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AMHERST, Mass. September 21, 2018 – A new analysis of thousands of very small earthquakes in the San Bernardino basin near the San Andreas and San Jacinto faults suggests that the unusual deformation of some – they are moving differently than expected – may be due to "Deep creep" 10 km below the surface of the Earth, say geoscientists at the University of Massachusetts at Amherst.
The new understanding should support more refined assessments of fault load and risk of seismic failure in the region, they add. Write in the current online Letters of geophysical researchJennifer Beyer, a PhD student, and her adviser, Michele Cooke, professor of geoscience, say the enigmatic behavior is found in about one-third of the hundreds of earthquakes recorded between two major earthquakes and their importance.
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Cooke says, "These small earthquakes are a very useful dataset, and if we pay more attention than in the past to the details they tell us, we can learn more about active fault behaviors. we understand better the loading which leads to great earthquakes damageable.
Over the last 36 years, the authors point out that seismic stations have recorded the deformation style of thousands of small earthquakes in the San Bernardino California Basin. They state, "The results of this study demonstrate that small earthquakes occurring near and between faults may have very different deformation styles than large earthquakes caused by active faults. This means that scientists should not use the information recorded by these small earthquakes in the San Bernardino Basin to predict the loading of the San Andreas and San Jacinto faults nearby.
Cooke explains that the usual type of loophole in the area is called a keystroke, where movement is one of the blocks that creep into each other. The least common type, with an "abnormal sense of slippage," is a prolonged flaw, where the movement between the blocks is like a wave moving away from the beach, a block moving away and increasing the flaw. "These only happen in this small area and nobody knew why," she says. "We did the modeling that helps explain the enigmatic data."
This is an area in which Cooke, an expert in 3D fault modeling, has done her own research and is familiar with the field of research as a whole. So she decided to model what's going on. She began with a hypothesis based on her earlier 3D modeling in the region that had reproduced the long-term deformation over thousands of years.
"I noticed that this basin was an extension in these models, unlike the surrounding areas of waste," she says. "The extension was limited inside the basin just like the configuration of abnormal earthquakes. This gave me an idea of what maybe these faults were not blocked as they should be between large earthquakes, but at depths less than 10 km, they crawled.
"The typical way we look for creep is to use GPS stations on either side of the fault. Over time, you may notice that there is movement; the defects are slowly diverging. The problem here is that the San Andreas and San Jacinto faults are so close together that GPS can not solve creep problems. That's why no one has seen this before. The traditional way of detecting it could not do it.
Cooke adds, "In this article we have shown that there is a way to have these tiny earthquakes quite next to the San Jacinto Fault below 10 km, where deep creep can occur. produce. We show that this is plausible and can explain the enigmatic earthquakes nearby. The model may not be perfectly correct, but it is consistent with the observations. "
As noted, this work has implications for the assessment of the default load, say Beyer and Cooke. Until now, seismologists have assumed that the faults in the region are blocked – there is no creep – and they use data from all small earthquakes to deduce the loading on the primary faults. However, Cooke and Beyer write that "scientists should not use the information recorded by these small earthquakes in the San Bernardino basin to predict the loading of the nearby San Andreas and San Jacinto faults".
Cooke adds, "Our catalog of earthquakes is increasing every year; we are seeing smaller and smaller each year, so we thought why not take advantage of the networks we created and look at them in more detail. We do not want to wait for the mistakes to move in a devastating earthquake, we want to take advantage of all the weaker earthquakes that occur all the time to understand how San Andreas and San Jacinto are charged. If we can understand how they are loaded, perhaps we can better understand when these defects are likely to break. "
This study was funded by the Southern California Earthquake Center, which is funded through cooperation agreements with the National Science Foundation and the US Geological Survey.
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