1957 Andreanof Islands earthquake
|Date||9 March 1957|
|Origin time||14:22:33 (UTC) |
|Magnitude||8.6 Mw |
|Depth||16 mi (25 km) |
|Areas affected||Aleutian Islands & Hawaii|
|Total damage||$5 million |
|Max. intensity||VIII (Severe) |
The 1957 Andreanof Islands earthquake took place on March 9 with a moment magnitude of 8.6 and a maximum 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. Total losses were around $5 million.
The 2,500 mi (4,000 km) Aleutian Subduction Zone is the convergent boundary of the Pacific and North American Plates. This oceanic trench runs from the Kuril Subduction Zone in the west to the Yakutat Collision Zone in the east. At each end of the subduction zone are right-lateral transform faults, including the Queen Charlotte Fault in the east, and a similar structure at the far west end of the arc near Attu Island.
Because the shock occurred before the World Wide Standardised Seismological Network was in operation, few instruments captured 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 750 mi (1,200 km) aftershock zone was the largest that had ever been observed.
In Alaska, the earthquake caused severe damage to roads and buildings on Adak Island, but no lives were lost. Two bridges and some oil and fuel-related structures at the dock were also destroyed there. On Umnak Island, a concrete mixer and some docks were lost. Prompt warnings from the Seismic Sea Wave Warning System were credited with preventing greater damage or loss of life.
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. On the island of Kauai, the wave height reached 52 feet (16 m). By comparison, the effects were considered about twice as strong as that of the 1946 Aleutian Islands earthquake and resulting tsunami. About 50 homes were flooded on the north shore of Oahu and significant effects were seen in Waialua Bay. Buildings and bridges were also affected in Haleiwa.
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