[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

[budj]

<...“deep creep,” ...>

Probably a geophysical cross reference to the politicians.

[ETL]

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

[allendale]

Rather drastic way to sever the epicenter of hedonistic decadence from the rest of America.

[ETL]

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