2. Earthquakes!

3. An earthquake, or seismic event, is a sudden movement of Earth’s crust that releases energy.

4. Stress and Strain You might think of Earth’s mantle and crust as solid and unbending. However, forces applied to solid rock at high temperatures and over a long time can bend, or deform, rocks. Earthquakes occur when stress (force) causes strain (bending) that exceed the ability of the rocks to resist breakage.

5. What you feel as an earthquake is the sudden movement of the ground, which releases stored energy.

6. Where do Earthquakes occur? Earthquakes do not occur randomly. Earthquakes occur at cracks in Earth’s crust along which there is movement. These cracks are called faults. Faults are usually zones of weakness. The places most likely to break are those that have broken in the past.

7. Earthquakes occur when the ground suddenly shifts along geologic faults such as this small fault near Kingman, Arizona. The displacement here is small, but major faults can offset many miles.

8. Perhaps the most active fault in the United States is the San Andreas Fault system, which extends from the Gulf of California north past Los Angeles and through the San Francisco area.

9. The focus of an earthquake is the place underground where the rock breaks and shifts. Energy waves (shaking) radiates from the focus. Directly above the focus is the epicenter, where the energy waves first reach the surface.

10. Earthquakes 101