The September 12, 1986 earthquake

M6.5near Akhiok, Alaska

A magnitude 6.5 earthquake 19.4 miles deep, 29.7 miles off Kodiak Island Borough. The 187th strongest in Alaska’s record since 1900, and the strongest within 50 miles since a magnitude 6.8 on June 19, 1986. 8 aftershocks of magnitude 2.5 or more followed within 30 days.

The earthquake

Magnitude: 6.5; moment magnitude (mw)
Depth: 19.4 mi; 31.3 km below the surface
Local time: Friday, September 12, 1986; 15:57 AKDT; 23:57 UTC
Epicentre: 56.20°N, 153.41°W; 29.7 mi from Kodiak Island Borough
In Alaska’s record: 187th; strongest of 124,625 since 1900
Within 50 miles: 1; county; more on this date

USGS ANSS Comprehensive Earthquake Catalog, event usp0002y12 (“95 km SSE of Akhiok, Alaska”). The USGS event page carries the ShakeMap, felt reports and the official summary.

Aftershocks

Earthquakes of magnitude 2.5 or more within 20 miles of the epicentre in the 30 days after.

Aftershocks in 30 days

5.9

Largest aftershock, Sep 16, 1986

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

Kodiak Island BoroughAK · 29.7 mi

Earthquakes around the epicentre

493 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

June 19, 1986M6.8
April 10, 1957M6.7
December 21, 1917M6.6
April 11, 1978M6.6
March 28, 1964M6.5
March 28, 1964M6.5
March 29, 1964M6.5
April 4, 1964M6.5
June 15, 1953M6.5
November 29, 1952M6.5

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