1957 Andreanof Islands earthquake

The 1957 Andreanof Islands earthquake occurred at 04:22 local time on March 9 with a moment magnitude estimated at 8.6 and a maximum Modified Mercalli intensity of VIII (Severe). It occurred south of the Andreanof Islands group, which is part of the Aleutian Islands arc. The event occurred along the Aleutian Trench, the convergent plate boundary that separates the Pacific plate and the North American plates near Alaska. A basin-wide tsunami followed, with effects felt in Alaska and Hawaii, and strong waves recorded across the Pacific rim. Total losses were around $5 million ().

Tectonic setting

File:Alaska earthquakes.jpg

Map showing the tectonics and seismicity of Alaska

The Aleutian Islands lie between Kamchatka and mainland Alaska. They were formed as the result of the long convergent boundary that accommodates the subduction of the oceanic Pacific plate underneath the continental North American plate. This oceanic trench runs from the Kuril-Kamchatka Trench in the west to the Yakutat Collision Zone in the east. Most of the trench ruptured in a sequence of earthquakes from east to west.Template:Sfn Earthquakes in 1938, 1946, 1948, and 1965 generally progressed westward with smaller earthquakes filling in any gaps.Template:Sfn At each terminus of the subduction zone, convergence ends in favor of right-lateral transform faults.Template:Sfn In the west, convergence becomes increasingly oblique until the Commander Islands where faulting is nearly completely strike-slip—a 2017 earthquake was associated with this tectonic setting.Template:Sfn The plate boundary ends at the Kuril-Kamchatka Trench.Template:Sfn In the east, the Pacific plate continues to subduct underneath the North American plate until the Yakutat microplate.Template:Sfn There, a transition from subduction to strike-slip faulting exists.Template:Sfn When this transition ends, faulting is completely right-lateral transform and is largely accommodated along the Queen Charlotte Fault.Template:Sfn

Earthquake

The seismic intensity peaked at VIII (Severe) on the Modified Mercalli intensity scale at Adak and Umnak.Template:Sfn As the shock occurred before the World Wide Standardised Seismological Network was in operation, few instruments recorded the event, and its mechanism is not understood well as a result. Some effort was made with the limited data to gain an understanding of the rupture area and the distribution of slip. One aspect of the event that was certain was that the aftershock zone was one of the largest that had ever been observed,Template:SfnTemplate:Sfn comparable to the approximate lower rupture length limit of the 2004 Indian Ocean earthquake. The aftershock zone may slightly overlap other ruptures, however there is minimal overlap between the aftershocks of the 1946 Aleutian Islands earthquake to the east and the 1965 Rat Islands earthquake to the west.Template:Sfn Studies of the event differ on rupture characteristics. Some suggest a rupture zone greater than , stretching from Amchitka Pass in the east to Unimak Pass in the west.Template:Sfn Other studies have the rupture area at a significantly longer .Template:Sfn Yet other studies conclude that the entirety of the aftershock area ruptured in the earthquake, for a total rupture length of .Template:Sfn The western portion of the rupture stopped at Bowers Ridge.Template:Sfn Studies also disagree on whether the easternmost area near Unalaska ruptured. Some of the early scientific papers conclude that this area remained unruptured during the event and remains a seismic gap.Template:Sfn Others, especially ones written decades after the fact, conclude that slip did occur here,Template:Sfn but signals from it were blocked by the coda of the main slip.Template:Sfn However, the amount of slip is not agreed upon. Some studies support a low amount of slip,Template:Sfn while others conclude that there was large amounts of slip in this area,Template:Sfn up to .Template:Sfn A maximum slip of was estimated in the eastern portion of the rupture.Template:Sfn If the eastern portion of the megathrust did rupture, then a magnitude of Template:M is more reflective of the event.Template:Sfn The tsunami created by the earthquake suggests a (Template:M) event.Template:Sfn The earthquake was previously assigned magnitude 9.1 and subsequent analysis have revised it to magnitude 8.6.

Tsunami

Teletsunami observationsTemplate:Sfn

Location	Recorded height (m)
Hāʻena, Kauai County, Hawaii, Hawaii	16.1
Wainiha, Hawaii, Hawaii	11.6
Pololu Valley, Hawaii	9.8
Oahu, Hawaii	7.0
Fagasā, American Samoa	1.5
Crescent City, California	0.7
San Diego, California	0.2

Tsunami waves were reported in far way places such as in Chile.Template:Sfn The tsunami's strength led to suspicion that a landslide may have contributed to its severity, but there is no evidence of a landslide.Template:Sfn A submarine landslide is considered inconsistent with the wave patterns recorded, and the high wave heights could be explained by large amounts of near trench slip.Template:Sfn

Alaska

Wave heights were the highest in Alaska. On Unimak Island, waves reached as high as . Also on Unimak, near the Scotch Cap Lighthouse that was destroyed in the 1946 earthquake, run up heights of were observed.Template:Sfn Trappers Cove recorded wave heights of .Template:Sfn At Sand Bay, the tsunami reached .Template:Sfn Dutch Harbor in Unalaska recorded waves of , Massacre Bay in Attu recorded waves up to high and Sitka had waves reaching .Template:Sfn At Yakutat run-ups measured , while Women's Bay, Kodiak, Seward, and Juneau had recorded tsunami heights of .Template:Sfn

Hawaii

On the island of Kauai, the wave height reached at Haena.Template:Sfn In northern Oahu, wave heights reached .Template:Sfn Various areas around Big Island recorded tsunami waves with heights ranging ,Template:Sfn including a reading of at Hilo. In Kahului, Maui, tide gauges recorded waves up to .Template:Sfn Coconut Island was submerged by .Template:Sfn

California

Crescent City recorded a tsunami wave of .Template:Sfn Los Angeles recorded run-ups of , Santa Monica experienced a -high tsunami, while Anaheim Bay had waves.Template:Sfn San Francisco's tide gauge recorded run-ups of .Template:Sfn In San Diego, a wave caused minor damage, however the tide gauge only recorded a wave high.Template:Sfn Other tide gauges across the state recorded run-up heights of .Template:Sfn

Elsewhere

At Fagasā, American Samoa, tsunami run-up heights reached .Template:Sfn Pago Pago recorded wave heights of , however the amplitude of the wave was .Template:Sfn Midway recorded tsunami waves up to high.Template:Sfn Wake Island recorded amplitudes of , Kwajalein and Enewetak recorded heights of .Template:Sfn Johnston Atoll experienced waves of , while waves less than were recorded at Guam and Chuuk Lagoon.Template:Sfn In Mexico, the tidal gauge in Ensenada, Baja California recorded the strongest waves at .Template:Sfn Many countries in Central America also recorded tsunami run-ups including at Puntarenas, Costa Rica, at Puerto San José, Guatemala, and waves at La Unión, El Salvador.Template:Sfn Peru and Chile were favorably oriented for large waves from the tsunami, and as a result strong waves were recorded. In Peru, the strongest wave heights of were recorded at Matarani, with other coastal areas recording wave heights of .Template:Sfn Valparaíso, Chile recorded wave heights of , which were the highest across the country.Template:Sfn Across the rest of the country, wave heights of , , , and were recorded at Arica, Antofagasta, Caldera, and Talcahuano, respectively.Template:Sfn

Damage

Prompt warnings from the Seismic Sea Wave Warning System were credited with preventing major damage or loss of life.Template:Sfn The earthquake caused severe damage to roads and buildings on Adak including a crack in size,Template:Sfn however there were no deaths.Template:Sfn Two bridges and some oil and fuel-related structures at a dock were also destroyed there.Template:Sfn On Umnak, a concrete mixer and some docks were lost.Template:Sfn At Chernofski, Trappers Cove, and Vsevidof, strong waves drowned sheep.Template:Sfn Oil pipelines were damaged at Sand Bay.Template:Sfn Many boats were damaged from strong waves.Template:Sfn

The tsunami caused twice the damage the tsunami of the 1946 earthquake did.Template:Sfn In Hawaii, damage was much more extensive, including two indirect fatalities that occurred when a pilot and photographer were killed while attempting to document the tsunami's arrival from an airplane. About 50 homes were flooded on the north shore of Oahu and significant effects were seen in Waialua Bay. Buildings and bridges were also flooded in Haleiwa.Template:Sfn In Hilo, the tsunami damaged buildings. The total damage cost was over $5 million ($46 million in 2017).

See also

• List of earthquakes in 1957
• List of earthquakes in Alaska
• List of earthquakes in the United States
• Mount Vsevidof

References

Sources

External links

• USGS Earthquake Hazards Program – Andreanof Islands, Alaska, Magnitude 8.6
• Tsunami! – 1957 Aleutian tsunami Template:Webarchive
• The March 9, 1957 Aleutian Tsunami – George Pararas-Carayannis
• Template:EQ-isc-link