There are two types of earthquakes that occur near the plate boundary off the coast of Pacific Ocean. First are the interplate earthquakes caused by the slipping movement at the boundary of the subducting Philippine Sea Plate and the land plate. The other type covers those that occur within the subducting Philippine Sea Plate.

There are also earthquakes that occur at deeper locations of 200-400 km and which are related to the subducting Pacific Plate from the Japan Trench underneath the Japanese archipelago. These are considered unlikely to cause any significant damage in the Kinki region.

1) Interplate earthquakes caused by the subducting Philippine Sea Plate

Great earthquakes at the M 8 level are known to have recurred along the Nankai Trough from historical accounts. The source region for many of these earthquakes covers a wide area from the vicinity of the Nankai Trough in the Pacific Ocean to areas that incorporate parts of land. Therefore, in many cases, there is both damage caused by seismic ground motion over a wide area and damage by a tsunami along the Pacific Ocean coast. This tsunami damage has not been limited to the coastal areas of Mie or Wakayama Prefectures, and has even extended to the interior of Osaka Bay (Osaka Wan).

Historical accounts indicate that great earthquakes have occurred along the Nankai Trough from an area off the coast of Tokai to an area off the coast of Shikoku in roughly 100-150 year intervals.

The area in which these great earthquakes occur is somewhat defined. Those earthquakes whose source region lies only in the open sea from Shikoku to the Kii Peninsula are called "Nankai Earthquakes". Earthquakes whose source region lies further east are called "Tokai Earthquakes". The forthcoming Tokai Earthquakes, the occurrence of which cause great concern, are those with a source region along the Suruga Trough. Compared to the Tokai Earthquakes of the past, this source region is much smaller.

Great earthquakes along the Nankai Trough have occurred either simultaneously or in a series in adjoining source regions. Many of those that have continued in a series have started off the coast of Tokai area and later moved west to off the coast of Nankai area. For example, these great earthquakes can occur over a period of several months or years, such as the 1944 Tonankai Earthquake (M 7.9) followed two years later by the 1946 Nankai Earthquake (M 8.0). In other cases, this series can occur over a very short time, such as the Ansei Tokai Earthquake (M 8.4) of December 23, 1854, followed 32 hours later on December 24 by the Ansei Nankai Earthquake (M 8.4). In addition, two earthquakes have occurred almost simultaneously off the coast of Tokai and Nankai. Examples are the 1605 Keicho Earthquake (M 7.9) and the 1707 Hoei Earthquake (M 8.4), which are thought to have occurred over the entire Tokai and Nankai sea areas.

Compared to earthquakes in other regions of Japan, these earthquakes occur at well-understood intervals. The seismic ground motion and size of the tsunami differ greatly with each occurrence, however. For example, the 1605 Keicho Earthquake generated a tsunami that struck the Pacific Ocean coast from the Kanto region to Kyushu. Yet, there was almost no recorded damage from seismic ground motion. Therefore, it seems that this was a "tsunami earthquake" ("slow earthquake" or "low-frequency earthquake") whose fault slipped more slowly than that of a normal earthquake.

A look at the crustal movement in the Kinki region shows that there was northwest-southeast extension in the southern Kii Peninsula during the period in which the 1946 Nankai Earthquake occurred (Fig.7-6A). It is also known that vertical movement, including uplifts of 50 cm, accompanied this earthquake in Cape Shionomisaki (Siono Misaki) and other locations. These phenomena suggest that this earthquake caused substantial uplift on the Pacific Ocean side (southeast side) of the land plate on which the Kii Peninsula is located. There is flat terraced topography (coastal terraces) on the coast at southern Wakayama Prefecture, which was formed by upheavals caused by great earthquakes occurring along the Nankai Trough for more than 100,000 years. About 125,000 years ago, the coastline is known to have been near Kushimoto, but this site has now been lifted to a height of about 60 m above sea level.

For more details on great earthquakes occurring along the Nankai Trough, refer to 8-1 (1).

2) Earthquakes within the subducting Philippine Sea Plate

The Philippine Sea Plate is subducting under the Kinki region from the Nankai Trough. No destructive earthquakes are known to have occurred in shallow locations within the plate recently subducted near the Nankai Trough. Somewhat larger earthquakes occur at depths deeper than 30 km on land. Destructive earthquakes of the past include the 1952 Yoshino Earthquake (M 6.8, a depth of 60 km), which produced damage centering on Nara Prefecture. The hypocenter for this type of earthquake is somewhat deep, so the destruction can spread across a wide area. The 1899 earthquake in the southeastern Kii Peninsula (M 7.0, also known as the Kii Yamato Earthquake) may have been of this type.