The Nankai earthquakes (南海地震, Nankai jishin) are major megathrust earthquakes that affect the Nankaidō (Southern Sea Circuit) region of Japan, west of the Tōnankai region (Southeastern Sea) and Tōkai region (Eastern Sea), and are caused by ruptures in the Nankai zone of the Nankai megathrust, specifically segments A and/or B. They occur with a return period of 100 to 200 years, and there is a high probability of one in the 21st century, as the most recent was the 1946 Nankaidō earthquake. These have high destructive potential, and thus are a focus of earthquake preparation.

These are a class of Nankai megathrust earthquakes, frequently occurring in combination with, or following, a rupture of segments C and/or D (Tōnankai zone, yielding Tōnankai earthquakes) and sometimes segment E (Tōkai zone, yielding Tōkai earthquakes). They are also known as Nankaidō earthquakes (南海道地震, Nankaidō jishin).

History

Historical Nankai earthquakes are as follows; some of these coincided with or followed earthquakes in the Tōnankai or Tōkai zones, and the 1498 Meiō Nankaidō earthquake may or may not have involved the Nankai zone:

The 1946 Nankai earthquake (南海地震 Nankai jishin) was a great earthquake in Nankaidō, Japan. It occurred on December 21, 1946 at 04:19 JST (December 20, 19:19 UTC). The earthquake measured between 8.1 and 8.4 on the moment magnitude scale, and was felt from Northern Honshū to Kyūshū. It occurred almost two years after the 1944 Tōnankai earthquake, which ruptured the adjacent part of the Nankai megathrust.

Geology

The Nankai Trough is a convergent boundary where the Philippine Sea Plate is being subducted beneath the Eurasian Plate. Large earthquakes have been recorded along this zone since the 7th century, with a recurrence time of 100 to 200 years.

Earthquake

The 1946 Nankaido earthquake was unusual in its seismological perspective, with a rupture zone estimated from long-period geodetic data that was more than twice as large as that derived from shorter period seismic data. In the center of this earthquake rupture zone, scientists used densely deployed ocean bottom seismographs to detect a subducted seamount 13 kilometres (8 mi) thick by 50 kilometres (31 mi) wide at a depth of 10 kilometres (6 mi). Scientists propose that this seamount might work as a barrier inhibiting brittle seismogenic rupture.

The 1605 Nankai earthquake occurred at about 20:00 local time on 3 February. It had an estimated magnitude of 7.9 on the surface wave magnitude scale and triggered a devastating tsunami that resulted in thousands of deaths in the Nankai and Tōkai regions of Japan. It is uncertain whether there were two separate earthquakes separated by a short time interval or a single event. It is referred to as a tsunami earthquake, in that the size of the tsunami greatly exceeds that expected from the magnitude of the earthquake.

Background

The southern coast of Honshu runs parallel to the Nankai Trough, which marks the subduction of the Philippine Sea Plate beneath the Eurasian Plate. Movement on this convergent plate boundary leads to many earthquakes, some of them of megathrust type. The Nankai megathrust has five distinct segments (A-E) that can rupture independently, the segments have ruptured either singly or together repeatedly over the last 1,300 years.Megathrust earthquakes on this structure tend to occur in pairs, with a relatively short time gap between them although in the 1707 Hōei earthquake all segments are thought to have ruptured at once. In 1854 there were two earthquakes a day apart and there were similar earthquakes in 1944 and 1946. In each case, the northeastern segment ruptured before the southwestern segment. In the 1605 event, there is evidence for two distinct earthquakes, but they are not distinguished by all historical sources and some seismologists suggest that only the Nankai segment of the megathrust ruptured.

The 1498 Nankai earthquake (明応地震 Meiō Jishin) occurred off the coast of Nankaidō, Japan, at about 08:00 local time on 20 September 1498. It had a magnitude estimated at 8.6 M_S and triggered a large tsunami. The death toll associated with this event is uncertain, but between 26,000 and 31,000 casualties were reported. A tsunami caused by the Meiō Nankaidō earthquake washed away the building housing the statue of the Great Buddha at Kōtoku-in in Kamakura.

Tectonic setting

The southern coast of Honshū runs parallel to the Nankai Trough, which marks the subduction of the Philippine Sea Plate beneath the Eurasian Plate. Movement on this convergent plate boundary leads to many earthquakes, some of them of megathrust type. The Nankai megathrust has five distinct segments (A-E) that can rupture independently, the segments have ruptured either singly or together repeatedly over the last 1,300 years.Megathrust earthquakes on this structure tend to occur in pairs, with a relatively short time gap between them. In addition to the two events in 1854, there were similar earthquakes in 1944 and 1946. In each case, the northeastern segment ruptured before the southwestern segment. In the 1498 event, the earthquake is thought to have ruptured segments C, D and E and possibly A and B. If both parts of the megathrust ruptured, the events were either simultaneous, or close enough in time, to not be distinguished by historical sources.