Deep below California’s San Andreas Fault lurks rock-melting forces that are driving tremors underneath a segment 800 mile stretch of the sliding boundary, reveals a new study from the University of Southern California.
To investigate how earthquakes happen, the team looked more than 43 miles below the surface and discovered rocks, friction and fluids play key roles in ruptures.
Researchers studied the section of Parkfield, which has been hit with quakes of magnitude 6 seven times since 1857, and found that after a big earthquake ends, the tectonic plates glide in a way that is a recipe for disaster.
The sliding of the plates causes friction that pushes temperatures above 650 degrees Fahrenheit, which in turn changes rocks from solid to more fluid-like.
Without the rocks as a barrier, the plates slide more until they slip past each other rapidly – triggering an earthquake.
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The team studied the section of Parkfield, which has been hit with quakes of magnitude 6 seven times since 1857, and found that after a big earthquake ends, the tectonic plates glide in a way that is a recipe for disaster
Sylvain Barbot, assistant professor of Earth sciences at the USC Dornsife College of Letters, Arts and Sciences, said: ‘Most of California seismicity originates from the first 10 miles of the crust, but some tremors on the San Andreas Fault take place much deeper.’
‘Why and how this happens is largely unknown. We show that a deep section of the San Andreas Fault breaks frequently and melts the host rocks, generating these anomalous seismic waves.’
The study, conducted in collaboration with Lifeng Wang of the China Earthquake Administration in China, chose the region of Parkfield because it is one of the world’s most intensively monitored epicenters.
This area has been hit with magnitude 6 earthquakes in in 1857, 1881, 1901, 1922, 1934, 1966 and 2004, according to the U.S. Geological Survey.
The team developed mathematical models and laboratory experiments using rocks, along with simulations of Parkfield that shows fault activity a great depths over the past 300 years. The researchers observed that, after a big quake ends, the tectonic plates that meet at the fault boundary settle into a go-along, get-along phase
However, smaller tremblors have been recorded at greater depths about every few months – leading experts to wonder what could be happening deep below the surface?
The team developed mathematical models and laboratory experiments using rocks, along with simulations of Parkfield that shows fault activity a great depths over the past 300 years.
The researchers observed that, after a big quake ends, the tectonic plates that meet at the fault boundary settle into a go-along, get-along phase.
The plates glide past each other and although disturbance on the surface is minimal, deep below it is setting the stage for an earthquake.
Friction is created by the motion that gradually pushes out heat – temperatures can reach 650 degrees Fahrenheit.
The surrounding rocks lose their solidity and become more fluid-like, which makes the bedrock unstable and allows the plates to slide past each other rapidly – causing an earthquake.
Friction is created by the motion that gradually pushes out heat – temperatures can reach 650 degrees Fahrenheit. The surrounding rocks lose their solidity and become more fluid-like, which makes the bedrock unstable and allows the plates to slide past each other rapidly – causing an earthquake
Scientists typically focus on movement in the top of Earth’s crust, anticipating that its motion in turn rejiggers the rocks deep below. Pictured is an aerial view of a surface crack along the San Andreas Fault south of San Francisco
‘Just like rubbing our hands together in cold weather to heat them up, faults heat up when they slide. The fault movements can be caused by large changes in temperature,’ Barbot said.
‘This can create a positive feedback that makes them slide even faster, eventually generating an earthquake.’
Scientists typically focus on movement in the top of Earth’s crust, anticipating that its motion in turn rejiggers the rocks deep below.
For this study, the scientists looked at the problem from the bottom up.
‘It’s difficult to make predictions,’ Barbot added, ‘so instead of predicting just earthquakes, we’re trying to explain all of the different types of motion seen in the ground.’
WHAT ARE THE ‘SLOW EARTHQUAKES’ DETECTED IN THE SAN ANDREAS FAULT?
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 two magnitude 7.9 earthquakes, in 1857 in Fort Tejon and 1906 in San Francisco.
The scientists also suggest a mechanism that might cause the stop-and-go movements.
The central San Andreas Fault (green) is flanked by sections (red) that are far more active. New research, however, shows that the earth movements along this central section have not been smooth and steady, as previously thought
‘Fault rocks contain a fluid phase that’s trapped in gaps between particles, called pore spaces,’ Dr Khoshmanesh said.
‘Periodic compacting of fault materials causes a brief rise in fluid pressure, which unclamps the fault and eases the movement.’
The two scientists used synthetic aperture radar data from orbit for the years 2003 to 2010.
This data let them map month-to-month changes in the ground along the central part of the San Andreas fault.
They combined the detailed ground-movement observations with seismic records into a mathematical model.
The model let them explore the driving mechanism of slow earthquakes and their link to big nearby quakes.
From 2003 to 2010 (bottom scale), portions of the fault at different distances from Parkfield (left scale) moved at varying rates. Red shows periods when the movement was greater than average, blue when it was less
‘We found that this part of the fault has an average movement of about three centimeters a year, a little more than an inch,’ Dr Khoshmanesh said.
‘But at times the movement stops entirely, and at other times it has moved as much as 10 centimetres a year, or about four inches.’
The picture of the central San Andreas Fault emerging from their work suggests that its stick-and-slip motion resembles on a small timescale how the other parts of the San Andreas Fault move.
They note that the new observation is significant because it uncovers a new type of fault motion and earthquake-triggering mechanism, which is not accounted for in current models of earthquake hazards used for California.
Dr Shirzaei said: ‘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.’
He added that accurate estimates of this varying hazard are essential to include in operational earthquake-forecasting systems.