San Andreas Fault: Scientists Find 'Slow Earthquakes' That Could Trigger More Destructive Quakes

The central section of the famous San Andreas fault in California is characterized by a smooth and steady creeping motion, which safely releases energy and has reduced the chances of a big earthquake over the past several decades. At least, that's what geologists have long considered to be true.

However, a study published in the journal Nature Geoscience has demonstrated that the movement of plates along this central section has not been as smooth and steady as previously thought.

Researchers from Arizona State University (ASU) found that instead of a smooth and steady motion, the fault is marked by small "stick-and-slip" movements—sometimes referred to as "slow earthquakes"—which release energy over a period of hours to months, rather than seconds to minutes like a typical quake.

These slow earthquakes often go completely unnoticed by people, however, they have the potential to trigger larger, more destructive earthquakes, according to the researchers, much like the magnitude 6.0 event that struck Parkfield, California in 2004.

"What looked like steady, continuous creep was actually made of episodes of acceleration and deceleration along the fault," Mostafa Khoshmanesh, a graduate research assistant at ASU's School of Earth and Space Exploration (SESE) and lead author of the study, said in a statement.

The researchers found that movement on the fault tended to occur every year, or every other year, and usually lasted several months before stopping entirely. The average movement was about an inch a year, but sometimes this figure jumped up to four inches.

"These episodic slow earthquakes lead to increased stress on the locked segments of the fault to the north and south of the central section," Manoochehr Shirzaei, an assistant professor at the SESE and co-author the paper, said in the statement.

Both the northern and southern sections have seen large, destructive earthquakes in the past, but the central section has remained mostly quiet.

For example, the sections to the north and south were responsible for two powerful 7.9 magnitude earthquakes in 1857 (Fort Tejon) and 1906 (San Francisco). The latter event was one of the worst natural disasters in American history, causing the deaths of up to 3,000 people and destroying more than 80% of the city of San Francisco.

To make their findings, the scientists used radar data collected between the years 2003 and 2010, enabling them to track movements in the ground from month-to-month. This information was combined with seismic activity records to create a mathematical model that allowed the researchers to learn more about the processes behind slow earthquakes and how they relate to larger quakes in the surrounding area.

The new findings are significant because they reveal a new type of fault motion and a trigger mechanism for earthquakes that have not been accounted for in Californian quake hazard models.

"Based on our observations, we believe that seismic hazard in California is something that varies over time and is probably higher than what people have thought up to now," Shirzaei said. "He adds that accurate estimates of this varying hazard are essential to include in operational earthquake forecasting systems."

This new data could influence current models which suggest that there is around a 75% chance of a magnitude 7 or greater earthquake occurring in both northern and southern California within the next 30 years