Nankai Megathrust Earthquakes
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megathrust earthquake 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 earthq ...
s that occur along the ''Nankai megathrust'' – the fault under the
Nankai Trough The is a submarine trough located south of the Nankaidō region of Japan's island of Honshu, extending approximately offshore. The underlying fault, the ''Nankai megathrust,'' is the source of the devastating Nankai megathrust earthquakes, ...
– which forms the plate interface between the
subducting Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second pla ...
Philippine Sea plate The Philippine Sea plate or the Philippine plate is a tectonic plate comprising oceanic lithosphere that lies beneath the Philippine Sea, to the east of the Philippines. Most segments of the Philippines, including northern Luzon, are part of ...
and the overriding Amurian Plate (part of the Eurasian plate), which dips beneath southwestern
Honshu , historically known as , is the largest of the four main islands of Japan. It lies between the Pacific Ocean (east) and the Sea of Japan (west). It is the list of islands by area, seventh-largest island in the world, and the list of islands by ...
,
Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
. 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 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 histor ...
. The earthquakes occur with a
return period A return period, also known as a recurrence interval or repeat interval, is an average time or an estimated average time between events such as earthquakes, floods, landslides, or river discharge flows to occur. The reciprocal value of return p ...
of about 90–200 years, and often occur in pairs, where a rupture along one part of the fault is followed by a rupture elsewhere on the fault, notably the 1854 Ansei-Tōkai earthquake and the
1854 Ansei-Nankai earthquake The 1854 Nankai earthquake occurred at about 16:00 local time on 24 December. It had a magnitude of 8.4 and caused a damaging tsunami. More than 30,000 buildings were destroyed and there were at least 3,000 casualties. It was the second of the ...
the next day, and the 1944 Tōnankai earthquake, followed by the
1946 Nankaidō earthquake The 1946 Nankai earthquake (昭和南海地震 Shōwa period, Shōwa Nankai jishin) was a great earthquake in Nankaidō, Occupation of Japan, Japan. It occurred on December 21, 1946, at 04:19 Japan Standard Time, JST (December 20, 19:19 UTC). Th ...
. In one recorded case (the
1707 Hōei earthquake The 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 Hons ...
) the fault ruptured along its entire length. All of these great earthquakes have resulted in damaging
tsunami A tsunami ( ; from , ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and underwater explosions (including detonations, ...
s, which are particularly damaging due to the Japanese population being concentrated on the
Taiheiyō Belt The , also known as the Tōkaidō corridor, is the megalopolis in Japan extending from Ibaraki Prefecture in the northeast to Fukuoka Prefecture in the southwest, running for almost . Its estimated population as of 2011 was about 80 million. The ...
, especially the coastal cities of
Tokyo Tokyo, officially the Tokyo Metropolis, is the capital of Japan, capital and List of cities in Japan, most populous city in Japan. With a population of over 14 million in the city proper in 2023, it is List of largest cities, one of the most ...
,
Yokohama is the List of cities in Japan, second-largest city in Japan by population as well as by area, and the country's most populous Municipalities of Japan, municipality. It is the capital and most populous city in Kanagawa Prefecture, with a popu ...
and
Osaka is a Cities designated by government ordinance of Japan, designated city in the Kansai region of Honshu in Japan. It is the capital of and most populous city in Osaka Prefecture, and the List of cities in Japan, third-most populous city in J ...
, the three most populous cities in Japan. The area remains seismically active, and future earthquakes are anticipated, with a high risk of a Nankai earthquake in the near future, which could be potentially very damaging.


Tectonic setting

The
Nankai Trough The is a submarine trough located south of the Nankaidō region of Japan's island of Honshu, extending approximately offshore. The underlying fault, the ''Nankai megathrust,'' is the source of the devastating Nankai megathrust earthquakes, ...
is the surface expression of the
subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...
zone between the Philippine Sea and Amur plates. Honshu itself is formed from the
island arc Island arcs are long archipelago, chains of active volcanoes with intense earthquake, seismic activity found along convergent boundary, convergent plate tectonics, tectonic plate boundaries. Most island arcs originate on oceanic crust and have re ...
developed over the subducting plate. The megathrust boundary extends about 700 km from the southern end of Kyūshū to the triple junction with the Okhotsk microplate near
Mount Fuji is an active stratovolcano located on the Japanese island of Honshu, with a summit elevation of . It is the highest mountain in Japan, the second-highest volcano on any Asian island (after Mount Kerinci on the Indonesian island of Sumatra), a ...
. At its southwestern end, there is another triple junction, where the overriding plate becomes the Okinawa plate.


Megathrust geometry

The megathrust dip increases from about 5° near the surface to 10° as it passes beneath the coast of Honshu. Analysis of seismic reflection data suggests that some of the displacement is carried by a splay fault dipping at about 25°. This 'megasplay' fault system has caused an unusually thick section of fluid-rich sedimentary rocks to be deeply underthrust. The presence of this 'weak' zone may lead to shallow coseismic rupture along the megasplay faults during megathrust earthquakes, explaining the large tsunamis created by these events.


Historical seismicity

The Nankai megathrust is thought to have caused at least 12 major earthquakes in the last ca. 1300 years. The pattern of historical seismicity reveals that the megathrust surface is segmented, with five separate zones of rupturing identified, conventionally labelled A–E, from west to east. Earthquakes involving the A+B segments are generally referred to as ''Nankai'' (literally ''South Sea'') earthquakes, C+D ''Tonankai'' (literally ''Southeast Sea'') earthquakes and C+D+E ''Tokai'' (literally ''East Sea'') earthquakes. The earthquake repeat intervals are generally in the range 90–200 years. On all but one occasion, rupture of segment C (±D ±E) has been followed by rupture of segments A+B within a few years. This behaviour has been reproduced by modelling the viscoelastic response of the megathrust fault plane with lateral variations in both convergence rate and frictional properties.


Future earthquake risk

The northeasternmost part of the megathrust, segment E, has not ruptured since 1854. A future great earthquake involving rupture along this and possibly other segments has been proposed as a major risk for the southern coast of Honshu. In 1999, the likelihood of the occurrence of a great earthquake in the Tokai area in the 2000–2010 period was estimated to be in the range of 0.35–0.45. Despite the uncertainty of when such an earthquake will occur, local authorities are already taking action to prepare residents for what they regard as an inevitability. Following the Hyūga-nada earthquake in August 2024, the
Japan Meteorological Agency The Japan Meteorological Agency (JMA; ''気象庁, Kishō-chō'') is a division of the Ministry of Land, Infrastructure, Transport and Tourism dedicated to the Scientific, scientific observation and research of natural phenomena. Headquartered ...
(JMA) warned that the risks of a megathrust earthquake on the Nankai Trough is "relatively higher than usual", and issued a "caution" warning for the first time in Japanese history. Meanwhile, the agency also stressed that such earthquake was not imminent, though the probability was higher than usual.


Potential effects

The Japanese government estimates that a major earthquake on the Nankai Trough would cause ¥169.5 trillion (US$1.191 trillion) in direct damage and ¥50.8 trillion (US$357 billion) in economic losses for the following year. A study by the Japan Society of Civil Engineers in 2018 estimated that the long-term damage from the earthquake could result in ¥1.240 trillion (US$8.7 billion) in economic losses over a 20-year period. It is predicted that the economic damage is likely to be 10 times higher than for the
2011 Tōhoku earthquake and tsunami On 11 March 2011, at 14:46:24 Japan Standard Time, JST (05:46:24 UTC), a  9.0–9.1 Submarine earthquake, undersea megathrust earthquake occurred in the Pacific Ocean, east of the Oshika Peninsula of the Tōhoku region. It lasted approx ...
. The government estimated a worst-case scenario death toll as high as 323,000, with ¥220.3 trillion (US$1.547 trillion) in economic damage. In 2019 the estimate was revised to 231,000 deaths and ¥213.7 trillion (US$1.501 trillion) in economic damage, due to improved earthquake resistance.


List of occurrences

All the dates shown in the table use the
Gregorian calendar The Gregorian calendar is the calendar used in most parts of the world. It went into effect in October 1582 following the papal bull issued by Pope Gregory XIII, which introduced it as a modification of, and replacement for, the Julian cale ...
. Some sources use the
Julian calendar The Julian calendar is a solar calendar of 365 days in every year with an additional leap day every fourth year (without exception). The Julian calendar is still used as a religious calendar in parts of the Eastern Orthodox Church and in parts ...
for the earlier earthquakes in the list.


See also

*
List of earthquakes in Japan This is a list of earthquakes in Japan with either a magnitude greater than or equal to 7.0 or which caused significant damage or casualties. As indicated below, magnitude is measured on the Richter scale (''ML'') or the moment magnitude scale ('' ...


References

{{DEFAULTSORT:Nankai Megathrust Earthquakes Earthquake clusters, swarms, and sequences Megathrust earthquakes in Japan