1946 Aleutian Islands earthquake

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1946 Aleutian Islands earthquake
Tsunami large.jpg
People fleeing the approaching tsunami in Hilo, Hawaii
1946 Aleutian Islands earthquake is located in Alaska
Anchorage
Anchorage
Juneau
Juneau
Fairbanks
Fairbanks
Adak
Adak
White Horse
White Horse
King Salmon
King Salmon
Unalaska
Unalaska
Anadyr
Anadyr
St. Paul
St. Paul
Nome
Nome
1946 Aleutian Islands earthquake
UTC time1946-04-01 12:29:01
ISC event898313
USGS-ANSSComCat
Local dateApril 1, 1946 (1946-04)
Local time02:29
Magnitude7.4 Ms,[1] 8.6 Mw,[2] 9.3 Mt[3]
Depth15 km (9.3 mi)[2]
Epicenter53°29′N 162°50′W / 53.49°N 162.83°W / 53.49; -162.83Coordinates: 53°29′N 162°50′W / 53.49°N 162.83°W / 53.49; -162.83[2]
TypeMegathrust
Areas affectedHawaii, Alaska
United States
Max. intensityVI (Strong)[2]
TsunamiUp to 42 m (138 ft) at Unimak Island, Alaska[1]
Casualties165–173[4]

The 1946 Aleutian Islands earthquake occurred near the Aleutian Islands, Alaska on April 1, 1946. The shock had a moment magnitude (Mw ) of 8.6, a tsunami magnitude Mt  of 9.3, and a surface-wave magnitude (Ms ) of only 7.4, and a maximum Mercalli intensity of VI (Strong).[1][3] It resulted in 165–173 casualties and over $26 million in damage. The seafloor along the fault was elevated, triggering a Pacific-wide tsunami with multiple destructive waves at heights ranging from 45–138 ft (14–42 m). The tsunami obliterated the Scotch Cap Lighthouse on Unimak Island, 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.[5]

Tectonic setting[edit]

Map showing the tectonics and seismicity of Alaska

The Aleutian Islands are a group of 14 large and 55 smaller volcanic islands situated between mainland Alaska and Kamchatka.[6] They are situated along the Aleutian Trench as they were formed by the active subduction of the oceanic Pacific plate underneath the continental crust of the North American plate.[7] The western portion of the trench becomes more and more oblique until near the Commander Islands where faulting becomes nearly exclusively strike-slip and causes earthquakes such as the 2017 Komandorski Islands earthquake.[8] The trench ends where the Kuril-Kamchatka Trench begins in the west.[8] 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.[9] The collision totally ends slightly further east, where the Queen Charlotte Fault becomes the dominant fault of the tectonic regime.[9] This specific area of the Aleutian Islands is thought to have ruptured in the 1585 Aleutian Islands earthquake.[10]

Earthquake[edit]

The earthquake struck at 2:29 am local time on April 1, 1946, at a shallow depth of 15 km (9.3 mi). 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 Ms 7.4, however further research uncovered that this was truly a much larger event, but with a hidden seismic signature.[1] A size of Mw 8.6 fits this event best, with an estimated slip between 9 m (30 ft) and 12.7 m (42 ft).[1][11] This event shows the classic signs of a tsunami earthquake 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.[1] 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, Hilo, and the average of stations in California. Even excluding the values given by Hilo (the highest Mt ), the tsunami magnitude is at least 9.1.[3] 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, rather than the P wave 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.[1]

Tsunami[edit]

At Unimak Island, tsunami run-up reached 42 m (138 ft). 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.[1] Waves reportedly traveled across the ocean at 500 miles per hour (430 kn; 800 km/h) and measured 55 feet (17 m) high, crest to trough. The wave reached Kauai, Hawaii 4.5 hours after the quake, and Hilo, Hawaii 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 as well. Waves there demolished 77 homes and many other buildings.[12] 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, Oregon, and California.[5]

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.[13] The large-scale destruction prompted the creation of the Seismic Sea Wave Warning System, which became the Pacific Tsunami Warning Center in 1949.[14]

See also[edit]

References[edit]

  1. ^ a b c d e f g h Okal, Emile; López, Alberto M. (June 2006). "A seismological reassessment of the source of the 1946 Aleutian 'tsunami' earthquake". Geophysical Journal International. 165 (3): 835–849. Bibcode:2006GeoJI.165..835L. doi:10.1111/j.1365-246X.2006.02899.x. Archived from the original on September 29, 2022. Retrieved August 8, 2022.
  2. ^ a b c d National Geophysical Data Center. "Comments for the Significant Earthquake". Archived from the original on August 7, 2022. Retrieved September 10, 2011.
  3. ^ a b c Abe, Katsuyuki (1979). "Size of great earthquakes of 1837–1974 inferred from tsunami data". Journal of Geophysical Research. 84 (B4): 1561–1568. Bibcode:1979JGR....84.1561A. doi:10.1029/JB084iB04p01561. Retrieved August 8, 2022.
  4. ^ USGS (September 4, 2009), PAGER-CAT Earthquake Catalog, Version 2008_06.1, United States Geological Survey, archived from the original on July 17, 2020, retrieved August 3, 2018
  5. ^ a b "1946 Aleutian Tsunami". www.usc.edu. Archived from the original on January 12, 2016. Retrieved February 2, 2016.
  6. ^ "Aleutian Islands". Britannica. Britannica. Archived from the original on September 12, 2019. Retrieved August 7, 2022.
  7. ^ "Aleutian Trench". Archived from the original on February 25, 2014. Retrieved August 7, 2022.
  8. ^ a b Lay, Thorne; Ye, Lingling; Bai, Yefei; Cheung, Kwok Fai; Kanamori, Hiroo; Freymueller, Jeffrey; Steblov, Grigory M.; Kogan, Mikhail G. (December 8, 2017). "Rupture Along 400 km of the Bering Fracture Zone in the Komandorsky Islands Earthquake ( M W 7.8) of 17 July 2017: 2017 Mw 7.8 Komandorsky Is. Earthquake". Geophysical Research Letters. 44: 1–4. doi:10.1002/2017GL076148. S2CID 55917937. Retrieved August 8, 2022.
  9. ^ a b McCann, William (1980). "Yakataga gap, Alaska: Seismic history and earthquake potential". Science. 207 (4437): 1309–1314. Bibcode:1980Sci...207.1309M. doi:10.1126/science.207.4437.1309. JSTOR 1683431. S2CID 128624810. Archived from the original on July 3, 2022. Retrieved July 3, 2022.
  10. ^ Butler, Rhett; Burney, David; Walsh, David (September 1, 2014). "Paleotsunami evidence on Kaua'i and numerical modeling of a great Aleutian tsunami". Geophysical Research Letters. 41 (19): 6795–6802. Bibcode:2014GeoRL..41.6795B. doi:10.1002/2014GL061232. S2CID 32932011. Retrieved August 8, 2022.
  11. ^ Tape, Carl; Lomax, Anthony (June 18, 2022). "Aftershock Regions of Aleutian-Alaska Megathrust Earthquakes, 1938–2021". Journal of Geophysical Research: Solid Earth. 127 (7): 127. Bibcode:2022JGRB..12724336T. doi:10.1029/2022JB024336. S2CID 249857173. Retrieved August 9, 2022.
  12. ^ "A History of Tsunamis in Hawaii". March 11, 2011. Archived from the original on September 29, 2018. Retrieved September 29, 2018.
  13. ^ Kanamori, H. (1972). "Mechanism of tsunami earthquakes" (PDF). Physics of the Earth and Planetary Interiors. 6 (5): 346–359. Bibcode:1972PEPI....6..346K. doi:10.1016/0031-9201(72)90058-1. Archived from the original (PDF) on February 22, 2016. Retrieved February 15, 2016.
  14. ^ The April 1, 1946 Earthquake and Tsunami in the Aleutian Islands, George Pararas-Carayannis, archived from the original on January 19, 2019, retrieved February 9, 2007

External links[edit]