The Big One could trigger series of large earthquakes, study finds
New research released Wednesday suggests that the shaking from "the Big One," the long-predicted major earthquake on the San Andreas fault, could trigger additional large temblors on nearby faults, intensifying the overall seismic impact.
The study suggests that such a quake "could presage a flurry of 'other Big Ones' on other faults," said USC earth sciences professor James Dolan, "as stresses related to the original San Andreas fault earthquake are redistributed on other faults throughout Southern California."
The study, presented by Dolan at a meeting of the Seismological Society of America in Pasadena, focuses on whether earthquakes are generated in "super cycles." A super cycle refers to when a large number of quakes rupture on a single fault system in a relatively short period of time in seismic terms, over a matter of decades or a few centuries.
The concept of more than one Big One in a lifetime might feel outlandish to Californians today. But it wasn't so long ago when this state had more powerful earthquakes more frequently. The San Andreas fault, for example, suffered two major ruptures in the 19th century: a quake of about magnitude 7.5 in 1812 and a much worse, 7.9 temblor in 1857.
The San Andreas fault in Southern California has been quiet since. And the region hasn't had a true Big One — a quake greater than 7.7 — since 1857.
"This period of relative seismic quietude, during which we have been releasing in earthquakes far less energy than we have been storing from relative tectonic plate motions, cannot last forever," Dolan said. "At some point, we will need to start releasing all of this pent-up energy stored in the rocks in a series of large earthquakes."
To better understand where quakes happen and when they hit, Dolan and his colleagues focused on Southern California's second largest fault, the Garlock fault, which stretches along the northern edge of the Mojave Desert and intersects the San Andreas. Previous studies showed that between AD 250 and 1550, the Garlock fault produced four large earthquakes, probably between magnitudes 7.5 and 7.7.
But there were periods of quiet. One lasted about 3,500 years before the year 250. A new quiet period began after 1550; the Garlock fault has not had quakes large enough to break the surface in nearly half a millennium.
Dolan and his team found that during its active time, the Garlock fault was moving nearly 20 millimeters a year. That's much faster than the long-term annual average of that fault, between 5 millimeters to 7.5 millimeters.
The calculations show how earthquake faults can essentially stop moving for thousands of years, and then go into a relatively hyperactive super cycle, unleashing seismic strain.
The results of this study can be applied to other faults, Dolan said. "We're not focused especially on the seismic threat posed by the Garlock," he said. "This study focuses on the deeper scientific significance, the more general importance of how faults interact with one another over long time and distance scales, and fundamentally on helping us to understand how faults store and release energy.
"These are issues of absolutely basic importance for our understanding of seismic threats from all faults," Dolan said.
The Garlock fault happened to be a good example to focus on because of the presence of data that show both the timing of prehistoric temblors and the rate at which the fault stores up seismic stress and releases energy through earthquakes.
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Earthquake 'super-cycle' patterns on the Garlock fault
6 hours ago
A new look at slip rate data and geologic evidence for ancient earthquakes on the central Garlock fault suggest that seismic activity along the fault may be controlled in part by "super-cycle" changes in strain that occur on thousand-year timescales.
The findings are part of an increasing body of evidence that suggests there may be large-scale coordination of earthquakes in time and space, which can cause large earthquakes to cluster in time along a single fault system, for instance. The Garlock fault runs along the northern border of the Mojave Desert in southern California. Although the immediate region around the fault is not heavily populated, earthquakes along the fault could impact most of southern California.
James Dolan of University of Southern California and colleagues' new look at the Garlock fault found that a cluster of four earthquakes during the late Holocene, about 500 to 2000 years ago, occurred at a time when the average slip rate on the fault was twice as fast as the long-term average slip rate. Previous paleoseismic results show, however, that this cluster was preceded by a 3000-year lull of very small or no slip. This "on-off" behavior of the Garlock indicates that the fault may go through "super-cycles" of strain, where the strength of the fault waxes and wanes over thousands of years, the researchers say. Overall, the earthquake cycles in the area may be caused by this type of super-cycle influencing the strength of many different faults in the region, including the San Andreas, Garlock and the Eastern California Shear Zone faults.
Dolan will present his research on April 22 at the annual meeting of the Seismological Society of America (SSA) in Pasadena, Calif.
http://phys.org/news/2015-04-earthquake-super-cycle-patterns-garlock-fault.html
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Earthquake potential where there is no earthquake history
11 hours ago
It may seem unlikely that a large earthquake would take place hundreds of kilometers away from a tectonic plate boundary, in areas with low levels of strain on the crust from tectonic motion. But major earthquakes such as the Mw 7.9 2008 Chengdu quake in China and New Zealand's 2011 Mw 6.3 quake have shown that large earthquakes do occur and can cause significant infrastructure damage and loss of life. So what should seismologists look for if they want to identify where an earthquake might happen despite the absence of historical seismic activity?
Roger Bilham of the University of Colorado shows that some of these regions had underlying features that could have been used to identify that the region was not as "aseismic" as previously thought. Some of these warning signs include debris deposits from past tsunamis or landslides, ancient mid-continent rifts that mark the scars of earlier tectonic boundaries, or old fault scarps worn down by hundreds or thousands of years of erosion.
Earth's populated area where there is no written history makes for an enormous "search grid" for earthquakes. For example, the Caribbean coast of northern Colombia resembles a classic subduction zone with the potential for tsunamigenic M>8 earthquakes at millennial time scales, but the absence of a large earthquake since 1492 is cause for complacency among local populations. These areas are not only restricted to the Americas. Bilham notes that in many parts of Asia, where huge populations now reside and critical facilities exist or are planned, a similar historical silence exists. Parts of the Himalaya and central and western India that have not had any major earthquake in more than 500 years could experience shaking at levels and durations that are unprecedented in their written histories.
Bilham will present his research on April 22 at the annual meeting of the Seismological Society of America (SSA) in Pasadena, Calif.
http://phys.org/news/2015-04-earthquake-potential-history.html
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