Fact - that area of Idaho has a bunch of fault lines. Quakes are very common but 99% are too small to be felt. The '83 Borah quake was the biggest ever recorded and this one was is 2d on the list. This pic shows the known fault lines. This one was near the top of the Lost River line and the Borah quake was farther south on the same line. Idaho is said to be the #4 state for total number of quakes but most are very tiny.

That whole area of fault lines is in remote mountain country with very few small towns. This map doesn't show state lines. The Red Rock and Centennial lines are in MT and the Teton is in WY. The rest are in ID.

[Linked Image]

This photo is just below Mt Borah, on the same Lost River fault line. It took one whopping quake to turn this piece of real estate on it's side like that.
[Linked Image from i.imgur.com]

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