Notable Earthquake of the Week

Clarkston Valley, Montana M6.6 Earthquake  

On June 27, 1925, a very strong earthquake measuring about M6.6 (M6-3/4) rocked the west-central region of Montana, about 30 miles northwest of Bozeman in Clarkston Valley.

The earthquake caused significant damage within the epicentral region and it was felt as far away as North Dakota, Wyoming, Washington, and Canada. The greatest damage occurred in Manhattan, Three Forks, and Lombard, where many buildings collapsed, numerous chimneys were thrown from roofs, and walls were extensively cracked.

Photograph to the upper right is of the Methodist Church in Three Forks, Montana. The tall twelve-inch-thick brick wall was not tied in at the second story and the mortar failed (BSSA, 1926- By J. T. Pardee; Courtesy of USGS - see photo links below).

Rock falls and landslides were common in the mountainous regions, some of which blocked roads and railroad tracks. One rock fall near Lombard, which nearly missed a passing train, blocked rail transportation, and cost railroads an estimated one million dollars to repair.

Seismological analysis of the main shock suggested a focal depth of about 9 km with a subsurface rupture length of approximately 12 km, with oblique-normal slip. The earthquake is believed to have occurred along the Clarkston Valley fault and despite the significant size and shallow depth, no primary surface fault ruptures were ever discovered.

The main jolt was preceded by a sizable foreshock, probably registering about M6, and several aftershocks, the largest of which measured M4.8 on July 10.

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