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.
The study area is in the dry bed of prehistoric Lake Cahuilla at Coachella, Calif. The lake has been dry since about 1715, according to timelines found in early travelers' descriptions of the area. Researchers found that the lakebed was full of water six times in the study period.
"We now have the best chronology of these lakes that has ever existed," said Weldon, who knows the area well from previous work.
As a doctoral student, he was part of a Cal Tech team led by Kerry Sieh that studied a nearby site in the late 1970s and early 1980s. Seismologists have cited the team's never-fully published findings often. Weldon's return to the region began when researchers were granted access to three 26-foot-deep trenches dug to determine Alquist-Priolo earthquake fault zones as required under California law to assure that human dwellings are not built on fault lines. The trenches, two of which exposed the fault, provide direct access to layers of lake sediments and alluvial deposits.
The Lake Cahuilla Basin, 132 miles from downtown Los Angeles, is separated from the Gulf of California to the south by the expansive, ever-changing delta of the Colorado River. The current body of water in the lake's southern basin, called the Salton Sea, was born in 1905, when heavy rain and snowmelt in the Colorado River drainage led to the collapse of an intake canal built for irrigation purposes just south of Yuma, Ariz. The river then poured into the sink.
The observation that the last lake and last quake were at about 1700 AD and that there have been seven earthquakes and seven lakes during the approximately past 1,000 years have led to the hypothesis that the filling or emptying of each lake triggered earthquake events by changing the pressure on the fault plane below. The new study, Weldon said, shoots down that idea.
The data show that earthquakes occurred in all scenarios: when the lake was filling, while it was full, when it was draining and even when it had long been dried up. In fact, researchers found, the last quake in about 1690 occurred when the lake was full, just before it drained. Calculations of how long the lake would take to dry, if shut off from its Colorado River source coupled with travelers' journals, cited in the study, actually provide witness to the lake's disappearance. "If anything, the earthquake made the lake go away," Weldon said.
"The most popular hypothesis has been that the filling of the lake causes an earthquake, or the draining of the lake causes a quake," he said. "Neither can be true based on where we've found. There's probably no relationship, but if you want to say there is a relationship, it could be that the quakes make or unmake the lakes."
That scenario, he added, could mean that earthquakes have at times shifted the Colorado River's pathway into or away from Lake Cahuilla's bed, perhaps by shaken driven lateral spreading and collapse of its riverbanks
Seismic activity has been common in the Imperial Valley south of the Salton Sea, which is the continuation of the plate boundary to the south but not part of the actual fault, Weldon noted.
"At some point, this area will get kicked by shaking from one of the many quakes that happen south of the San Andreas Fault," he said. "It will rupture northward along the fault. When it comes into the San Bernardino Valley, seismic energy will be directed by a series of basins, including the Los Angeles Basin, into the most highly populated part of Southern California"
The U.S. Geological Survey supported the research. A National Science Foundation Graduate Research Fellowship also supported Philibosian, who was the study's lead author.About the University of Oregon
Contact: Jim Barlow, director of science and research communications, 541-346-3481, email@example.com
Source: Ray Weldon, professor and head, department of geological sciences, 541-346-4584, firstname.lastname@example.orgLinks:
Southern California Earthquake Center: http://www.scec.org/
UO Science on Facebook: http://www.facebook.com/UniversityOfOregonScience
Jim Barlow | Newswise Science News
New research calculates capacity of North American forests to sequester carbon
16.07.2018 | University of California - Santa Cruz
Scientists discover Earth's youngest banded iron formation in western China
12.07.2018 | University of Alberta
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.
Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
18.07.2018 | Life Sciences
18.07.2018 | Materials Sciences
18.07.2018 | Health and Medicine