5.6 quake in western Montana

A moderate earthquake of magnitude 5.6 struck westernmost Montana late last night local time, causing light damage and widespread shaking. It appears to have been an event caused by normal faulting, the same mechanism that, multiplied over millions of years, has dropped the valleys and raised the mountains of the whole Mountain West. Follow developments, including the "did you feel it?" shape map, at the USGS special page for this quake. Learn how I knew the cause, based on the focal mechanism diagram, here.