On July 6, 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. 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 kilometers in length and up to 80 kilometers wide that extends from Helena southwest into eastern Idaho. Other notable events from the Lewis and Clark line include the magnitude 7.2 Hebgen Lake earthquake in August 1959. It triggered a massive landslide that resulted in more than 28 deaths, mostly in campgrounds around the lake.
Fortunately, the July 6 quake caused only minor damage and no reported injuries.