Megathrust earthquake


Megathrust earthquakes occur at subduction zones at destructive convergent plate boundaries, where one tectonic plate is forced underneath another, caused by slip along the thrust fault that forms the contact between them. These interplate earthquakes are the planet's most powerful, with moment magnitudes that can exceed 9.0. Since 1900, all earthquakes of magnitude 9.0 or greater have been megathrust earthquakes. No other type of tectonic activity of terrestrial origin has produced earthquakes of this scale.

Terminology

During the rupture, one side of the fault is pushed upwards relative to the other, and it is this type of movement that is known as thrust. They are a type of dip-slip fault. A thrust fault is a reverse fault with a dip of 45° or less. Oblique-slip faults have significant components of different slip styles. The term megathrust does not have a widely accepted rigorous definition, but is used to refer to an extremely large thrust fault, typically formed at the plate interface along a subduction zone such as the Sunda megathrust.

Areas

Megathrust earthquakes are almost exclusive to tectonic subduction zones and are often associated with the Pacific and Indian Oceans. These subduction zones are not only responsible for megathrust earthquakes, but are also largely responsible for the volcanic activity associated with the Pacific Ring of Fire.
Since the earthquakes associated with these subduction zones deform the ocean floor, they often generate a significant series of tsunami waves. Subduction zone earthquakes are also known to produce intense shaking and ground movements for significant periods of time that can last for up to 5-6 minutes.
In the Indian Ocean region, the Sunda megathrust is located where the Indo-Australian Plate is subducting under the Eurasian Plate and extends off the coasts of Myanmar, Sumatra, Java and Bali before terminating off the northwestern coast of Australia. This subduction zone was responsible for the 2004 Indian Ocean earthquake and tsunami.
In Japan, the Nankai megathrust under the Nankai Trough is responsible for Nankai megathrust earthquakes and associated tsunamis.
In the North America, the Juan de Fuca Plate is subducting under the North American Plate creating the Cascadia subduction zone which stretches from Middle Vancouver, British Columbia to Northern California. This subduction zone was responsible for the 1700 Cascadia earthquake. The Aleutian Trench, of the southern coast of Alaska and the Aleutian Islands, where the North American Plate overrides the Pacific Plate, has generated many major earthquakes throughout history, several of which generated Pacific-wide tsunamis, including the 1964 Alaska earthquake; at magnitude 9.2, it remains the largest recorded earthquake in North America, and the second largest earthquake instrumentally recorded in the world.
The largest recorded megathrust earthquake was the 1960 Valdivia earthquake, estimated magnitude 9.4-9.6, centered off the coast of Chile along the Peru-Chile trench, where the Nazca Plate is subducting under the South American Plate. This megathrust region has regularly generated extremely large earthquakes historically, the largest megathrust event within the last 20 years being the magnitude 8.8 2010 Chile earthquake.
A study reported in 2016 found that the largest megathrust quakes are associated with downgoing slabs with the shallowest dip, so-called flat slab subduction.

Examples

Examples of megathrust earthquakes are listed in the following table.
EventEstimated Magnitude
Tectonic Plates InvolvedOther Details/Notes
0365 Crete earthquake8.0+African Plate subducting beneath the Aegean Sea Plate
  • The quake generated a large tsunami in the eastern Mediterranean Sea and caused significant vertical displacement in the island of Crete.
  • Death toll in the "many thousands" according to Roman historian Ammianus Marcellinus.
869 Sanriku earthquake8.6–9.0Pacific Plate subducting beneath the Okhotsk Plate
  • Slip length: 200 km over
  • Slip width: 85 km over
  • Led to a death toll of around 1000 people and completely destroyed the town of Tagajō.
    • 1575 Valdivia earthquake8.5Nazca Plate subducting beneath the South American Plate
  • Caused the dam at Laguna de Anigua to rupture and burst. Leading to the deaths of "many Native people" in Valdivia, Chile.
    • 1700 Cascadia earthquake8.7–9.2Juan de Fuca Plate subducting beneath the North American Plate
  • Slip length: 1000 km
  • Slip motion: 20 m
  • According to Native American oral traditions from the affected area, the quake and tsunami struck during a "winter evening" after many had gone to bed, wiping out several villages around Pachina Bay. A woman named Anacla Aq Sop was said to have been the only survivor of her entire tribe.
  • The generated tsunami affected Japan; since it was not associated with a local earthquake, Japanese records called it an “orphan” tsunami.
    • 1707 Hōei earthquake8.6–9.3Philippine Sea Plate subducting beneath the Eurasian Plate
  • Duration: approximately 10 minutes
  • Slip length: maybe 600 and 700 km
  • Around 29,000 houses were destroyed, over 5,000 people died and it was the most powerful earthquake in Japanese history until 2011.
  • The quake also triggered one of the three largest landslides ever seen in Japan.
  • It might also have changed the pressure levels inside the Mount Fuji magma chamber as the volcano erupted merely 49 days later on 16 December 1707.
    • 1730 Valparaíso earthquake9.1–9.3Nazca Plate subducting beneath the South American Plate
  • Slip length: 600 and 800 km
  • Slip motion: 10 and 14 m
  • Caused destruction over a large area, though very few deaths were reported as a result of the earthquake and following tsunami. This was due to both a strong foreshock which made people leave their homes before the quake, as well as the inhabitants running to higher ground after seeing the water recede.
    • 1737 Kamchatka earthquake8.3–9.0Pacific Plate subducting beneath the Okhotsk Plate
  • Duration: 15 minutes
  • Depth: 40 km
  • Slip length: maybe 700 km over
  • No reported deaths or damage.
    • 1746 Lima-Callao earthquake8.6–8.8Pacific Plate subducting beneath the Nazca Plate
  • Caused severe damage to all Peruvian ports and cities around the central Peruvian coastline.
  • Earthquake completely destroyed the city of Lima. A mere 25 out of 3,000 buildings were left standing but miraculously only 1,141 of its ~60,000 citizens lost their lives.
  • The tsunami which followed 30 minutes later obliterated the city of Callao, less than 200 of its ~5,000 citizens survived the event.
  • The port town of Pisco was also destroyed after having been rebuilt further inland after a previous tsunami in 1687.
    • 1755 Lisbon earthquake8.5–9.0Hypothesized to be part of a young subduction zone but origin still debated; related to the Azores–Gibraltar Transform Fault
  • Produced tsunami waves around 20 metres high.
  • Tsunami and firestorm destroyed much of Lisbon.
  • Destruction rampant in the Algarve severely damaging villages, towns and several coastal forts.
  • Between 40,000 and 50,000 people lost their lives in Portugal, Spain and Morocco.
  • Grave of Nuno Álvares Pereira was destroyed.
  • The royal archives disappeared together with detailed records of the voyages of Vasco da Gama.
  • Ópera do Tejo burned to the ground and the Royal Ribeira Palace was destroyed.
  • Only recorded megathrust earthquake in the Atlantic Ocean.
    • 1868 Arica earthquake8.5–9.0Nazca Plate subducting beneath the South American Plate
  • Slip length: 600 km
  • Resulted in near complete destruction of many cities and towns in the southern parts Peru.
  • The port city of Pisco was razed.
  • The tsunami that followed caused severe damage not only in Peru but also in Hawaii and caused the only recorded deaths by a tsunami in New Zealand.
  • Led to the deaths of over 25,000 people.
    • 1877 Iquique earthquake8.5–9.0?Nazca Plate subducting beneath the South American Plate
  • Slip length: 420 and 450 km
  • Caused much damage in the Chilean regions of Tarapacá and Antofagasta.
  • A 10 m tsunami washed up along 500 km of Chilean and Peruvian coastline.
  • Biggest loss of life contributed to the event was in Fiji where ~2,000 people died in the tsunami.
    • 1906 Ecuador–Colombia earthquake8.8Nazca Plate subducting beneath the South American Plate
  • Slip length: 500 and 600 km
  • The greatest damage from the tsunami occurred on the coast between Río Verde, Ecuador and Tumaco, Colombia.
  • Estimates of the number of deaths caused by the tsunami vary between 500 and 1,500.
    • 1923 Great Kantō earthquake8.2Philippine Sea Plate subducting beneath the Okhotsk Plate
  • Earthquake struck at lunchtime when people were preparing food over open fires, because of this many large blazes broke out, some of which turned into raging firestorms.
  • Around 38,000 people were all burned alive at once when a fire tornado struck an old Clothing Depot in downtown Tokyo where they had taken shelter following the quake.
  • The extreme fires were caused by a strong typhoon bringing violent winds into Tokyo Bay around the same time of the earthquake.
  • Landslides were triggered, one of which pushed the entire village of Nebukawa and a stationary train with ~100 passengers into the sea.
  • Over 570,000 homes were destroyed, leaving an estimated 1.9 million homeless.
  • The total number of people who perished in the catastrophe was between 105,385 and 142,800.
    • 1932 Jalisco earthquakes8.2Rivera Plate and Cocos Plate subducting beneath the North American Plate
  • Duration: 95 seconds
  • Depth: 60 km
    • 1942 Peru earthquake8.2Nazca Plate subducting beneath the South American Plate
    1946 Nankaidō earthquake8.1Philippine Sea Plate subducting beneath the Eurasian Plate
  • Slip length: maybe 300 km
    • 1952 Kamchatka earthquake9.0Pacific Plate subducting beneath the Okhotsk Plate
  • Depth: 30 km
  • Slip length: maybe 600 km over
    • 1960 Great Chilean earthquake9.5Nazca Plate subducting beneath the South American Plate
  • Duration: 5–6 minutes
  • Depth: 33 km
  • Slip length: 850 and 1000 km
  • Slip width: 200 km
  • Slip motion: 20 m
    • 1963 Kuril Islands earthquake8.5Pacific Plate subducting beneath the Eurasian Plate
    1964 Alaska earthquake 9.2Pacific Plate subducting beneath the North American Plate
  • Duration: 4–5 minutes
  • Depth: 25 km
  • Slip length: 800 and 850 km
  • Slip width: 250 km
  • Slip motion: 23 m
    • 1965 Rat Islands earthquake8.7Pacific Plate subducting beneath the North American Plate
    1985 Mexico City earthquake8.0Cocos Plate subducting beneath the North American Plate
    2001 southern Peru earthquake8.4Nazca Plate subducting beneath the South American Plate
  • Depth: 33 km
  • Slip length: 300 km
  • Slip width: 120 km
    • 2003 Hokkaido earthquake8.3Pacific Plate subducting beneath the Okhotsk Plate
    2004 Sumatra-Andaman earthquake 9.1–9.3India Plate subducting beneath the Burma Plate
  • The total vertical displacement measured by sonar survey is about 40 m in the vicinity of the epicenter and occurred as two separate movements which created two large, steep, almost vertical cliffs, one above the other.
  • Duration: 8–10 minutes
  • Depth: 30 km
  • Slip length: 1000 and 1300 km
  • Slip width: 180 km
  • Slip motion: 33 m
    • 2005 Nias–Simeulue earthquake8.6Indo-Australian Plate subducting beneath the Eurasian Plate
    2006 Kuril Islands earthquake8.3Pacific Plate subducting beneath the Okhotsk Plate
    2010 Chile earthquake8.8Nazca Plate subducting beneath the South American Plate
  • Depth: 35 km
  • Slip length: 500 km
  • Slip width: 200 km
    • 2011 Tōhoku earthquake and tsunami9.1Pacific Plate subducting beneath the Okhotsk Plate
  • Duration: 6 minutes
  • Depth: 29 km
  • Slip length: 500 km
  • Slip width: 200 km
  • Slip motion: 20 m
    • 2013 Okhotsk Sea earthquake8.3Pacific Plate subducting beneath the Okhotsk Plate
    2014 Iquique earthquake8.2Nazca Plate subducting beneath the South American Plate
  • Depth: 20.1 km
  • Slip length: 170 km
  • Slip width: 70 km
    • 2015 Illapel earthquake8.3Nazca Plate subducting beneath the South American Plate
  • Depth: 25 km
  • Slip length: 260 km
  • Slip width: 80 km