The March 20, 1999 earthquake
M6.9near Adak, Alaska
A magnitude 6.9 earthquake 20.5 miles deep, 4.7 miles off Aleutians West Census Area. The 72nd strongest in Alaska’s record since 1900, and the strongest within 50 miles since a magnitude 7.3 on June 10, 1996. 3 aftershocks of magnitude 2.5 or more followed within 30 days.
The earthquake
- Magnitude
- 6.9
- moment magnitude (mwc)
- Depth
- 20.5 mi
- 33.0 km below the surface
- Local time
- Saturday, March 20, 1999
- 00:47 HST; 10:47 UTC
- In Alaska’s record
- 72nd
- strongest of 124,625 since 1900
USGS ANSS Comprehensive Earthquake Catalog, event usp00094ud (“78 km WSW of Adak, 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.
3
Aftershocks in 30 days
4.7
Largest aftershock, Mar 28, 1999
0
On the first day
0
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 · 4.7 mi
Earthquakes around the epicentre
3,427 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 9, 1996M7.9
- February 13, 1905M7.6
- July 7, 1929M7.3
- June 10, 1996M7.3
- March 14, 1957M7.1
- March 12, 1957M7.1
- July 5, 1929M7.0
- May 1, 1971M7.0
- March 11, 1957M7.0
- February 6, 1971M6.9
USGS ANSS Comprehensive Earthquake Catalog. Local time is the time zone of the county nearest the epicentre. Magnitudes are as the catalogue reports them.