9 Characteristics of seismic activity in the Kyushu-Okinawa region


There are various types of destructive earthquakes in the Kyushu region. These include the earthquakes that occur at sea, such as the Hyuganada Sea , and earthquakes that occur at shallow locations on land. Earthquakes with M 7 or so frequently occur in the area near Hyuganada Sea. In addition to damage from seismic ground motion, damage is also caused by accompanying tsunami on the Pacific coast of Kyushu. Great M 8 or so earthquakes also occur in the area near the Nansei Shoto Islands (Nansei Syoto). One example was the 1911 earthquake (M 8.0) near Amami Oshima (Amami Osima). Many destructive M 6 to 7 earthquakes have occurred in shallow locations on land. Since the Meiji period (beginning in 1868) alone, these have included the 1889 earthquake at Kumamoto (M 6.3), the 1914 earthquake at Sakurajima Island (Sakurajima) (M 7.1), the 1922 earthquake on the Shimabara Peninsula (Simabara Hanto) (M 6.9), and the 1968 Ebino Earthquake (M 6.1). Damage is also known to be caused here by earthquakes in surrounding areas, such as the 1946 Nankai Earthquake (M 8.0). Additionally, tsunami generated by earthquakes in foreign countries have caused damage, such as the 1960 Chile Earthquake Tsunami.

The destructive earthquakes in the Okinawa region include those that occur in the open sea off the Pacific coast. The 1771 Yaeyama Earthquake Tsunami (M 7.4) reportedly killed nearly 12,000 people in the Sakishima Islands (Sakisima Syoto). Few records exist of destructive earthquakes occurring before the end of the 19th century on Okinawa Island (Okinawa To) and the Kerama Islands, but damage was caused by the 1911 earthquake near Amami Oshima (M 8.0). Also, damage has been known to occur in the Okinawa region from a tsunami caused by earthquakes overseas, such as the 1960 Chile Earthquake Tsunami. Fig.9-1, Fig.9-2 and Fig.9-3 show the known primary destructive earthquakes in the Kyushu-Okinawa region.

Seismic activity in the Kyushu-Okinawa region is roughly divided into two types. First are earthquakes that occur in the Pacific Ocean near the plate boundary subducting toward land from the Nankai Trough and the Nansei Islands Trench (Nansei Syoto Trench). Second are shallow earthquakes occurring on land at a depth of 20 km or less.

The Philippine Sea Plate is approaching the Kyushu-Okinawa region from the southeast at a rate of about 5 to 7 cm a year. This plate is subducting beneath the Kyushu-Okinawa region from the Nankai Trough and the Nansei Islands Trench. The seismic activity in the Kyushu region that accompanies this subduction occurs at a depth of about 60 km near the coastline from Miyazaki Prefecture to Kagoshima (Kagosima) Prefecture. This activity can be observed at a depth of almost 200 km in the western land areas (Fig.9-4). In the Okinawa region, the seismic activity that accompanies this subduction of the Philippine Sea Plate occurs at depths below 200 km.

The topography of the Kyushu region is characterized by volcanoes that bisect the island almost horizontally from east to west from Beppu Bay (Beppu Wan) to the Shimabara Peninsula. These volcanoes include Mt. Kuju, Mt. Aso, and Mt. Unzen. Many short active faults also lie in an east-west direction. These active faults are normal faults that consistent with extension in a north-south direction there, and move in a direction that causes the surface to subside. This is a unique zone in Japan, where force is usually exerted to cause compression (reverse faults and strike-slip faults). The crustal movement also extends in a north-south direction (Fig.9-7). This zone is called the Beppu-Shimabara rift valley. Shallow earthquakes on land frequently occur in this zone or in the surrounding area. The Okinawa Trough runs parallel to the Nansei Shoto Islands at the sea floor on the northwest side (East China Sea side) of the islands, which correspond to an extension of the Beppu-Shimabara rift valley to the southwest. Seabed surveys show that this zone has many normal faults. Many of the earthquakes that occur in the Okinawa Trough are shallow earthquakes on normal faults, as are those in the Beppu-Shimabara rift valley. There are volcanoes in the southern part of the Kyushu region, including Kirishima and Sakurajima. Seismic activity is also seen in this area. Active faults can be found on the Tanegashima Island, the Yakushima Island and at the southern part of Okinawa Islands, and on the Ishigaki Island (Isigakijima), but no earthquakes are known to occur on these active faults. Fig.9-5 and Fig.9-6 provide bird's eye views of the topography and distribution of the active faults in the Kyushu-Okinawa region from the southeast and the northwest.

A look at the recent seismic activity in the Kyushu-Okinawa region shows that a destructive earthquake (M 7.1) occurred in 1984 in the Hyuganada Sea area. In 1987, another earthquake (M 6.6) in the Hyuganada Sea resulted in one fatality, while others in October and December 1996 (both M 6.6) caused slight damage. Earthquakes of M 6.6 and M 6.5 occurred in October 1995 near Amami Oshima, causing slight damage on Kikaijima Island and other locations. Tsunami accompanied these earthquakes. Several destructive earthquakes occurred in shallow locations on land. These include the 1975 earthquakes at the north edge of Mt. Aso (Maximum M 6.1), the earthquake in central Oita Prefecture in the same year (M 6.4), the 1984 earthquake swarm on the western area of the Shimabara Peninsula (Maximum M 5.7), the 1994 earthquake in northern Kagoshima Prefecture (M 5.7) and the 1997 earthquakes in northwestern Kagoshima Prefecture (M 6.3, M 6.2). Earthquake swarms began in September 1992, primarily in northwestern Iriomotejima Island (Iriomote Jima). An earthquake of M 5.0 in October of that year caused slight damage.