1935 October 31 18:37:47 UTC
This aftershock was almost as severe as the main tremor on October 19. Two people were killed at Helena, and about $1 million in property damage occurred, bringing the death toll from this series to four and the damage total to $4 million. (Ref. 512 reports a total of 6 deaths and $5.5 million damage). It intensified the damage in all the towns and damaged structures weakened by previous shocks. Most residents described it as sharper and more pronounced than the main earthquake on October 19.
Many buildings previously damaged were demolished, including the new Helena High School and the Kessler Brewery. Damage was most severe in the neighborhood of the City Hall on Main Street and in the residential district on 9th Street. On the west side of town, damage from this shock was more severe than that caused by the October 19 earthquake. Damage to frame buildings was slight, except to their chimneys and brick-veneer facing.
The ground in Helena Valley again was cracked. Water was observed spurting 30 cm or more from cracks, and dust was emitted from others. All chimneys in this neighborhood were downed, and a bridge was shifted slightly. Several tombstones turned over in the Resurrection Cemetery, about 5 km north of Helena. Also felt in parts of Idaho, Washington, Wyoming, and 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.