An earthquake is defined as the "perceptible trembling to violent shaking of the ground, produced by the sudden displacement of rocks below the Earth's surface." Rocks respond to stress (being squeezed or pulled apart) near the Earth's surface by breaking. Where the rocks break and move, we call it a fault. The buildup of tectonic forces and release of stress on individual faults is what causes quakes. Higher stresses lead to larger earthquakes.
There are five potentially active volcanoes in Oregon: Crater Lake, Mount Hood, Mount Jefferson, Newberry Caldera, and the Three Sisters region.
Find the latest Hazards Assessment Reports and Maps for these volcanoes at the USGS website.