Tectonics and recent seismicity near Flathead Lake, Montana

Since 1900, more than 290 earthquakes have been reported near Flathead Lake, Montana. Surprisingly, none has exceeded magnitude 5 to 512. Most recent earthquake swarms appear to result from east-west or northwest-southeast extension along short fault segments west and north of the lake. Major normal faults like the Swan and Mission faults east of the lake may pose higher risk, but they appear dormant today. Deformation of sediments in Flathead Lake may be caused by several large earthquakes more than 10,000 years ago but is more probably due to glacial processes accompanying the last retreat of the Cordilleran ice sheet.