Massive M8.2 earthquake hits near the coast of Alaska, tsunami warnings issued

A massive shallow earthquake registered by the USGS as M8.2 hit near the coast of Alaska at 06:15 UTC on July 29, 2021. The agency is reporting a depth of 32.2 km (20 miles). EMSC is reporting it as M8.2 at depth of 30 km (18.6 miles). This is the strongest earthquake in the world since M8.2 off the coast of Fiji on August 19, 2018.

The epicenter was located 90.9 km (56.5 km) ESE of Perryville, Alaska, United States.

There are about 400 people living within 100 km (62 miles).

2 000 people are estimated to have felt very strong shaking, 1 000 moderate, and 25 000 light.

As of 07:31 UTC, a Tsunami Warning is in effect for SOUTH ALASKA AND THE ALASKA PENINSULA – Pacific coasts from Hinchinbrook Entrance, Alaska (145 km / 90 miles E of Seward) to Unimak Pass, Alaska (138 km / 80 miles NE of Unalaska); and ALEUTIAN ISLANDS – Unimak Pass, Alaska (128 km / 80 miles NE of Unalaska) to Samalga Pass, Alaska (48 km / 30 miles SW of Nikolski).

A Tsunami Advisory is in effect for SOUTHEAST ALASKA – The inner and outer coast from Cape Decision, Alaska (138 km / 85 miles SE of Sitka) to Cape Fairweather, Alaska (128 km / 80 miles SE of Yakutat), SOUTH ALASKA AND THE ALASKA PENINSULA – Pacific coasts from Cape Fairweather, Alaska (128 km / 80 miles SE of Yakutat) to Hinchinbrook Entrance, Alaska (145 km / 90 miles E of Seward); ALEUTIAN ISLANDS – Samalga Pass, Alaska (48 km / 30 miles SW of Nikolski) to Amchitka Pass, Alaska (201 km / 125 miles W of Adak) including the Pribilof Islands.

Recommended actions

by NWS National Tsunami Warning Center in Palmer, AK:

Actions to protect human life and property will vary within tsunami warning areas and within tsunami advisory areas.

If you are in a tsunami warning area, evacuate inland or to higher ground above and beyond designated tsunami hazard zones or move to an upper floor of a multi-story building depending on your situation.

If you are in a tsunami warning or advisory area:

• Move out of the water, off the beach, and away from harbors, marinas, breakwaters, bays and inlets.
• Be alert to and follow instructions from your local emergency officials because they may have more detailed or specific information for your location.
• If you feel a strong earthquake or extended ground rolling take immediate protective actions such as moving inland and/or uphill preferably by foot.
• Boat operators: Where time and conditions permit, move your boat out to sea to a depth of at least 55 m (180 feet). If at sea avoid entering shallow water, harbors, marinas, bays, and inlets to avoid floating and submerged debris and strong currents.
• Do not go to the shore to observe the tsunami.
• Do not return to the coast until local emergency officials indicate it is safe to do so.

IMPACTS

Impacts will vary at different locations in the warning and in the advisory areas.

If you are in a tsunami warning area;

• A tsunami with damaging waves and powerful currents is possible.
• Repeated coastal flooding is possible as waves arrive onshore, move inland, and drain back into the ocean.
• Strong and unusual waves, currents and inland flooding can drown or injure people and weaken or destroy structures on land and in water.
• Water filled with floating or submerged debris that can injure or kill people and weaken or destroy buildings and bridges is possible.
• Strong and unusual currents and waves in harbors, marinas, bays, and inlets may be especially destructive.

If you are in a tsunami advisory area:

• A tsunami with strong waves and currents is possible.
• Waves and currents can drown or injure people who are in the water.
• Currents at beaches and in harbors, marinas, bays, and inlets may be especially dangerous.

If you are in a tsunami warning or advisory area:

• Some impacts may continue for many hours to days after arrival of the first wave.
• The first wave may not be the largest so later waves may be larger.
• Each wave may last 5 to 45 minutes as a wave encroaches and recedes.
• Coasts facing all directions are threatened because the waves can wrap around islands and headlands and into bays.
• Strong shaking or rolling of the ground indicates an earthquake has occurred and a tsunami may be imminent.
• A rapidly receding or receded shoreline, unusual waves and sounds, and strong currents are signs of a tsunami.
• The tsunami may appear as water moving rapidly out to sea, a gentle rising tide like flood with no breaking wave, as a series of breaking waves, or a frothy wall of water.

Tsunami update:

All tsunami warnings, advisories and watches have been canceled.

A tsunami was generated by this event, but no longer poses a threat. However, some areas may continue to see small sea level changes. The determination to re-occupy hazard zones must be made by local officials, NWS NTWC in Palmer, AK said at 09:29 UTC.

The maximum observed tsunami height was in Old Harbor, Alaska at 09:05 UTC – 21 cm (0.7 feet).

Image credit: TW/SAM, Google

The USGS issued a Green alert for shaking-related fatalities and economic losses. There is a low likelihood of casualties and damage.

Overall, the population in this region resides in structures that are resistant to earthquake shaking, though vulnerable structures exist. The predominant vulnerable building types are unreinforced brick masonry and reinforced masonry construction.

A Yellow alert was issued for economic losses. Some damage is possible and the impact should be relatively localized. Estimated economic losses are less than 1% of GDP of the United States. Past events with this alert level have required a local or regional level response.

The earthquake occurred as the result of thrust faulting at shallow depth on the subduction zone interface between the Pacific plate and North America plate, where the Pacific plate begins subducting to the northwest beneath Alaska.

Large earthquakes in this region are common, USGS said.

The same subduction zone hosted the second-largest earthquake recorded on modern instrumentation in March 1964 (M9.2 Alaska earthquake). M8.2 event occurred in a very similar location to today's earthquake in 1938. 

The quake was followed by M6.1 at 06:19 UTC, M6.2 at 06:23, and M5.6 at 06:32.

Aftershocks forecast

The USGS advised everyone to be aware of the possibility of aftershocks, especially when in or around vulnerable structures such as unreinforced masonry buildings.

The agency estimates the chance of more aftershocks as follows:

Within the next 1 Week until 06:40 UTC on August 5, 2021:

• The chance of an earthquake of magnitude 3 or higher is > 99 %, and it is most likely that as few as 380 or as many as 33 000 such earthquakes may occur in the case that the sequence is re-invigorated by a larger aftershock.
• The chance of an earthquake of magnitude 5 or higher is > 99 %, and it is most likely that as few as 3 or as many as 330 such earthquakes may occur.
• The chance of an earthquake of magnitude 6 or higher is 87 %, and it is most likely that as few as 0 or as many as 33 such earthquakes may occur.
• The chance of an earthquake of magnitude 7 or higher is 36 %, and it is most likely that as few as 0 or as many as 4 such earthquakes may occur.

Estimated population exposure to earthquake shaking

Selected cities exposed

Regional seismicity

Featured image credit: TW/SAM, Google