Stresses build along edges of the plates until part of the crust suddenly gives way in a violent movement. This shaking of the crust is called an earthquake. The crust breaks along uneven lines called faults. Geologists locate these faults and determine which are active and inactive. This helps identify where the greatest earthquake potential exists.
Most faults mapped by geologists, however, are inactive and have no earthquake potential.
When the crust moves abruptly, the sudden release of stored force in the crust sends waves of energy radiating outward from the fault. Internal waves quickly form surface waves, and these surface waves cause the ground to shake. Buildings may sway, tilt, or collapse as the surface waves pass. The constant interaction of crustal plates in western North America still creates severe earthquakes. Idaho is situated where the Basin and Range and Rocky Mountain geomorphic provinces meet.