1935 10 19 04:48 UTC (local 10/18)
This is the main earthquake of the 1935 series of shocks at Helena. Two people were killed by falling bricks, several were injured, and property damage was estimated at about $3 million. The earthquake damaged about 300 buildings, of which more than 200 lost their chimneys. Damage was most severe in 2 and 2 1/2-story brick houses on alluvial soil in northeast Helena, but severe damage also occurred in the southern business section of Helena. Downed chimneys and cracked plaster were common throughout the city, and in sections, almost all chimneys were destroyed. Gables commonly were damaged, regardless of the structural material used or the location of the building.
The most severely damaged structure in the area was the Helena High School, constructed a few months earlier, in August 1935. Many large buildings were damaged heavily, including the City Hall, Kessler Brewery, and St. Joseph's Orphanage, but damage was slight to structures on solid bedrock (e.g., the State Capitol, Federal Building, and St. Helena Cathedral). In general, wood buildings covered with wood siding and structures having a framework of reinforced concrete or steel sustained little damage. Tombstones in all the cemeteries in the area were twisted or overturned.
The ground cracks observed were shallow, narrow, surface cracks in alluvial material caused by shaking of the ground, and none represent slip along the fault plane. East of town, water flowed from the cracks that formed in the ground. The largest crack, a maximum of 13 centimeters wide and 91 meters long, was observed on the gravel road leading into the Stanchfield Gun Club.
Changes in the volume of flow of many wells and springs occurred. The most noted change was an increase in flow of springs or the formation of new springs. Seven Mile Creek, which was almost dry before the earthquake, was about 1.5 meters wide and 30 to 45 centimeters deep when it was observed on Oct. 31. Also felt in parts of Idaho, Washington, Wyoming, and in adjacent areas of Canada.
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.