The February 26, 1987 earthquake

M6.9near Unalaska, Alaska

A magnitude 6.9 earthquake 6.2 miles deep, centred in Aleutians West Census Area. The 72nd strongest in Alaska’s record since 1900, and the strongest within 50 miles since a magnitude 6.9 on March 23, 1980. 28 aftershocks of magnitude 2.5 or more followed within 30 days.

Watch the sequence unfold

Magnitude 6.9 · February 26, 1987

How the ground moved

Every recorded earthquake in the sequence, in order, through the month after.

About this replay

Local time (AKST)—Historical replay — logged times

Main shock—

Aftershocks—of 28 in 30 days

Largest so far—

Sequence milestones

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

Main shockMagnitude 6.9 at 6.2 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 22 miles are counted, the length of fault a magnitude 6.9 typically breaks

Source: U.S. Geological Survey ComCat catalogue, every event it holds within 22 miles of the epicentre from seven days before to 30 days after. Times as logged, shown in local time (AKST); the clock runs faster as the sequence slows. Towns are today’s Census places.

The earthquake

Magnitude: 6.9; moment magnitude (mw)
Depth: 6.2 mi; 10.0 km below the surface
Local time: Thursday, February 26, 1987; 23:31 AKST; 08:31 UTC on Feb 27, 1987
Epicentre: 53.47°N, 167.29°W; In Aleutians West Census Area
In Alaska’s record: 72nd; strongest of 124,625 since 1900
Within 50 miles: 1; county; more on this date

USGS ANSS Comprehensive Earthquake Catalog, event usp00032u7 (“67 km SW of Unalaska, Alaska”). The USGS event page carries the ShakeMap, felt reports and the official summary.

Aftershocks

Earthquakes of magnitude 2.5 or more within 22 miles of the epicentre in the 30 days after.

Aftershocks in 30 days

5.4

Largest aftershock, Mar 16, 1987

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

Aleutians West Census AreaAK · inside

Earthquakes around the epicentre

1,238 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

July 2, 1965M7.8
March 23, 1980M6.9
March 28, 1957M6.8
January 12, 1955M6.8
January 12, 1955M6.5
March 14, 1957M6.4
January 8, 1957M6.2
September 4, 1942M6.2
October 31, 1985M6.2
June 12, 1972M6.1

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