The July 28, 2021 earthquake

M8.2Chignik, Alaska

A magnitude 8.2 earthquake 21.7 miles deep, 45.3 miles off Lake and Peninsula Borough. The 6th strongest in Alaska’s record since 1900, and the strongest within 50 miles since a magnitude 8.2 on November 10, 1938. 628 aftershocks of magnitude 2.5 or more followed within 30 days.

Watch the sequence unfold

Magnitude 8.2 · July 28, 2021

How the ground moved

Every recorded earthquake in the sequence, in order, from the foreshocks to a month after.

About this replay

Local time (AKDT)—Historical replay — logged times

Main shock—

Aftershocks—of 628 in 30 days

Largest so far—

Sequence milestones

EarthquakesSized by magnitude; each flares as it strikes, then fades

Main shockMagnitude 8.2 at 21.7 miles deep

Shaking frontsThe faster and slower seismic waves, drawn at typical speeds of 6 and 3.5 km per second, not measured ones

Sequence areaEarthquakes within 170 miles are counted, the length of fault a magnitude 8.2 typically breaks

Source: U.S. Geological Survey ComCat catalogue, every event it holds within 170 miles of the epicentre from seven days before to 30 days after. Times as logged, shown in local time (AKDT); the clock runs faster as the sequence slows. Towns are today’s Census places.

The earthquake

Magnitude: 8.2; moment magnitude (mww)
Depth: 21.7 mi; 35.0 km below the surface
Local time: Wednesday, July 28, 2021; 22:15 AKDT; 06:15 UTC on Jul 29, 2021
Epicentre: 55.36°N, 157.89°W; 45.3 mi from Lake and Peninsula Borough
In Alaska’s record: 6th; strongest of 124,625 since 1900
Within 50 miles: 1; county; more on this date

USGS ANSS Comprehensive Earthquake Catalog, event ak0219neiszm (“2021 Chignik, Alaska Earthquake”). The USGS event page carries the ShakeMap, felt reports and the official summary.

Aftershocks

Earthquakes of magnitude 2.5 or more within 170 miles of the epicentre in the 30 days after.

628

Aftershocks in 30 days

6.9

Largest aftershock, Aug 14, 2021

259

On the first day

In the 7 days before

The search radius is the surface rupture length expected for an earthquake of this magnitude (Wells and Coppersmith, 1994), at least 20 and at most 600 miles, so a great earthquake's sequence is counted along its whole fault.

Counties and cities within 50 miles

Distance from the epicentre to each county boundary (zero when inside) and to each city centre.

Counties

Lake and Peninsula BoroughAK · 45.3 mi

Earthquakes around the epicentre

512 earthquakes of magnitude 3 or more within 50 miles of this epicentre since 1973.

Earlier earthquakes of magnitude 5 or more within 50 miles, strongest first

November 10, 1938M8.2
July 21, 2020M7.8
November 16, 1938M6.9
February 12, 1979M6.7
July 28, 1934M6.6
November 10, 1938M6.6
August 5, 1927M6.2
July 28, 1927M6.2
January 1, 1985M6.0
November 2, 1930M5.8

USGS ANSS Comprehensive Earthquake Catalog. Local time is the time zone of the county nearest the epicentre. Magnitudes are as the catalogue reports them.