Megathrust earthquakes occur at convergent plate boundaries, where one tectonic plate is forced underneath another. The earthquakes are caused by slip along the thrust fault that forms the contact between the two plates. These interplate earthquakes are the planet's most powerful, with moment magnitudes (Mw) that can exceed 9.0. Since 1900, all earthquakes of magnitude 9.0 or greater have been megathrust earthquakes.
The Nankai Trough is a submarine trough located south of the Nankaidō region of Japan's island of Honshu, extending approximately 900 km (559 mi) offshore. The underlying fault, the Nankai megathrust, is the source of the devastating Nankai megathrust earthquakes, while the trough itself is potentially a major source of hydrocarbon fuel, in the form of methane clathrate.
The Ansei great earthquakes were a series of major earthquakes that struck Japan during the Ansei era (1854–1860):
The 1946 Nankai earthquake was a great earthquake in Nankaidō, Japan. It occurred on December 21, 1946, at 04:19 JST. 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.
Nankai megathrust earthquakes are great megathrust earthquakes that occur along the Nankai megathrust – the fault under the Nankai Trough – which forms the plate interface between the subducting Philippine Sea plate and the overriding Amurian Plate, which dips beneath southwestern Honshu, Japan. The fault is divided into five segments in three zones, which rupture separately or in combination, and depending on location, the resulting earthquakes are subdivided by zone from west to east into Nankai earthquakes, Tōnankai earthquakes, and Tōkai earthquakes.
The Nankai earthquakes are major megathrust earthquakes that affect the Nankaidō region of Japan, west of the Tōnankai region and Tōkai region, 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 and sometimes segment E. They are also known as Nankaidō earthquakes.
The 1854 Tōkai earthquake was the first of the Ansei great earthquakes (1854–1855). It occurred at about 09:00 local time on 23 December 1854. It had a magnitude of 8.4 and caused a damaging tsunami. More than 10,000 buildings were destroyed and there were at least 2,000 casualties.
The 1707 Hōei earthquake struck south-central Japan at around 13:45 local time on 28 October. It was the largest earthquake in Japanese history until it was surpassed by the 2011 Tōhoku earthquake. It caused moderate-to-severe damage throughout southwestern Honshu, Shikoku and southeastern Kyūshū. The earthquake, and the resulting destructive tsunami, caused more than 5,000 casualties. This event ruptured all of the segments of the Nankai megathrust simultaneously, the only earthquake known to have done this, with an estimated magnitude of 8.6 or 8.7 . It possibly also triggered the last eruption of Mount Fuji 49 days later.
The 1730 Valparaíso earthquake occurred at 04:45 local time on July 8. It had an estimated magnitude of 9.1–9.3 and triggered a major tsunami with an estimated magnitude of Mt 8.75, that inundated the lower parts of Valparaíso. The earthquake caused severe damage from La Serena to Chillan, while the tsunami affected more than 1,000 km (620 mi) of Chile's coastline.
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, and 1854. Given the historic regularity of these earthquakes, Kiyoo Mogi in 1969 pointed out that another great shallow earthquake was possible in the "near future".
The 1855 Edo earthquake was the third Ansei Great Earthquake, which occurred during the late-Edo period. It occurred after the 1854 Nankai earthquake, which took place about a year prior. The earthquake occurred at 22:00 local time on 11 November. It had an epicenter close to Edo, causing considerable damage in the Kantō region from the shaking and subsequent fires, with a death toll of 7,000–10,000 people and destroyed around 14,000 buildings. The earthquake had a magnitude of 7.0 on the surface wave magnitude scale and reached a maximum intensity of XI (Extreme) on the Mercalli intensity scale. The earthquake triggered a minor tsunami.
The 1498 Meiō earthquake struck off the coast of Nankaidō, Japan, at approximately 08:00 local time on September 20, 1498. With an estimated magnitude of 8.6 , it triggered a massive tsunami. The exact death toll from this event remains uncertain, but reports range from 5,000 to 41,000 casualties. The tsunami, caused by the Meiō Nankaidō earthquake, washed away the building that housed the statue of the Great Buddha at Kōtoku-in in Kamakura, although the statue itself remained intact.
The 1944 Tōnankai earthquake occurred at 13:35 local time on 7 December. It had an estimated magnitude of 8.1 on the moment magnitude scale and a maximum felt intensity of greater than 5 Shindo. It triggered a large tsunami that caused serious damage along the coast of Wakayama Prefecture and the Tōkai region. Together, the earthquake and tsunami caused 3,358 casualties.
The 1605 Keichō 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.
The 1979 Tumaco earthquake occurred at on 12 December with a moment magnitude of 8.2 and a maximum Mercalli intensity of IX (Violent). The epicenter was just offshore from the border between Ecuador and Colombia, near the port city of Tumaco. It triggered a major tsunami, which was responsible for most of the estimated 300–600 deaths. The hardest hit area was Colombia's Nariño Department.
The 684 Hakuho earthquake took place in Japan in 684, and is described in the history book Nihon Shoki from the 8th century. The earthquake took place on 26 November 684, in the 13th year of the reign of Emperor Tenmu. It caused the under-sea inundation of approximately 10 km2 (3.9 sq mi) of rice fields.
The Nemuro-Oki earthquake in scientific literature, occurred on June 17 at 12:55 local time. It struck with an epicenter just off the Nemuro Peninsula in northern Hokkaidō, Japan. It measured 7.8–7.9 on the moment magnitude scale (Mw ), 8.1 on the tsunami magnitude scale (Mt ) and 7.4 on the Japan Meteorological Agency magnitude scale (MJMA ).
The Shōnai offshore earthquake occurred at around 14:00 on December 7, 1833. It struck with an epicenter in the Sea of Japan, off the coast of Yamagata Prefecture, Japan. A tsunami was triggered by the estimated MJMA 7.5–7.7 earthquake. One hundred and fifty people were killed and there was severe damage in the prefecture.
The 1940 Shakotan earthquake occurred on August 2 at 00:08:22 JST with a moment magnitude (Mw ) of 7.5 and maximum JMA seismic intensity of Shindo 4. The shock had an epicenter off the coast of Hokkaido, Japan. Damage from the shock was comparatively light, but the accomanying tsunami was destructive. The tsunami caused 10 deaths and 24 injuries on Hokkaido, and destroyed homes and boats across the Sea of Japan. The highest tsunami waves were recorded at the coast of Russia while along the coast of Hokkaido, waves were about 2 m.
