'Slow earthquakes' on San Andreas Fault raise risks of massive quakes

BY LEAH WILLIAMS
Posted on JUL 22, 2019

A comprehensive study of well-known California fault discovers a new kind.

New interpretations by two Arizona State University geophysicists reveal that the earth movements along a central section have not been smooth and steady, as previously estimated.

The activity has been a sequence of small stick-and-slip movements referred to as "slow earthquakes" which release energy over a period of months.
Researchers say they can trigger large destructive quakes in their surroundings. One such quake was the magnitude 6 event that shook Parkfield in 2004.

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

"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.

Episodic slow earthquakes result in prolonged stress on the locked segments of the fault to the north and south of the central section.

Shirzaei notes that these flanking sections experienced two magnitudes 7.9 earthquakes, in 1857 (Fort Tejon) and 1906 (San Francisco).

The scientists also suggest a mechanism that might cause the stop-and-go movements.

"Fault rocks contain a fluid phase that's trapped in gaps between particles, called pore spaces," Khoshmanesh said. "Periodic compacting of fault materials causes a brief rise in fluid pressure, which unclamps the fault and eases the movement.