shallow locations on land include the 1872 Hamada Earthquake (M 7.1) and the 1943 Tottori Earthquake (M 7.2). The Tottori Earthquake was a large earthquake that occurred directly beneath an urban area and caused extensive damage. Destructive earthquakes of M 7 or so have also occurred in the area near the Akinada Sea (Akinada) and the Iyonada Sea (Iyonada). Damage is also known to be caused in this region by earthquakes in surrounding areas, such as the Hyuganada Sea , by tsunami generated by earthquakes occurring in the eastern margin of the Sea of Japan (Japan Sea), and by tsunami generated by earthquakes occurring overseas, such as the 1960 Chile Earthquake Tsunami. Fig.8-1 and Fig.8-2 show the known primary destructive earthquakes in the Chugoku-Shikoku region.

Seismic activity in the Chugoku-Shikoku region can be roughly divided into two types. First are earthquakes that occur near the plate boundary subducting toward land from the Nankai Trough. Second are those occurring in shallow locations on land at depths of 20 km or less. Earthquakes also occur in somewhat deeper locations, such as the Akinada Sea and Iyonada Sea earthquakes from the western part of the Seto Inland Sea to the area near the Bungo Channel.

The Philippine Sea Plate is approaching the Chugoku-Shikoku region from the southeast at a speed of about 5 cm a year. This plate is subducting from the Nankai Trough in the Pacific Ocean toward the Chugoku-Shikoku region. Seismic activity accompanied the subduction of this plate occurs to depths of about 40 km on the north coast of central Shikoku (Fig.8-3A), but it is observed to occur in deeper locations to the west on the southern coast of the western Chugoku area (Fig.8-3B). Earthquakes in the somewhat deeper locations in the western part of the Seto Inland Sea are believed to occur because the subduction of the Philippine Sea Plate has reached this area.

One of the most prominent topological features of the Chugoku region is the Chugoku mountainous area stretching from east to west. These are generally gently sloping mountains. There are few active faults in this region, and records indicate few large earthquakes have occurred in shallow land areas here. This region is also characterized by slight crustal movement compared with the rest of the Japanese archipelago (Fig.8-5). However, two large earthquakes have occurred here since the Meiji period (beginning in 1868): the Hamada Earthquake and the Tottori Earthquake. There are some mountainous areas on Shikoku with extremely steep slopes, such as Mt. Ishizuchi and Mt. Tsurugi. At the northern edge of these mountains, the Median Tectonic Line(Chuo-kozosen), which is a major boundary in terms of geological structure, runs roughly east to west. These mountainous areas directly adjoin plains, such as the Niihama Plain (Niihama Heiya), with the Median Tectonic Line as the boundary. Also, narrow sections of plains (the Tokushima Plain(Tokusima Heiya)) generally enter the mountainous areas along this tectonic line. Thus, in the Shikoku region the Median Tectonic Line forms very pronounced topographical features, and is also an active fault with an extremely high level of activity. No other active faults of high activity are known to exist in Shikoku in addition to this one. Fig.8-4 provides a bird's eye view from the southeast and the northwest of the topography of the Chugoku-Shikoku region and the distribution of active faults.

A look at recent seismic activity in the Chugoku-Shikoku region and surrounding area shows that no destructive earthquakes have occurred along the Nankai Trough in the Pacific Ocean since the 1946 Nankai Earthquake. Several M 5 or 6 earthquakes have occurred at shallow locations on land in the Chugoku region, and resulted in local damage. Examples include the M 5 to 6 earthquakes that occurred in central Shimane Prefecture in 1977 and 1978 near Mt. Sanbe, the M 5 to 6 earthquakes that occurred near the border of Tottori and Shimane Prefectures in 1989 and 1991, the 1987 earthquake in central Yamaguchi Prefecture (M 5.2), the 1991 earthquake at the Suonada Sea (Suonada) (M 6.0), and the 1997 earthquake near the border of Yamaguchi and Shimane Prefectures (M 6.1). Since the 1955 earthquake in southern Tokushima Prefecture (M 6.4), no destructive earthquakes have occurred in shallow locations in the Shikoku region. In the areas surrounding Chugoku and Shikoku regions, there was the 1995 Southern Hyogo Prefecture Earthquake (Hyogo-ken-Nanbu Earthquake) (M 7.2), causing slight damage in Tokushima and Kagawa Prefectures. Earthquakes occurred in 1968 and 1979 at somewhat deeper locations in the western part of the Seto Inland Sea. These were M 6 level earthquakes, causing slight damage. Also, the 1968 earthquake at the Hyuganada Sea (M 7.5) resulted in damage in Kochi and Ehime Prefectures.