1935 October 12 07:50 UTC
This was the first destructive foreshock of the 1935 series of earthquakes at Helena on October 12, 18, and 31. An estimated total of $4 million in property damage was caused by the series. The fault zone from which the tremors originated lies on the south side of Prickly Pear Valley, north and northeast of Helena. The heavy damage sustained was due to the fact that the shocks occurred almost directly beneath Helena and that each successive shock further damaged previously weakened buildings.
Damage in East Helena from the first tremor was confined to toppled chimneys, broken plaster, and cracked windows. Cracks formed in walls of buildings in Helena, but damage was generally slight. Chimneys toppled at Fort Harrison, plaster was torn from walls, and a hospital was damaged. Total property damage, about $50,000, generally was confined to a few structures.
Abridged from Seismicity of the United States, 1568-1989 (Revised), by Carl W. Stover and Jerry L. Coffman, U.S. Geological Survey Professional Paper 1527, United States Government Printing Office, Washington: 1993.