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Science Friday: 7.0 Earthquake in Chile; 4 Day Old NY Times Article Warns of Building Stress | LA Long Overdue for Major Quake

Weird. I was actually working on a post on earthquakes using the Times article and the Science Daily article when news broke about the earthquake in Chile. And in the Times article they quote Stefano Lorito, a geophysicist with the National Institute of Geophysics and Vulcanology in Rome who said that last years earthquake added to the stress on the fault rather than relieving it. Update: Now the USGS says it was 6.8


Quake With A Preliminary Magnitude Of 7.0 Strikes Chile

http://www.npr.org/blogs/thetwo-way/2011/02/11/133688214/quake-with-a-preliminary-magnitude-of-7-0-strikes-chile



Stress of Sliding Plates Builds Near Chile

By HENRY FOUNTAIN
Published: February 7, 2011

    When a magnitude-8.8 earthquake struck off the coast of Chile last February, geophysicists and seismologists were not surprised. The quake’s epicenter was on a roughly 200-mile stretch of a fault where stresses had been building for nearly two centuries, and experts had expected that one day the strain would be relieved in a cataclysmic event.

    But as scientists have pored over volumes of data from what may turn out to be the best-studied major earthquake yet, they have concluded that the ground movement during the quake did not relieve the stresses as anticipated.

    The greatest seismic slip was outside the 200-mile segment, known as the Darwin gap since Charles Darwin happened to witness the last earthquake along it, in 1835.

    “The pattern of slip was quite different from what would have been expected,” said Stefano Lorito, a geophysicist with the National Institute of Geophysics and Vulcanology in Rome. While there was one area of slip south of the epicenter that was within the Darwin gap, he said, the area of greatest movement was north of the gap in an area where an 8.0 earthquake occurred in 1928.

    Dr. Lorito said the findings show that “there is a fraction of the gap that probably did not rupture.” In addition, he said, the 2010 quake may have added to the stress in the unruptured area, increasing the odds of another major earthquake, although it would probably not be as large as the one last year.

    [...]

http://www.nytimes.com/2011/02/08/science/08quake.html?_r=1&ref=science



Los Angeles Basin Long Overdue for Major Earthquake: Lake-Effect Theory Sinks, but Quake Timing Questions Go on

    ScienceDaily (Feb. 10, 2011) — A chronology of 1,000 years of earthquakes at the southern end of the San Andreas Fault nixes the idea that lake changes in the now-dry region caused past quakes. However, researchers say, the timeline pulled from sediment in three deep trenches confirms that this portion of the fault is long past the expected time for a major temblor that would strongly shake the Los Angeles Basin.

    The new study, appearing in the February issue of the Bulletin of the Seismological Society of America, doesn't change existing thinking about the threat of a major quake -- potentially measuring 7.0 to 8.0 on the Richter scale -- for southern California. It does, however, provide the first published documentation of much-discussed data that have emerged in the last three decades from an area that is now rapidly being built up and populated, just north of the Salton Sea.

    Projections of such a quake in recent years led to the nation's largest-ever drill, the Great Southern California ShakeOut, last year. The 2011 ShakeOut is set for Oct. 20. There's even a video projection of the quake's probable route created by the Southern California Earthquake Center. The last earthquake to originate from the area occurred in about 1690.

    The new study, said co-author Ray Weldon, professor and head of the department of geological sciences at the University of Oregon, documents that the south end of the San Andreas fault has gone perhaps 140 years longer than the average 180 years between quakes.

    "We have dated the last five to seven prehistoric earthquakes of the southernmost 100 kilometers (about 60 miles) of the San Andreas Fault, which is the only piece of the fault that hasn't ruptured in historical times," Weldon said. "If you were there in about 1690, when the last earthquake occurred, the odds of getting to 2010 without an earthquake would have been 20 percent or less."

    Weldon stopped short of concluding that a major earthquake is due or overdue, saying that data from this study and other recent work may just as well point to unknowns in current earthquake-modeling techniques.

    The seven earthquake events, including the two possible temblors, were placed between 905-961 AD, 959-1015 (possible), 1090-1152, 1275-1347, 1320-1489 (possible), 1588-1662 and 1657-1713, based on analyses of seismic structures preserved in the sediment in the three trenches and 82 radiocarbon dates drawn from 61 samples of organic material.

    Weldon and co-authors -- former UO graduate student Belle Philibosian, now pursuing a doctorate at the California Institute of Technology (Cal Tech), and Thomas Fumal of the U.S. Geological Survey, who died in December -- concluded there is a high probability of rupture in the fault because of a likely buildup of tectonic stress.

    [...]

http://www.sciencedaily.com/releases/2011/02/110210122941.htm