Earthquakes usually occur along fracture zones in Earth's crust, where large tectonic plates slide past one another and become locked. Stress builds up over long periods and is suddenly released in the form of an earthquake. In Southern California, the San Andreas and San Jacinto faults are among the most significant of these zones, accommodating most of the plate motion in the region.

Earthquakes usually occur along fracture zones in Earth’s crust, where large tectonic plates slide past one another and become locked. Stress builds up over long periods and is suddenly released in the form of an earthquake. In Southern California, the San Andreas and San Jacinto faults are among the most significant of these zones, accommodating most of the plate motion in the region.

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