The scientists also suggest a mechanism that might cause the stop-and-go movements. He points out that these flanking sections experienced two magnitude 7.9 earthquakes, in 1857 (Fort Tejon) and 1906 (San Francisco). 'These episodic slow earthquakes lead to increased stress on the locked segments of the fault to the north and south of the central section,' Shirzaei said. 'We found that movement on the fault began every one to two years and lasted for several months before stopping,' said Manoochehr Shirzaei, assistant professor in SESE and co-author of the paper. He is the lead author of a Nature Geoscience paper reporting on the research. 'What looked like steady, continuous creep was actually made of episodes of acceleration and deceleration along the fault,' said Mostafa Khoshmanesh, a graduate research assistant in ASU's School of Earth and Space Exploration (SESE). One such quake was the magnitude 6 event that shook Parkfield in 2004. Although these slow earthquakes pass unnoticed by people, the researchers say they can trigger large destructive quakes in their surroundings. Instead, the activity has been a sequence of small stick-and-slip movements - sometimes called 'slow earthquakes' - that release energy over a period of months.

New research by two Arizona State University geophysicists, however, shows that the earth movements along this central section have not been smooth and steady, as previously thought. Geologists have long thought that the central section of California's famed San Andreas Fault - from San Juan Bautista southward to Parkfield, a distance of about 90 miles - has a steady creeping movement that provides a safe release of energy.Ĭreep on the central San Andreas during the past several decades, so the thinking goes, has reduced the chance of a big quake that ruptures the entire fault from north to south. JDetailed study of well-known California fault finds new kind of movement not accounted for in earthquake forecasting

Unrequited Toil by Calvin Schermerhorn