Nav: Home

Geoscientists find unexpected 'deep creep' near San Andreas, San Jacinto faults

September 18, 2018

AMHERST, Mass. - A new analysis of thousands of very small earthquakes that have occurred in the San Bernardino basin near the San Andreas and San Jacinto faults suggests that the unusual deformation of some - they move in a different way than expected - may be due to "deep creep" 10 km below the Earth's surface, say geoscientists at the University of Massachusetts Amherst.

The new understanding should support more refined assessments of fault loading and earthquake rupture risk in the region, they add. Writing in the current online Geophysical Research Letters, doctoral student Jennifer Beyer and her advisor, geosciences professor Michele Cooke say the enigmatic behavior is seen in about one third of the hundreds of tiny quakes recorded during the lull between big damaging quakes, and their possible significance had not been appreciated until now.

Cooke says, "These little earthquakes are a really rich data set to work with, and going forward if we pay more attention than we have in the past to the details they are telling us, we can learn more about active fault behavior that will help us better understand the loading that leads up to large damaging earthquakes."

Over the past 36 years, the authors point out, seismic stations have recorded the style of deformation for thousands of small earthquakes in California's San Bernardino basin. They state, "Findings of this study demonstrate that small earthquakes that occur adjacent to and between faults can have very different style of deformation than the large ground rupturing earthquakes produced along active faults. This means that scientists should not use the information recorded by these small earthquakes in the San Bernardino basin to predict loading of the nearby San Andreas and San Jacinto faults."

Cooke explains that the usual type of fault in the region is called a strike-slip fault, where the motion is one of blocks sliding past each other. The less common kind, with "anomalous slip-sense," is an extending fault, where the motion between blocks is like a wave pulling away from the beach, one block dropping at an angle away from the other, "extending" the fault. "These only occur in this one small area, and nobody knew why," she points out. "We did the modeling that helps to explain the enigmatic data."

This is an area where Cooke, an expert in 3D fault modeling, has done research of her own and where she is familiar with the broader research field, so she decided to try to model what is happening. She began with a hypothesis based on her earlier 3D modeling in the area that had replicated long-term deformation over thousands of years.

"I noticed that this basin was in extension in those models unlike the surrounding regions of strike-slip," she says. "The extension was limited to within the basin just like the pattern of the anomalous extensional earthquakes. That gave me a clue that maybe those faults weren't locked as they should be between big earthquakes, but that at depths below 10 km, they were creeping."

"The typical way we look for creep is to use GPS stations set up on each side of the fault. Over time, you can note that there is movement; the faults are creeping slowly apart. The problem here is that the San Andreas and the San Jacinto faults are so close together that the GPS is unable to resolve if there is creep or not. That's why no one had seen this before. The traditional way to detect it was not able to do so."

Cooke adds, "In this paper we've shown that there is a way to have these weird tiny earthquakes all the time next to the San Jacinto Fault below 10 km, which is where deep creep may be happening. We show that it's plausible and can account for nearby enigmatic earthquakes. The model may not be perfectly correct, but it's consistent with observations."

As noted, this work has implications for assessing fault loading, Beyer and Cooke point out. Until now, seismologists have assumed that faults in the region are locked - no creep is taking place - and they use data from all the little earthquakes to infer loading on the primary faults. However, Cooke and Beyer write, "scientists should not use the information recorded by these small earthquakes in the San Bernardino basin to predict loading of the nearby San Andreas and San Jacinto faults."

Cooke adds, "Our earthquake catalog is growing every year; we can see smaller and smaller ones every year, so we thought why not take advantage of the networks we've built and we can look at them in more detail. We don't want to wait around for the faults to move in a damaging earthquake, we want to take advantage of all the tinier earthquakes happening all the time in order to understand how the San Andreas and San Jacinto are loaded. If we can understand how they are being loaded maybe we can understand better when these faults will going to rupture."
-end-
This research was supported by the Southern California Earthquake Center, which is funded by cooperative agreements with the National Science Foundation and U.S. Geological Survey.

University of Massachusetts at Amherst

Related Earthquakes Articles:

Can a UNICORN outrun earthquakes?
A University of Tokyo Team transformed its UNICORN computing code into an AI-like algorithm to more quickly simulate tectonic plate deformation due to a phenomenon called a ''fault slip,'' a sudden shift that occurs at the plate boundary.
Earthquakes in slow motion
A survey of slow-slip events in Cascadia reveals new insight into the recently discovered phenomenon.
Earthquakes can be predicted five days ahead
An international team of researchers, which includes physicists from HSE University and the RAS Space Research Institute (IKI), have discovered that, with an impending earthquake, the parameters of internal gravity waves (IGWs) can change five days before a seismic event.
Stanford researchers explain earthquakes we can't feel
Researchers have explained mysterious slow-moving earthquakes known as slow slip events with the help of computer simulations.
Solved: How tides can trigger earthquakes
Some earthquakes along mid-ocean ridges are linked with low tides, but nobody could figure out why.
Measuring iceberg production with earthquakes
An international team led by French researchers from the CNRS and Paris Diderot University came up with the idea of using earthquakes generated when icebergs break away -- felt hundreds of kilometres off -- to measure this ice loss.
Injection wells can induce earthquakes miles away from the well
A study of earthquakes induced by injecting fluids deep underground has revealed surprising patterns, suggesting that current recommendations for hydraulic fracturing, wastewater disposal, and geothermal wells may need to be revised.
Earthquakes can be weakened by groundwater
Researchers from EPFL and the Ecole Normale Supérieure in Paris have found that the presence of pressurized fluid in surrounding rock can reduce the intensity of earthquakes triggered by underground human activities like geothermal energy production.
UH researchers report new understanding of deep earthquakes
Researchers from the University of Houston have for the first time reported a way to analyze seismic wave radiation patterns in deep earthquakes to suggest global deep earthquakes are in anisotropic rocks.
International collaboration studies the predictability of earthquakes
At four centers in California, New Zealand, Europe and Japan -- and in countless labs across the globe -- CSEP's experiments and its rigorous testing procedures have shed light on the predictability of earthquakes, according to a special focus section published June 13 in Seismological Research Letters.
More Earthquakes News and Earthquakes Current Events

Top Science Podcasts

We have hand picked the top science podcasts of 2019.
Now Playing: TED Radio Hour

Risk
Why do we revere risk-takers, even when their actions terrify us? Why are some better at taking risks than others? This hour, TED speakers explore the alluring, dangerous, and calculated sides of risk. Guests include professional rock climber Alex Honnold, economist Mariana Mazzucato, psychology researcher Kashfia Rahman, structural engineer and bridge designer Ian Firth, and risk intelligence expert Dylan Evans.
Now Playing: Science for the People

#541 Wayfinding
These days when we want to know where we are or how to get where we want to go, most of us will pull out a smart phone with a built-in GPS and map app. Some of us old timers might still use an old school paper map from time to time. But we didn't always used to lean so heavily on maps and technology, and in some remote places of the world some people still navigate and wayfind their way without the aid of these tools... and in some cases do better without them. This week, host Rachelle Saunders...
Now Playing: Radiolab

Dolly Parton's America: Neon Moss
Today on Radiolab, we're bringing you the fourth episode of Jad's special series, Dolly Parton's America. In this episode, Jad goes back up the mountain to visit Dolly's actual Tennessee mountain home, where she tells stories about her first trips out of the holler. Back on the mountaintop, standing under the rain by the Little Pigeon River, the trip triggers memories of Jad's first visit to his father's childhood home, and opens the gateway to dizzying stories of music and migration. Support Radiolab today at Radiolab.org/donate.