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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. (See location map) 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 seismograph, he commented on the number of events and noted people should always be prepared for significant aftershocks, sometimes equal to or larger than the main shock.
- Red Rock Valley experiences a number of microseisms throughout the year and occasionally 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 - 24 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.
Update: 10:00 a.m., August 20, 1999.
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