GRCC seismometer records Montana earthquake

On July 6, 2017 GRMI, the seismometer in the geology lab of Calkins, recorded a magnitude 5.8 earthquake near Helena in western Montana. Buried in the seismic trace of the larger earthquake were two aftershocks, one measuring 4.9 and the other 4.5 in magnitude. In the seismogram, we can see many aftershocks associated with the magnitude 5.8 earthquake. This swarm of earthquakes occurred as the result of strike-slip (horizontal) faulting along the Lewis and Clark line, a broad zone of faulting about 400 km in length, and up to 80 km wide that extends from Helena, Montana southwest into eastern Idaho. Other notable events from the Lewis and Clark line include the August 1959, magnitude 7.2 Hebgen Lake earthquake, the largest historic earthquake in this region. The Hebgen Lake earthquake triggered a massive landslide that resulted in more than 28 fatalities, mostly in campgrounds around the lake. Fortunately, this most recent quake caused only minor damage and no reported injuries. For more information about the earthquake visit the U.S. Geological Survey’s website.

Readings from GRCC's seismometer on July 6 show a lot of activity on the 40-minute mark.