Geologists have long thought that the central section of California’s famous San Andreas Fault – from San Juan Boutista to Parkfield on the south side, about 90 miles (145 km) – is a steady creep movement that is a safe movement of energy. Provides release.
Creep over central San Andreas during the last several decades, so thinking has reduced the likelihood of a major earthquake, which would break the entire fault from north to south.
However, new research suggests that the Earth’s move along this central section has not been as smooth and stable as before.
Research by two Arizona State University geophysicists found that the activity had been a sequence of small stick and slip movements – sometimes called ‘slow earthquakes’ – that released energy over a period of months.
Although these slow earthquakes are not noticed by people, experts say they can trigger large devastating earthquakes around them.
Synthetic aperture radar data from 2003 to 2010 allowed researchers to map the average rate of movement for the central section of the San Andreas Fault (black line). Red appears to the southeast and blue appears to the northwest.
One such earthquake was the six events that rocked Parkfield in 2004.
“The steady, sustained creep was actually made up of episodes of acceleration and deceleration,” said Mustafa Khomenesh, a graduate research assistant at ASU’s School of Earth and Space Exploration (SSE).
‘Based on current time-independent models, Northern and Southern California have a 75 percent chance of seven or more earthquakes of magnitude within the next 30 years.’
He is the lead author of a Nature Geoscience paper reporting on research.
“We found that the movement started every one to two years at fault and lasted for several months before stopping.” ‘
Shirzei said, “These slow slowing earthquakes increase the stress on accidentally closed sections in the central part of north and south.”
He explains that these flanking classes experienced two strong earthquakes in 1857, Fort Tejon in 1857 and San Francisco in 1906.
Scientists also suggest a mechanism that may be the cause of stop-and-go movements.
The central San Andreas Fault (green) is surrounded by sections (red) that are far more active. However, new research suggests that the Earth’s movement along this central section has not been as smooth and stable as before.
Dr. “Fault rocks have a fluid state that is trapped in the gaps between the particles, which is called the hole space,” Khomnesh said.
‘Periodic compacting of the fault material causes a brief increase in fluid pressure, which exposes the fault and makes movement easier.’
Both scientists used synthetic aperture radar data from orbit from 2003 to 2010.
This data lets them map month-to-month changes in the plain along the central part of the San Andreas Fault.
He combined detailed geo-movement observations with seismic records into a mathematical model.
The model revealed to him the driving mechanism of slow-moving earthquakes and large earthquakes nearby.
From 2003 to 2010 (lower scale), fault fractions moved at different rates from Parkfield (left scale) at different distances. Red shows periods when movement was above average, blue when it was low
“We found that this part of the defect has an average speed of about three centimeters a year, a little over an inch,” Dr. Khomnesh said.
‘But sometimes the movement stops altogether, and at other times it has increased about 10 centimeters or about four inches in a year.’
Photographs of the central San Andreas Fault stem from his work, suggesting that the stick-and-slip motion looks at a short period of time as to how other parts of the San Andreas Fault move.
They note that the new observation is important because it uncovers a new type of fault motion and earthquake-triggering mechanism, which is not accounted for in the current model of earthquake hazards used for California.
Dr. Shirazai said: ‘Based on our observations, we believe that the seismic hazard in California is something that changes over time and is probably higher than what people have thought so far.’
He stated that accurate estimates of this different hazard are needed to be included in operational earthquake-forecasting systems.