(1)Earthquakes near the plate boundary off the coast of Pacific Ocean


The Philippine Sea Plate is subducting underneath the Kyushu-Okinawa region from the Nankai Trough and Nansei Islands Trench in the Pacific Ocean off the coast of Kyushu and Okinawa region (Fig.9-4).

There are two types of earthquakes that occur near the plate boundary from the Pacific Ocean to the coastal areas. First are interplate earthquakes caused by the slipping at the boundary of the subducting Philippine Sea Plate and the land plate. Second are those that occur in somewhat deep locations within the subducted Philippine Sea Plate.

1) Interplate earthquakes caused by the subduction of the Philippine Sea Plate

Many of the M 7 or so earthquakes that occur near Hyuganada Sea are interplate earthquakes caused by the subducting Philippine Sea Plate. Examples of these earthquakes are those occurring in 1961 (M 7.0), in 1968 (M 7.5), and in 1984 (M 7.1). Earthquakes in the M 7 or so occur in this area once every 10 years to several decades. There are no records of great M 8 or larger earthquakes, however. The earthquakes that occur near Hyuganada Sea result in damage caused by the ground motion on the coastline in this area. A tsunami also causes damage when the source region of the earthquake is shallow.

There are many remarkable examples of destructive earthquakes occurring near the Nansei Islands Trench. The 1771 Yaeyama Earthquake Tsunami (M 7.4) and the 1911 earthquake near Amami Oshima (M 8.0) occurred either when there was no network of seismic stations or the network was inadequate. It is unknown whether or not these were interplate earthquakes. At any rate, the large earthquakes that occur here are frequently accompanied by tsunami. Also, one theory holds that the tsunami accompanying the 1771 Yaeyama Earthquake was caused by a great landslide on the sea floor.

2) Earthquakes within the subducting Philippine Sea Plate

The two earthquakes near Amami Oshima in October 1995 (M 6.6, M 6.5) occurred in somewhat deep locations near a trench (20-40 km). These were earthquakes within the subducted Philippine Sea Plate. They occurred due to fault movement on a normal fault in which the subducted Philippine Sea Plate can break. They were accompanied by tsunami. It also has been suggested regarding the destructive earthquakes occurring near the Nansei Islands Trench in the past, that the 1911 earthquake near Amami Oshima (M 8.0) and others could have been of this type.

While rare, large earthquakes sometimes occur within subducted plates deep on land. Many of these earthquakes cause damage in a broad area. For example, the 1909 earthquake that occurred at a depth of about 150 km in western Miyazaki Prefecture (M 7.6) is known to have caused damage in distant Okayama and Hiroshima Prefectures.