The 1946 Aleutian Islands earthquake occurred near the

Aleutian Islands The Aleutian Islands (; ; ale, Unangam Tanangin,”Land of the Aleuts", possibly from Chukchi language, Chukchi ''aliat'', "island"), also called the Aleut Islands or Aleutic Islands and known before 1867 as the Catherine Archipelago, are a cha ...

Alaska Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S., ...

on April 1, 1946. The shock had a

moment magnitude The moment magnitude scale (MMS; denoted explicitly with or Mw, and generally implied with use of a single M for magnitude) is a measure of an earthquake's magnitude ("size" or strength) based on its seismic moment. It was defined in a 1979 pape ...

() of 8.6, a tsunami magnitude of 9.3, and a surface-wave magnitude () of only 7.4, and a maximum

Mercalli intensity The Modified Mercalli intensity scale (MM, MMI, or MCS), developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It measures the eff ...

of VI (''Strong''). It resulted in 165–173 casualties and over $26 million in damage. The seafloor along the fault was elevated, triggering a Pacific-wide

tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) 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 other underwater explo ...

with multiple destructive waves at heights ranging from . The tsunami obliterated the

Scotch Cap Light The Scotch Cap Light is a series of lighthouses located on the southwest corner of Unimak Island in Alaska. It was the first station established on the outside coast of Alaska. History In 1903, the Scotch Cap Light was built. The original lightho ...

house on

Unimak Island Unimak Island ( ale, Unimax, russian: Унимак) is the largest island in the Aleutian Islands chain of the U.S. state of Alaska. Geography It is the easternmost island in the Aleutians and, with an area of , the ninth largest island in the ...

, Alaska among others, and killed all five lighthouse keepers. Despite the destruction to the Aleutian Island Unimak, the tsunami had almost an imperceptible effect on the Alaskan mainland.

Tectonic setting

The Aleutian Islands are a group of 14 large and 55 smaller volcanic islands situated between mainland Alaska and

Kamchatka The Kamchatka Peninsula (russian: полуостров Камчатка, Poluostrov Kamchatka, ) is a peninsula in the Russian Far East, with an area of about . The Pacific Ocean and the Sea of Okhotsk make up the peninsula's eastern and wes ...

. They are situated along the

Aleutian Trench The Aleutian Trench (or Aleutian Trough) is an oceanic trench along a convergent plate boundary which runs along the southern coastline of Alaska and the Aleutian islands. The trench extends for from a triple junction in the west with the Ulakh ...

as they were formed by the active subduction of the oceanic

Pacific plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and Iza ...

underneath the continental crust of the

North American plate The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Pacific ...

. The western portion of the trench becomes more and more oblique until near the

Commander Islands The Commander Islands, Komandorski Islands, or Komandorskie Islands (russian: Командо́рские острова́, ''Komandorskiye ostrova'') are a series of treeless, sparsely populated Russian islands in the Bering Sea located about ea ...

where faulting becomes nearly exclusively

strike-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

and causes earthquakes such as the 2017 Komandorski Islands earthquake. The trench ends where the Kuril-Kamchatka Trench begins in the west. In the east, the trench extends for a couple thousand miles before reaching the transition zone near Yakataga responsible for creating earthquakes such as the 1899 Yakutat Bay earthquakes. The collision totally ends slightly further east, where the

Queen Charlotte Fault The Queen Charlotte Fault is an active transform fault that marks the boundary of the North American plate and the Pacific plate. It is Canada's right-lateral strike-slip equivalent to the San Andreas Fault to the south in California. The Queen Ch ...

becomes the dominant fault of the tectonic regime. This specific area of the Aleutian Islands is thought to have ruptured in the 1585 Aleutian Islands earthquake.

Earthquake

The earthquake struck at 3:29 am local time on April 1, 1946, at a shallow depth of . The earthquake was very large, but the maximum felt intensity was only a VI. The earthquake was originally thought to be a rather smaller earthquake of 7.4, however further research uncovered that this was truly a much larger event, but with a hidden seismic signature. A size of 8.6 fits this event best, with an estimated slip between and . This event shows the classic signs of a

tsunami earthquake In seismology, a tsunami earthquake is an earthquake which triggers a tsunami of significantly greater magnitude, as measured by shorter-period seismic waves. The term was introduced by Japanese seismologist Hiroo Kanamori in 1972. Such events a ...

as the surface-wave magnitude was very low compared to the moment and tsunami magnitudes, and the tsunami height was far larger than expected for the surface wave magnitude. The very high tsunami magnitude value is due to the sheer strength of the waves. It was calculated by using tsunami run up from

Honolulu Honolulu (; ) is the capital and largest city of the U.S. state of Hawaii, which is in the Pacific Ocean. It is an unincorporated county seat of the consolidated City and County of Honolulu, situated along the southeast coast of the island ...

, Hilo, and the average of stations in California. Even excluding the values given by Hilo (the highest ), the tsunami magnitude is at least 9.1. The earthquake was originally thought to be a strike-slip earthquake, and later a strike slip earthquake with a normal faulting component. These erroneous focal mechanisms were due to a lack of stations recording the earthquake. However, modeling of the

S wave __NOTOC__ In seismology and other areas involving elastic waves, S waves, secondary waves, or shear waves (sometimes called elastic S waves) are a type of elastic wave and are one of the two main types of elastic body waves, so named because th ...

, rather than the

P wave A P wave (primary wave or pressure wave) is one of the two main types of elastic body waves, called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any ...

helped to uncover both the true magnitude of the earthquake, as well as the widely accepted thrusting mechanism. This thrust mechanism is consistent with a subduction earthquake helping to relieve stress on the Aleutian Trench.

Tsunami

At Unimak Island, tsunami run-up reached . The tsunami earthquake aspect of this event helps to explain a portion of this massive height, however the rest must be attributed to a local submarine landslide. Waves reportedly traveled across the ocean at and measured high, crest to trough. The wave reached

Kauai, Hawaii Kauai, () anglicized as Kauai ( ), is geologically the second-oldest of the main Hawaiian Islands (after Niʻihau). With an area of 562.3 square miles (1,456.4 km2), it is the fourth-largest of these islands and the 21st largest island ...

4.5 hours after the quake, and

Hilo, Hawaii Hilo () is a census-designated place (CDP) and the largest settlement in Hawaii County, Hawaii, United States, which encompasses the Island of Hawaii. The population was 44,186 according to the 2020 census. It is the fourth-largest settlement i ...

4.9 hours later. In Hilo, 173 died, 163 were injured, 488 buildings were demolished and 936 more were damaged. Witnesses told of waves inundating streets, homes, and storefronts. Many victims were swept out to sea by receding water. The tsunami caused much damage in

Maui The island of Maui (; Hawaiian: ) is the second-largest of the islands of the state of Hawaii at 727.2 square miles (1,883 km2) and is the 17th largest island in the United States. Maui is the largest of Maui County's four islands, which ...

as well. Waves there demolished 77 homes and many other buildings. The residents of these islands were caught off-guard by the onset of the tsunami due to the inability to transmit warnings from the destroyed posts at Scotch Cap, and the tsunami is known as the April Fools' Day Tsunami in Hawaii because it happened on April 1. The effects of the tsunami also reached

Washington Washington commonly refers to: * Washington (state), United States * Washington, D.C., the capital of the United States ** A metonym for the federal government of the United States ** Washington metropolitan area, the metropolitan area centered o ...

Oregon Oregon () is a U.S. state, state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington (state), Washington, while the Snake River delineates much of it ...

, and

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

. The tsunami was unusually powerful for the size of the earthquake. The event was classified as a tsunami earthquake due to the discrepancy between the size of the tsunami and the relatively low surface-wave magnitude. The large-scale destruction prompted the creation of the Seismic Sea Wave Warning System, which became the

Pacific Tsunami Warning Center The Pacific Tsunami Warning Center (PTWC) is one of two tsunami warning centers that are operated by NOAA in the United States. Headquartered on Ford Island, HI, the PTWC is part of an international tsunami warning system (TWS) program and s ...

in 1949.

References

External links

*
Tsunami Animation: Unimak Island, Aleutian Islands, 1946
–

1946 Aleutians Tsunami
– Western States Seismic Policy Council

– ''

LiveScience Live Science is a science news website run by Future via Purch, which it purchased from Imaginova in 2009. Stories and editorial commentary are typically syndicated to major news outlets, such as Yahoo!, MSNBC, AOL, and Fox News.{{fact, date=Mar ...

'' *
Survivor audio interviewtranscript
{{DEFAULTSORT:Aleutian Islands earthquake, 1946 1946 earthquakes 1946 in Alaska 1946 in Hawaii 1946 tsunamis

Earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...

Megathrust earthquakes in Alaska 1946 Aleutian April 1946 events in the United States Tsunami earthquakes History of Hawaii

Tectonic setting

Earthquake

Tsunami

See also

References

External links