Fast Fault

The San Jacinto fault in Southern California began forming about 1.1 to 1.3 million years ago, much later than previously thought, suggests a new study. The late start means the grinding between the fault's rocky plates was speedier than thought.

Scientists, including Rebecca Dorsey of the University of Oregon, carried out detailed geologic mapping and analysis of sedimentary rocks from Pleistocene (12,000 to 1.8 million years ago) in the earthquake-prone region surrounding the San Jacinto fault -- a giant crack in Earth's crust that branches off the San Andreas fault.

They estimated the slip rate could be as high as 0.8 inches (20 millimeters) a year, a rate considered "fast" in a geologic sense. Previous studies concluded that fault has slipped about 15.5 miles (25 kilometers) in a right-lateral sense, at a rate of 0.4-0.5 inches (10-12 millimeters) a year during the last 2.0 million to 2.4 million years.

"Our findings suggest that the San Jacinto fault absorbs a large share of the relative motion between the Pacific and North American plates," Dorsey said. "This is important both for understanding the development of this active plate boundary and for helping to constrain estimates of seismic hazards in southern California."

-- LiveScience Staff

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Credit: Rebecca Dorsey