A massive earthquake struck off the coast of Alaska prompting a tsunami alert for a large part of the state and surrounding regions.
The seismic shock occurred on July 28, 2021, at 10:15 p.m. local time. Its epicenter was located about 90 kilometers away from Perryville, situated in the Alaska Peninsula and its depth was derived at about 47 kilometers. Preliminary data suggest that the shaking caused in Alaska was moderate and there were no immediate reports of severe infrastructure damage. Two powerful aftershocks with magnitudes of M6.2 and M5.6, respectively, struck within 30 minutes from the initial shock.
According to the United States Geological Survey (USGS), the M8.2 mainshock occurred on a thrust fault, dipping either shallowly to the northwest or steeply to the southeast, that is situated on or near the subduction zone interface between the Pacific and North American plates. In the location of interest, the Pacific plate is subducting under the North American plate at an annual rate of 64 millimeters. Earthquakes of such magnitude occupy a vast rupture area. An equivalent reverse faulting mechanism would typically activate an area of about 16,000km2. The current shock ruptured an area of around 200x100 km (20,000km2) with preliminary data suggesting a slippage of 3-4 meters.
Whilst little structural damage was reported, the seismic shock was felt as far as 2,400 kilometers away. Numerous people living near the epicentral area evacuated their residences as a precautionary measure and those in coastal areas reached higher grounds after a tsunami warning was issued. In King Cove, a city close to the epicenter of the quake located towards the southwestern side of the Alaska Peninsula (about 1,000 km from Anchorage), about 400 people evacuated their houses after the tsunami alert and found temporary shelters in a school gym. Paul Barker, the school's principal admits that such events are strange but not uncommon given the tectonic regime of the region. “...It’s not something you ever get used to, but it’s part of the job living here and being part of the community,” he said. Fortunately, no tsunami waves were reported on North American coasts with the higher recorded wave in Old Harbor, Kodiak Island, being slightly over 15 cm, therefore the warning was later canceled.
According to the major of Kodiak, Pat Branson, the current shock is the stronger to hit the region since the M9.2 Alaska Earthquake in 1964 which is the most powerful earthquake ever recorded in the history of North America. In 2018, a M7.0 earthquake hit Alaska with its epicenter located 13 kilometers northeast of Anchorage. The earthquake was less powerful than the current one but its proximity to communities resulted in significant infrastructure damage with, fortunately, no casualties. Another M7.8 seismic shock struck close to the current rupture area about a year earlier (observe the image above).
Sources: USGS, CNN, NYtimes, Guardian
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