2020 Alaska Peninsula earthquake

On July 21, at 10:12:44 local time, a magnitude 7.8 undersea earthquake struck ~65 miles south-southeast of the small town of Perryville, Alaska. It spawned a small tsunami that evacuated coastal residences, but caused no severe damage or injuries due to its location relative to the sparsely-populated Alaska Peninsula. It is, thus far, the largest earthquake in 2020 by magnitude[5]

2020 Alaska Peninsula earthquake
Anchorage
Kodiak
Perryville
UTC time2020-07-22 06:12:44
ISC event618259420
USGS-ANSSComCat
Local dateJuly 21, 2020
Local time10:12 a.m. AKDT
MagnitudeMww7.8
Depth28 km (17 mi)
Epicenter38.159°N 117.875°W / 38.159; -117.875
FaultAleutian subduction zone
TypeThrust[1]
Areas affectedAlaska
Max. intensityVII (Very Strong)[2]
Tsunamiyes
Aftershocks900+ (As of 12:00 AKDT August 11)[3][4]
Casualties0

Geology

The shallow earthquake was the result of thrust faulting[1] on or near the Aleutian subduction zone[6] where the Pacific plate subducts underneath the North American plate, forming the Aleutian Trench and Arc. The convergent boundary is one of the most active in the world, and was the epicenter for the Mw9.2 1964 Alaska earthquake; the largest recorded in North America, and the second largest in the world.[7] On average, the rate at which these plates converge is about 64 mm/year.[8]

Earthquake

The earthquake ruptured an area of about 120 mi × 60 mi (193 km × 97 km), equal to about 7,200 sq mi (19,000 km2). The estimated maximum slip along this fault surface is about 3 to 4 m (9.8 to 13.1 ft).[9] It was reportedly felt as far as Anchorage, more than 500 miles (800 km) away.[10][11]

Impact

There have been no reports of severe damage or injuries.[12][13] Mild damage occurred in Sand Point with reports of damaged docks, cracked roads and cement.[14]

Tsunami

A tsunami warning was issued by the NOAA for cities and towns within 200 miles from the epicenter, evacuating coastal residents to higher ground. The warning was rescinded about 4 hours later, as measured run-up heights of .24 metres (9.4 inches) were negligible.[15][16][17][11]

Aftershocks

Within the first 5 hours proceeding the mainshock, there were 6 aftershocks at or above M 4.5, including a mb 6.1 occurring less than 4 minutes later.[18][19] 6 days later on July 28, another Mww6.1 aftershock occurred west of the mainshock.[20][4] Both this, and the July 22 event are tied as the strongest aftershocks of the sequence.

References
  1. "USGS M7.8 focal mechanism". earthquake.usgs.gov. Retrieved August 14, 2020.
  2. "USGS Impact summary".
  3. "USGS aftershocks from 7.8 Alaska". earthquake.usgs.gov. Retrieved July 22, 2020.
  4. "Event Page | Alaska Earthquake Center". earthquake.alaska.edu. Retrieved August 11, 2020.
  5. "USGS worldwide earthquakes above M 7.5 from 1/1/2020 - 12/31/2020, largest to smallest". earthquake.usgs.gov. Retrieved July 22, 2020.
  6. "Event Page | Alaska Earthquake Center". earthquake.alaska.edu. Retrieved August 14, 2020.
  7. Carver, Gary; Plafker, George (2008). "Paleoseismicity and neotectonics of the Aleutian Subduction Zone—An overview". Washington DC American Geophysical Union Geophysical Monograph Series. 179: 43–63. doi:10.1029/179GM03.
  8. "Tectonic summary". earthquake.usgs.gov. Retrieved July 22, 2020.
  9. "Event Page | Alaska Earthquake Center". earthquake.alaska.edu. Retrieved August 14, 2020.
  10. "DYFI felt reports, USGS". earthquake.usgs.gov. Retrieved July 22, 2020.
  11. News, A. B. C. "Powerful quake jolts Alaska towns, produces small tsunami". ABC News. Retrieved July 22, 2020.
  12. AP (July 22, 2020). "7.8-magnitude quake rocks Alaska, tsunami warning canceled". ABC7 Los Angeles. Retrieved July 22, 2020.
  13. "Powerful 7.8 quake hits Alaska isles; tsunami threat over". WYFF. July 22, 2020. Retrieved July 22, 2020.
  14. McKenney, Hope. "Sand Point's Docks, Road To Harbor Damaged In Magnitude-7.8 Earthquake". www.kucb.org. Retrieved August 11, 2020.
  15. "Tsunami Event". www.ngdc.noaa.gov. Retrieved August 11, 2020.
  16. "Tsunami warning canceled hours after strong earthquake hit off Alaska". www.cbsnews.com. Retrieved July 22, 2020.
  17. title=Public Tsunami Message Number 5 (final) - NWS National Tsunami Warning Center Palmer AK|https://ntwc.ncep.noaa.gov/events/PAAQ/2020/07/22/qdux98/5/WEAK51/WEAK51.txt
  18. "USGS 6.1 Alaska (aftershock)". earthquake.usgs.gov. Retrieved July 22, 2020.
  19. "USGS aftershocks above M 4.5". earthquake.usgs.gov. Retrieved July 22, 2020.
  20. "USGS 6.1 aftershock #2". earthquake.usgs.gov. Retrieved August 11, 2020.
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