There are several reasons why we don’t need to be overly concerned about the Pit in the event of an earthquake, including the fact that there has been no significant seismic activity in nearby faults during the 28 years that the Earthquake Studies Office has been monitoring the area.
In the mining region, the recorded seismic activity is mainly caused by a few mining blasts per week, however, two landslides in the pit have been recorded in the last 28 years and a few very small, non-mining related earthquakes are recorded within Butte Silver-Bow county annually. Most seismic activity in Montana occurs outside of Silver-Bow County, and in the worst-case scenario, a large earthquake could cause landslides or sloughing in the Pit, but would not cause the Pit to overflow. Such an earthquake would cause considerably more damage to buildings and structures in the Uptown area than to the landscape or Pit.
After the initial PitWatch article on earthquakes in the Spring of 2005, there was a 5.6 magnitude earthquake centered near Dillon on July 25, 2005. This earthquake was felt in Butte, but there was no evidence of any sloughing or rise in water level in the Pit.
According to the Earthquake Studies Office, there have been approximately 20 very small earthquakes within 25 km of Butte in the past year, ranging in magnitude from -.2 to 1.9. Nineteen of these were less than magnitude 1.0, and only 4 were located within Silver-Bow County. Three of these events were non-mining related events in Butte and were not large enough to be felt.
There have been no reports, past or present, of any earthquake damage to the Pit and the last earthquake greater than magnitude 1.0 in the area was a 2.8 on October 9, 2005, located 3.3 kilometers west-southwest of Butte along Silver Bow Creek.