Deep Underground “Rock-Melting” Forces Explain Quakes on San Andreas Fault

Earth's Core

Rock-melting forces occurring much deeper in the Earth than previously understood appear to drive tremors along a notorious segment of California’s San Andreas Fault, according to new USC research that helps explain how quakes happen.

The study from the emergent field of earthquake physics looks at temblor mechanics from the bottom up, rather than from the top down, with a focus on underground rocks, friction and fluids. On the segment of the San Andreas Fault near Parkfield, Calif., underground excitations — beyond the depths where quakes are typically monitored — lead to instability that ruptures in a quake.

“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,” said Sylvain Barbot, assistant professor of Earth sciences at the USC Dornsife College of Letters, Arts and Sciences. “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 newly published study appears in Science Advances. Barbot, the corresponding author, collaborated with Lifeng Wang of the China Earthquake Administration in China.

The findings are significant because they help advance the long-term goal of understanding how and where earthquakes are likely to occur, along with the forces that trigger temblors. Better scientific understanding helps inform building codes, public policy and emergency preparedness in quake-ridden areas like California. The findings may also be important in engineering applications where the temperature of rocks is changed rapidly, such as by hydraulic fracturing.

Parkfield was chosen because it is one of the most intensively monitored epicenters in the world. The San Andreas Fault slices past the town, and it’s regularly ruptured with significant quakes. Quakes of magnitude 6 have shaken the Parkfield section of the fault at fairly regular intervals in 1857, 1881, 1901, 1922, 1934, 1966 and 2004, according to the U.S. Geological Survey. At greater depths, smaller temblors occur every few months. So what’s happening deep in the Earth to explain the rapid quake recurrence?

Using mathematical models and laboratory experiments with rocks, the scientists conducted simulations based on evidence gathered from the section of the San Andreas Fault extending up to 36 miles north of — and 16 miles beneath — Parkfield. They simulated the dynamics of fault activity in the deep Earth spanning 300 years to study a wide range of rupture sizes and behaviors.

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. For a spell, they glide past each other, a slow slip that causes little disturbance to the surface.

But this harmony belies trouble brewing. Gradually, motion across chunks of granite and quartz, the Earth’s bedrock, generates heat due to friction. As the heat intensifies, the blocks of rock begin to change. When friction pushes temperatures above 650 degrees Fahrenheit, the rock blocks grow less solid and more fluid-like. They start to slide more, generating more friction, more heat and more fluids until they slip past each other rapidly — triggering an earthquake.

“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.”

It’s a different way of looking at the San Andreas Fault. 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.”

The study was supported by grants from the National Natural Science Foundation of China (NSFC-41674067 and NSFC-U1839211) and the U.S. National Science Foundation (EAR-1848192).

8 Comments on "Deep Underground “Rock-Melting” Forces Explain Quakes on San Andreas Fault"

  1. “…, motion across chunks of granite and quartz, the Earth’s bedrock, …”

    That is a naive, misleading representation of the character of the crust of the Earth.

  2. Victoria Mendez | September 4, 2020 at 4:35 pm | Reply

    No. This is common sense what our fracking is harming the earth and contributing to the earthquakes and I have even started talking about the toxic chemicals they inject in the earth! We need to move away from our toxic love of oil and use energy that is less harmful to the earth! My husband is a geologist and has better understands than Most!

  3. Victoria Meñdez: Jello is hardly toxic. That is essentially what frac gel is. Just letting you know. Also, how are you going to power an economy without oil? You cannot cover the entire planet with solar panels and wind turbines. It would be a horrible planet to live on if you did, but that is only my opinion….

  4. Hey you brain dead scientist.
    Have you ever considered the add weight the seismic companies have done to this world?

    Use commonsense and realize, They altered the permeability of the earth, so there is less ground seepage and that holds back more water on each continent,

    What happens to a boat with extra weight?
    Come on you tell us uneducated people.

    You know that the tremendous weight of this extra water is going to cause extra pressures pushing the continents down even if it’s a small amount and it will cause untold problems where continents make their connections with each earthly piece,

    And do you think that will cause more earthquakes and volcanic activity

    Lets not use our brains
    We don’t know how these things happen

    Are you scared to blame the oil companies?

    There’s you major contributor to earthly problems
    INCLUDING THE PLASTIC POLLUTION THEY HAVE CAUSED

    WAKE UP AS YOU KNOW THEY ARE TO BLAME FOR A GREAT MAJORITY OF THESE PROBLEMS !

  5. I definitely agree that industrialisation is destroying our planet ,and disagree with ‘we don’t the cause and can’t predict ‘please wake up and THINK ‘The industrialisation is not part of the natural existence and so is contrary to nature so instead of finding causes this late start by finding ways of replacing oil and and finding other ways of conserving and protecting our planet not destroying it and saying we don’t know wats going on.

  6. Officer Alton Cook; G4S Rocky Mountain Division | September 5, 2020 at 6:18 pm | Reply

    There is more than ample Scientific data from ice and rock cores retrieving materials that were deposited millions of years ago proving beyond any doubt that Humans have undeniably changed the natural environment. It’s a Scientific fact and not a fake news headline. That does not mean that Natural Processes are not also taking place. It’s the combination of many processes working in synchronization that are exacerbating the effects that we are noticing. That said, it also does not mean that if we all buy the right things and just simply go all electric like in the TV Commercial that we will magically save the Environment. The painful reality is that it is forever changed and will never go back to how it was before. That does not mean that we are all going to die tomorrow. It does mean that the natural environment is changing and not in a good way. There will never be a magic button or a brilliant new technology that can save us any more than Super Man will swoop down at the perfect moment and pick you up. No, sorry… not going to happen. What that means is Humanity will continue to pollute, poison, and change everything it comes in contact with while at the same time trying to contend with the damage it has done and in the process create some interesting technologies. Will these technologies save us??… No. But we will continue on for a period and will learn that there will always be things that we can make better and things that we cannot undo. Inevitably it is Nature who always wins.

  7. Patricia Patricia holman | September 6, 2020 at 9:11 am | Reply

    Hmmmmm..when earth moves it causes friction like rubbimg hands together….geniuses with grants.

  8. Ms. Mendez, fracking is a shallow process, not even close to the depths talked about in this article.
    I just want to say that when they talk about rocks acting more liquid-like that that is a misrepresentation. They are no where near liquid like. A better way of looking at it is that they are more plastic like, deformable, but not like a liquid.

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