Nankaido Earthquake – Japan – December 20, 1946

The Nankai Trough was frequently hit in the past by powerful earthquakes.

Along a three hundred-mile long fault line known as the Nankai Trough, stretching from south of the Island of Kyushu to Kii-Hanto in the Island of Honshu, there is a long history of major earthquakes. On December 20, 1946, while Japan was recovering from all the devastation of World War II, it was hit once again with the Nankaido earthquake, a very powerful one of magnitude 8.1, along this line. From its epicenter offshore south of Kii-Hanto it triggered a twenty-foot high tsunami that reached the eastern shores of Kyushu where 1,400 people were killed, another 3,800 injured, and 12,000 houses completely destroyed.

The Philippine Sea Tectonic Plate is constantly moving at a rate of two inches a year toward the Nankai Trough as it subducts beneath Japan, part of the Eurasian Plate in this area. In this instance, since it was an earthquake of magnitude 8.1, it represented a massive disruption of the seafloor and, in contrast to past events it affected a bigger part of the trough. Japan is better acquainted with earthquakes, even ones of magnitude 8 or more, than almost any other nation on earth.

As recently as two years before this Nankaido earthquake this same fault line was struck by the Tonankai earthquake, which was of magnitude 8.1. It is rare for a specific location to experience two earthquakes of this magnitude within a two-year period. Both of these earthquakes were of the interplate kind. They occurred in the same area off Japan and their epicenters were located in the same general area.

Earthquakes have recurred in this area in the past. Historical data indicates that most of them were of magnitude 8 or greater and their recurrence rates have been one hundred to 150 years. Data for this in Japanese records goes back for more than 1,500 years so, like one or two other locations in Japan, there is something approaching predictability regarding future earthquakes.

There are four tectonic plates actively moving or resisting movement in and around Japan: the North American Plate (NAP) that includes part of northern Honshu and places north of Honshu, the Pacific Seas Plate (PSP) that dominates ocean areas south and east of central Honshu, and the two very large plates, the Pacific and the Eurasian, that lie east and west respectively of the NAP and PSP plates. Aftershocks are common and, in the case of the Nankaido earthquake, they continued for more than a month after the main shock with magnitudes of 6 or more. Conditions about a month after the initial shaking revealed a twenty-inch uplifting of the Kii-Hanto peninsula at Kushimoto.

The 1944 Tonankai earthquake also triggered a tsunami that affected the neighboring coasts. Wave heights reached thirty feet causing, like the Nankaido, numerous deaths and destruction of houses. Overall there were 1,250 fatalities, 2,970 injured, and 16,455 houses totally collapsed. There will be more earthquakes in the future in this very active multi-faulted part of Japan. A newspaper report dated September 7, 2004, noted that two strong earthquakes hit the Kii-Hanto region. Their magnitudes were not given.