Something Unexpected and Weird Is Happening Beneath California’s Deadliest Faults

A section of the San Andreas Fault on the Carrizo Plain in California.
Image: Doc Searls/Flickr

The detection of strange, unpredicted behavior deep below the surface near the San Andreas and San Jacinto faults suggests scientists have an incomplete understanding of the processes responsible for earthquakes in the region.

Over the past four decades, geoscientists have recorded thousands of small earthquakes in California’s San Bernardino basin near the San Andreas and San Jacinto faults. New research published in Geophysical Research Letters suggests many of these quakes, some of which occur at depths between 6 and 12 miles (10 and 20 kilometers), are exhibiting surprising deformation patterns. Instead of slipping in a horizontal manner, many of these earthquakes show vertical movement far below the surface.

This previously undetected movement, dubbed “deep creep,” suggests things aren’t happening beneath the San Andreas and San Jacinto faults in the ways we think they are. And that’s a problem, given the history of the region. Regions along these major faults are vulnerable to damaging earthquakes, the most recent being the 1989 Loma Prieta quake (in the Santa Cruz Mountains south of the Bay area), which registered a magnitude 6.9, killed over 60 people, and caused upward of $6 billion in damages.

Illustration depicting anomalous earthquakes—the deep creep—
near the San Andreas and San Jacinto faults in southern California.
Image: UMass Amherst/Michele Cooke

Surprisingly, one-third of all small earthquakes detected in the region since 1982 are of the deep creep variety, according to the authors of the new study, geoscientists Michele Cooke and Jennifer Beyer from the University of Massachusetts Amherst. These enigmatic slips are mostly occurring to the northeast of the San Jacinto Fault, and primarily below depths of 6 miles. This strange behavior, the authors say, was completely missed by scientists, the impacts of which now need to be determined.

“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 [the buildup of seismic pressure] that leads up to large damaging earthquakes,” said Cooke in a statement.

The deep creep earthquakes analyzed by Cooke and Beyer occur beside and between the two primary faults, and they produce distinctive deformations compared the ones produced by the large, ground-rupturing variety. The San Andreas and San Jacinto faults are strike-slip faults, and their movement can be compared to two blocks sliding horizontally past one another. The motion of the deep creep slips are noticeably different, with one block pulling away from the other at an angle—a wave-like movement that extends the fault. These anomalous slip-sense faults, as they’re called, only seem to occur in one small area, and the researchers were at a loss to explain why.

Normally, geoscientists use GPS to detect and measure seismic slips, where the location between GPS stations on each side of the fault shift in the wake of an earthquake. In this case, however, GPS could not be used because the smaller faults are too close together.

So, to figure out what was going on deep below, Cooke did what any good scientist would: She built a 3D model. Or more specifically, a computer simulation that does 3D fault modeling.

“That gave me a clue that maybe those [smaller] faults weren’t locked as they should be between big earthquakes, but that at depths below 10 kilometers, they were creeping,” said Cooke. “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 by Cooke, geoscientists has previously assumed that the smaller faults in this region were locked in place. So with no apparent creep going on below, scientists calculated the amount of load being placed onto the two primary faults by using data produced by the thousands of smaller quakes. This could carry potentially serious implications, as geoscientists now need to rethink the way energy is accumulating below the surface. As the authors write in study:

Speaking to Newsweek, John Vidale, director of the the Southern California Earthquake Center, said the study was “quite well founded,” and certain specialists may be “sad to see the interpretability of their models undercut.” However, Vidale said the study’s implications were “profoundly ambiguous,” and that Cooke and Beyer failed to flag anything particularly dangerous that experts haven’t identified before.

That said, it’s kinda scary for these researchers to find a previously undocumented process happening in this area. Ultimately, however, this is good news. Armed with this knowledge, scientists can now study these deep creeps further to improve our understanding of this seismically tumultuous region of California.

Correction: An earlier version of this article misstated the location of the 1989 Loma Prieta quake.

Research Letter

Off‐Fault Focal Mechanisms Not Representative of Interseismic Fault Loading Suggest Deep Creep on the Northern San Jacinto Fault

M. L. Cooke, J. L. Beyer
First published: 30 August 2018

Abstract

Within the San Bernardino basin, some focal mechanisms show normal slip that is inconsistent with the expected interseismic strike‐slip loading of the region. The discrepancy may owe to deep (>10‐km depth), creep along the nearby northern San Jacinto fault.

The enigmatic normal slip microseismicity occurs to the northeast of the fault and primarily below 10‐km depth, consistent with off‐fault deformation due to spatially nonuniform ongoing slip. Consequently, if these normal focal mechanisms are included in stress inversions from the seismic catalog, the results may provide inaccurate information about fault loading.

Here we show that off‐fault loading from models with deep interseismic creep on the northern San Jacinto fault match the first‐order pattern of observed normal slip focal mechanisms in the basin and that this deep creep cannot be detected with GPS data due to the proximity of the San Andreas fault.

Plain Language Summary

Over the past 36 years, seismic stations have recorded the style of deformation from thousands of small earthquakes in the San Bernardino basin, California. Within this basin, many earthquakes below 10‐km depth show deformation that does not match what we expect for this region during the current period between large damaging earthquakes along the San Jacinto and San Andreas faults.

Rather than showing expected horizontal slip, many of these earthquakes show vertical movement. We use crustal deformation models to show that vertical movement can be produced in the basin if the northern portion of the San Jacinto fault creeps at depth; this portion of the fault is constantly moving rather than locked, like the San Andreas fault.

Traditional GPS‐based approaches to detect deep creep do not work here because the faults are too close to one another. The 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.

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL078932

Deep creep means milder, more frequent earthquakes along Southern California's San Jacinto fault

November 8, 2009

Image result for earthquake san deep creep

With an average of four mini-earthquakes per day, Southern California's San Jacinto fault constantly adjusts to make it a less likely candidate for a major earthquake than its quiet neighbor to the east, the Southern San Andreas fault, according to an article in the journal Nature Geoscience.

"Those minor to moderate events along the San Jacinto fault relieve some of the stress built by the constantly moving tectonic plates," said Shimon Wdowinski, research associate professor at the University of Miami's Rosenstiel School of Marine and Atmospheric Science.

Previous estimates may have overstated the likelihood of a major event on the 140-mile long San Jacinto fault, which begins between Palm Springs and Los Angeles and runs south toward the Salton Sea east of San Diego. The US Geological Survey (USGS) is forecasting a 31 percent chance that an earthquake with a magnitude of 6.7 or higher on the Richter Scale will occur on the San Jacinto fault in the next 30 years. Only the San Andreas fault, with a 59 percent chance, is more likely to have a major event during the same period.

"Thirty-one percent is a high probability, when it comes to earthquake forecasting—the second highest in Southern California," said Wdowinski. "Our data show that the next significant event for the San Jacinto fault would probably be between 6.0 and 6.7. It doesn't sound like much, but in earthquake terms it is the difference between a major earthquake and a moderate event."

A magnitude 6.0 earthquake may be felt for dozens of miles from the epicenter, but building damage especially in California, due to strict building codes, would be minimal. As the magnitude approaches and passes 7.0, which is ten times stronger than an earthquake with a magnitude of 6.0, more serious property damage and loss of life may occur.

Wdowinski feels that the San Jacinto fault is not as dangerous as predicted, because "deep creep" releases elastic strain of the moving plates approximately six to ten miles beneath the surface. As a result, the accumulation of strain along the fault occurs in the upper six miles of crust, which may be released by more frequent, moderate earthquakes. However a major event can still occur on the San Jacinto fault, but with lower probability, if two segments of the fault rupture simultaneously.

https://phys.org/news/2009-11-deep-milder-frequent-earthquakes-southern.html#jCp

This is a continuation of the excerpt in post 2...

By contrast, the more famous Southern San Andreas fault to the east is locked some 10 miles down, throughout the entire seizmogenic crust. It has had very few earthquakes to release that strain but promises to release much more energy—a major earthquake—when a rupture occurs.

"It's like bending a stick," said Wdowinski. "You can bend it until it breaks and releases the energy. The San Jacinto fault [on the left in the figure below] is like a stick that has a cut in it. When you begin bending it and it breaks, less energy is released. Deep creep—evidenced by those small, more frequent earthquakes—in effect forms that small cut that reduces the release of energy when the rupture finally occurs. We are less likely to have the big energy release of a major earthquake because the energy is not allowed to build up."

The Southern San Andreas fault to the east is like a thicker stick without any stress-relieving cuts, which will snap with much greater force. USGS predicts that the San Andreas fault has a 59 percent chance of a major earthquake (greater than a magnitude of 6.7) in the next 30 years.

Aside from earthquakes, Wdowinski's primary research interest at the University of Miami is hydrology and water flow in wetlands and the Florida Everglades, in particular. The link between desert earthquakes and swamps is geodesy, the study of the earth's size, shape, orientation, gravitational field, and their variations over time. He uses satellite imaging and the Global Positioning System (GPS) to measure those slight changes.

"These are the new tools of geodesy," said Wdowinski, who co-authored a May 2009 paper in the journal Eos, Transactions, a publication of the American Geophysical Union. The article highlighted "Geodesy in the 21st Century", a look at how technological advances are benefiting the field and are applicable to many important societal issues, such as climate change, natural hazards, and water resources.

Source: University of Miami Rosenstiel School of Marine & Atmospheric Science

Here’s a recent piece from the same site (phys.org)...

Geoscientists find unexpected ‘deep creep’ near San Andreas, San Jacinto faults

September 18, 2018
Michele Cooke, University of Massachusetts Amherst

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 3-D 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 3-D 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.”

https://phys.org/news/2018-09-geoscientists-unexpected-deep-san-andreas.html#jCp

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.