[ETL]

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

[ETL]

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

November 8, 2009

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

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[JimRed]

That appears to be a good thing (for area residents, maybe not so much for those of us who would like to see the lib infested parts sink into the sea). Small and numerous releases of the stresses beats one huge one if you’re sitting on top of it!

[Bubba_Leroy]

Truck’s fault. Trump and fracking. Womyn and minorities will be hardest hit.

[JimRed]

That river clearly shows the movement over the past however-many years. It will keep moving.

[Bubba_Leroy]

Danged autocorrect.

Trump’s fault. Trump and fracking. Womyn and minorities will be hardest hit.

[budj]

<...“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. ...>

Wouldn’t it be nice if CLIMATE scientists took the same approach to their research efforts?
1 - theory
2 - observe nature
3 - evaluate results
4 - revise theory for better fit
5 - go back to (2)

[gundog]

Expected and normal can be pretty devastating when you’re talking about chunks of the Earth shifting.

[Joe 6-pack]

"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."

They should ask Bill Nye. He knows everything!

[gov_bean_ counter]

Funny thing about science and scientists...

The more they learn, the less they know.

[Flag_This]

"suggests scientists have an incomplete understanding..."

[gunnyg]

This IS the world, remember? WTF can ya ‘spect?
Nothing @ best!
++++++++++++++

GunnyG@PlanetWTF???
TRUMP.45IF???WeCanKeepHim?
+++++++++++++++++++++++++++++

[TADSLOS]

Let ‘Er Rip.

[moovova]

[mewzilla]

As noted by Cooke, geoscientists has previously assumed...

They never learn.

[Berlin_Freeper]

Southern California beach bum is my dream life.