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This view of the Wallace Creek (offset stream) is of perhaps the most famous feature on the San Andreas Fault. As the stream continues to progressively erode into its channel, right-lateral motion along the San Andreas Fault has moved the downstream portion of the channel northward. Sediments cut by Wallace Creek were radiocarbon dated at about 3800 years old, and the stream channel has been offset about 420 feet since then. Combined with information derived from modern earthquakes, offset along the fault shows a relative rate of motion in the in the Carrizo Plain is in the range of about 1.3 inches (34 mm) per year. Fault scarps and area uplifts show that there is a vertical component of the fault motion. Estimates range in 10 to 20 feet of horizontal slip for every foot of vertical motion (Seih & Wallace, 1987). Earthquake fault investigations in the Carrizo Plain show that as many as 130 streams in the park area display evidence of right-lateral slip. Wallace (1968) suggest that 40 of these channels display offset from the 1857 earthquake ranging from 20 to 50 feet (illustrating the variability of earthquake slip behavior). |
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