Much of coastal California rides on the Pacific plate, while the Sierran plate serves as a buffer zone of sorts for the North American plate, which carries the rest of the continental United States.
North of the 'big bend' in the San Andreas fault, the relative motion of the Pacific and Sierran plates in central California nearly parallels the San Andreas and related faults. In most places, the plates are converging at rates up to 3.3 millimeters (.13 inches) per year, horizontally shortening Earth's crust across the fault and raising California's coastal mountains.
"We found the greater the rate of convergence, the larger the size and extent of the mountains," said Argus.
The affected mountains include the Temblor and Diablo Ranges, those on the west flank of the Sacramento-San Joaquin Valley, others near the San Andreas fault system and those strictly near the coast. These ranges block drainage of the watershed comprising the Sierra Nevada and great central valley of California into the Pacific Ocean.
In contrast, he and Gordon found that just north of San Francisco, the Pacific and Sierran plates are slowly pulling apart at a rate of 2.6 millimeters (.1 inches) per year, opening a hole manifested as a topographic low in San Pablo Bay. Here, rivers originating in the Sierra Nevada mountains drain through the coastal mountains on their way to passage under the Golden Gate Bridge and out into the Pacific.
Argus and Gordon's study also addresses overall earthquake hazards in the region. They calculated the lateral rate of motion between the Pacific and Sierran plates at approximately 39 millimeters (about 1.5 inches) per year. This rate differs significantly from a previous estimate of 34 millimeters (about 1.3 inches) per year obtained by measuring and dating creek displacements across the San Andreas fault. The scientists attributed this difference to inelastic deformation, slip along other faults or both. These observations limit the total amount of strain that may be released in earthquakes along the fault system, Argus said.
The researchers also found a general relationship between the degree of convergence and the degree of stable sliding along the San Andreas and other northwest-striking strike-slip faults in central California. Where convergence rates are low or negative, sliding tends to be stable, manifesting itself as steady "creep" or small to moderate earthquakes; where convergence rates are high, the faults tend to be unstable, resulting in great earthquakes such as the 1906 San Francisco quake. In most cases, the stable fault sections move parallel to the direction of relative plate motion.
Argus and Gordon found prominent exceptions to this rule, however, that make their hypothesis at best a partial explanation for the observed distribution of locked and nonlocked fault sections. They speculate that other unknown factors are at work in these areas.
Based upon present rates of fault convergence and neglecting the effects of erosion, the two calculated the age of California's coastal ranges to be at least 3 to 6 million years, with the Diablo Range estimated at approximately 10 million years old. Most previous age estimates range from 1 to 3 million years.
This research was funded as part of NASA's Earth Science Enterprise, a long-term research effort dedicated to understanding how human-induced and natural changes affect our global environment.
JPL is a division of the California Institute of Technology in Pasadena.
