The July 30, 1972 earthquake
M7.6near Sitka, Alaska
A magnitude 7.6 earthquake 12.4 miles deep, 18.1 miles off Sitka City and Borough. The 17th strongest in Alaska’s record since 1900, and the strongest within 50 miles in the catalogue, which runs from 1900. 2 aftershocks of magnitude 2.5 or more followed within 30 days.
The earthquake
- Magnitude
- 7.6
- moment magnitude (mw)
- Depth
- 12.4 mi
- 19.9 km below the surface
- Local time
- Sunday, July 30, 1972
- 12:45 AHDT; 21:45 UTC
- In Alaska’s record
- 17th
- strongest of 124,625 since 1900
USGS ANSS Comprehensive Earthquake Catalog, event iscgem772060 (“55 km SW of Sitka, Alaska”). The USGS event page carries the ShakeMap, felt reports and the official summary.
Aftershocks
Earthquakes of magnitude 2.5 or more within 66 miles of the epicentre in the 30 days after.
2
Aftershocks in 30 days
6.1
Largest aftershock, Aug 4, 1972
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.
- Before the modern seismic networks of the 1970s many small aftershocks went unrecorded, so early sequences read short.
Counties and cities within 50 miles
Distance from the epicentre to each county boundary (zero when inside) and to each city centre.
Counties
- Sitka City and BoroughAK · 18.1 mi
Earthquakes around the epicentre
40 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
- October 30, 1949M6.2
USGS ANSS Comprehensive Earthquake Catalog. Local time is the time zone of the county nearest the epicentre. Magnitudes are as the catalogue reports them.