Distinctly Montana Magazine

D I S T I N C T L Y M O N T A N A M A G A Z I N E • W I N T E R 2 0 2 0 64 E A R T H Q U A K E S E A R T H Q U A K E S E A R T H Q U A K E S yea rs 3 30 YEARS IS NOT EVEN AN EYEBLINK IN GEOLOGIC HISTORY, BUT DO YOU SEE INCREASES IN EARTHQUAKE ACTIVITY OR SEVERITY OVER THAT PERIOD OF TIME? Earthquakes by their nature, are random events, which may see periods of increased or decreased activity. We have not observed any systematic increase in significant earthquake activity during the nearly four decades of seismic monitoring here at the Montana Bureau of Mines and Geology's Earthquake Studies Office. We do report more earthquakes now than we did several decades ago because the seismic monitoring network has expanded and improved, which allows us to de- tect and report more of the small-magnitude events that previously went unreported. However, the potential adverse effects of a major earthquake here in Montana are increasing through time because of the expanding infrastructure that accompanies our state's growing population. WHAT PARTS OF MONTANA SEEM TO HAVE THE GREATEST POTENTIAL EXPO- SURE TO DAMAGING QUAKE ACTIVITY THROUGH 2050? Western Montana has the highest earthquake hazard because of the Intermountain Seismic Belt, which extends from northwestern Montana to Yellowstone National Park and on southward through Salt Lake City, UT and into southern Nevada. e Montana portion of this ~150-mile-wide belt experiences frequent small (less than magnitude 2) earthquakes (5 to 10 per day on average) and occasional larger earth- quakes. All of Montana's major historic earthquakes have occurred in western Montana. While the vast majority of seismicity occurs there, eastern Montana is not immune from earthquakes as demonstrated by the occurrence of a magnitude 5.3 earthquake centered near Scobey on May 15, 1909. e Intermountain Seismic Belt passing through western Montana is clearly delineated on the U.S. Geological Survey's national seismic hazard map. PEOPLE DON'T GENERALLY THINK OF MONTANA AS AN EARTHQUAKE OR FAULT ZONE, BUT WE UNDERSTAND THAT THERE IS ONE FAULT IN PARTICU- LAR, THE MISSION FAULT, WHICH COULD PRODUCE A MAJOR SEISMIC EVENT OVER THE NEXT 30 YEARS. WHERE DOES THIS FAULT LIE, AND WHAT CONCERN DO YOU HAVE ABOUT A DAMAGING EVENT FROM THIS FAULT LINE? ere are about 80 faults in Montana that have the potential to pro- duce future earthquakes. Almost all of these faults lie within the Inter- mountain Seismic Belt in western Montana. One of the largest faults in northwestern Montana is the Mission Fault, which extends northward for 25 miles along the western base of the Mission Mountains roughly between the latitudes of St. Ignatius and Ronan. e Mission Fault is well-studied and these studies indicate that slip along this fault about 7,700 years ago produced a major earthquake. ese studies indicate that major earthquakes occur along the Mission Fault every 4,000 to 8,000 years. While nobody can predict earthquakes, the potential for a major earthquake in the Mission Valley is very real. A map of seismic shaking produced for a hypothetical scenario of a magnitude 6.9 earthquake on the Mission Fault suggests strong seismic shaking from Missoula to Kalispell and severe shaking between Arlee and Polson. M O N TA N A i n E A R T H Q U A K E S 3 M O N TA N A i n "THE HOLY GRAIL OF SEISMOLOGY IS EARTHQUAKE PREDICTION, HOWEVER OUR CURRENT UNDERSTANDING OF THIS VERY DIFFICULT PROBLEM MAKES THIS GOAL SEEM UNLIKELY IN THE NEXT THREE DECADES."

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