Poster of the Dillon, Montana Earthquake of 26 July 2005 - Magnitude 5.6

Tectonic Summary

Montana is one of the most seismically active States in the U.S. Since 1925, the State has experienced five shocks that reached intensity VIII or greater (Modified Mercalli Scale). During the same interval, hundreds of less severe tremors were felt within the State. Montana's earthquake activity is concentrated mostly in the mountainous western third of the State which lies within a seismic zone that includes western Montana, southeastern Idaho, western Wyoming, and central Utah (see Earthquake History of Montana). There are a variety of fault types represented in this seismic zone. Some faults clearly show evidence of being the source of many large-magnitude earthquakes in the past, and some faults do not. East-central Idaho and western Montana is characterized by long linear mountain ranges with intervening valleys. Geologically young faults bound most or all of these mountain blocks. Many of which are capable of producing large-magnitude earthquakes similar to the largest earthquakes that have occurred historically in the seismic zone (about magnitude 7.5). Seismologists have not yet determined the causative fault of the recent earthquake.

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