Exploring the San Andreas

Running the length of California, the San Andreas Fault is a 1200 km long continental strike-slip fault that serves as a transform boundary between the northwestward-sliding Pacific Plate and the southeastward-sliding North American plate (USGS). Friction along the plate boundary makes the region prone to earthquakes that have been recorded at magnitudes of up to 8.0 (SCEDC).

The San Andreas Fault can be observed in multiple locations across California, including three national parks: Point Reyes National Seashore, Golden Gate National Recreation Area, and Pinnacles National Park (National Park Service). A prime viewing location for the San Andreas is Elkhorn Scarp at Carrizo Plain National Monument, where the fault scarp cuts across the landscape like a raised scar.

Since lateral movement along a strike-slip fault does not result in any dip displacement, locating strike-slip faults on topographic maps is usually determined by observing truncated and offset features like streams or fold hinges (Bennison et al., 2011). Looking at a topographic map of the Elkhorn Scarp at Carrizo Plain, the San Andreas Fault is located by right-lateral offset of local streams as well as by the scarp itself, which is a clearly visible near-linear feature on maps.

Through 3-D modeling, the San Andreas Fault scarp is even more defined: a raised, linear structure that features stream offsets.