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 (145 km) - has a steady creeping movement that provides a safe release of energy.
Creep on the central San Andreas during the past several decades, so the thinking goes, has reduced the chance of a big quake that would rupture the entire fault from north to south.
New research, however, shows that the earth movements along this central section have not been smooth and steady, as previously thought.
Research by two Arizona State University geophysicists found the activity has been a sequence of small stick-and-slip movements - sometimes called 'slow earthquakes' - that release energy over a period of months.
Although these slow earthquakes pass unnoticed by people, experts say they can trigger large destructive quakes in their surroundings.
Synthetic aperture radar data for 2003 to 2010 let researchers team map the average rate of movement for the central section of the San Andreas Fault (black line). Red shows ground movement toward the southeast, and blue to the northwest
One such quake was the magnitude six event that shook Parkfield in 2004.
'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)
'Based on current time-independent models, there's a 75 per cent chance for an earthquake of magnitude seven or larger in both northern and southern California within next 30 years.'
He is the lead author of a Nature Geoscience paper reporting on the research.
'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.
'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.
He points out that these flanking sections experienced