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An earthquake registering a preliminary magnitude of Ml 5.3 (NEIC) was
detected 13:50:26 UTC (7:50 a.m. MDT), August 20, in southwestern
Montana. The focal depth was placed at a depth of 12 km and we do not
have a fault plane solution for this event as of yet.

The quake was centered in Red Rock Valley between the Clark Canyon and
Lime Reservoirs, near the small town of Dell. The quake was widely felt
in the valley and as far away as Helena, Butte, Billings, Montana,
Salmon and Boise, Idaho, and Yellowstone, Wyoming. Light items were
tossed from tables and shelves and some pictures were knocked from
walls. There have been no reports of broken windows or toppled chimneys.
There have been no reports of injuries, yet several people were
frightened by the shaking.

According to the Michael Stickney, chief seismologist for Montana Bureau
of Mines and Geology, about 2 dozen aftershocks have been recorded, the
largest of which measured in the upper M2 range. Looking at his drum
seismograph, he commented on the number of events and noted people
should always be prepared for significant aftershocks, sometimes equal
or larger than the main shock.

Red Rock Valley experiences a number of microseisms throughout the year
and occasional is the site of vigorous earthquake swarm activity. The
M5.3 temblor is the largest quake in Montana since a M5.0 near Flathead
Lake in northwestern Montana in 1975 - 44 years ago! A similar magnitude
quake struck close to today's epicenter in January 1960 and in 1897, was
perhaps the site of a destructive M6.7 temblor, but the epicenter for
that event is not well constrained.

A report on this and other recent earthquakes can be found on the
Seismo-Watch web page.

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Charles P. Watson, Consulting Geologist
Advanced Geologic Exploration/Seismo-Watch
P.O. Box 18012, Reno, Nevada 89511
Voice: 775-852-0992  /  Fax: 775-852-3226
mailto:watson@seismo-watch.com
http://www.seismo-watch.com