The Tōkai earthquakes are major earthquakes that have occurred regularly with a return period of 100 to 150 years in the Tōkai region of Japan. The Tōkai segment has been struck by earthquakes in 1498, 1605, 1707, 1854, and 1923. Given the historic regularity of these earthquakes, Kiyoo Mogi in 1969 pointed out that another great shallow earthquake was possible in the "near future". Given the magnitude of the last two earthquakes, the next is expected to have at least a magnitude scale of 8.0, with large areas shaken at the highest level in the Japanese intensity scale, 7. Emergency planners are anticipating and preparing for potential scenarios after such an earthquake, including the possibility of thousands of deaths and hundreds of thousands of injuries, millions of damaged buildings, and cities that include Nagoya and Shizuoka devastated. Concern has been expressed over the presence of the Hamaoka Nuclear Power Plant, close to the expected epicentre of a Tōkai earthquake. The Fukushima I Nuclear Power Plant was severely damaged after a large earthquake followed by a tsunami in 2011, causing a nuclear event of level 7, the highest on the scale. Shortly after the 2011 Tōhoku earthquake, new reports were released which indicated the significant likelihood of another magnitude 9 earthquake occurring elsewhere in Japan, this time on the Nankai Trough. The reports stated that if a 9.0 earthquake occurred on the Nankai Trough, the effects would be very serious. The quake itself would likely kill thousands, and a series of 34-meter tall tsunamis would impact areas from the Kantō region to Kyūshū, adding thousands to the death toll, and destroying Shizuoka, Shikoku, and other areas with large populations.
The pattern of historical seismicity reveals that the megathrust surface is segmented, with five separate zones of rupturing identified, conventionally labeled A-E, from west to east. Earthquakes involving the A+B segments are generally referred to as Nankai earthquakes, C+D Tōnankai earthquakes and E Tōkai earthquakes. These earthquakes repeat at intervals generally in the range of 90 - 200 years. On all but one occasion, rupture of segment C has been followed by rupture of segments A+B within a few years. This behavior has been reproduced by modeling the viscoelastic response of the megathrust fault plane with lateral variations in both convergence rate and frictional properties.
