'Slow earthquakes' on San Andreas Fault increase risk of large quakes

phys.org | 6/18/2018 | Staff
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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 80 miles—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 ruptures the entire fault from north to south.

Research - Arizona - State - University - Geophysicists

However new research by two Arizona State University geophysicists shows that the earth movements along this central section have not been smooth and steady, as previously thought.

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. Although these slow earthquakes pass unnoticed by people, the researchers say they can trigger large destructive quakes in their surroundings. One such quake was the magnitude 6 event that shook Parkfield in 2004.

Creep - Episodes - Acceleration - Deceleration - Fault

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

Slow - Earthquakes - Stress - Segments - Fault

"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 says. He points out that these flanking sections experienced two magnitude 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 - Fluid - Phase - Gaps

"Fault rocks contain a fluid phase that's trapped in gaps between particles, called pore spaces," Khoshmanesh says. "Periodic compacting of...
(Excerpt) Read more at: phys.org
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