Jangled nerves in California, as scientist at a loss to explain present Southern California quake swarm
August 29, 2012 – CALIFORNIA – An unusual swarm of hundreds of mostly small earthquakes has struck Southern California over the last three days and shaken the nerves of quake-hardy residents, but scientists say the cluster is not a sign a larger temblor is imminent. The earthquakes, the largest of which measured magnitude 5.5, began on Saturday evening and have been centered near the town of Brawley close to the state’s inland Salton Sea, said Jeanne Hardebeck, research seismologist for the U.S. Geological Survey. Scientists were monitoring the earthquake cluster, which continued on Tuesday, to see if it approaches the Imperial Fault, about three miles away. A destructive and deadly earthquake of magnitude 7.0 struck on that fault in 1940, she said. “We don’t have any reason to believe that the (earthquake) storm is going to trigger on the Imperial Fault, but there’s a minute possibility that it could,” Hardebeck said, adding that the swarm of quakes was not moving closer to that fault. The Brawley quake cluster, which is caused by hot fluid moving around in the Earth’s crust, is different than a typical earthquake, in which two blocks of earth slip past each other along a tectonic fault line. After that kind of an earthquake of magnitude 5.5 or above, there is a 5 percent chance a larger quake will follow, Hardebeck said. But she added the same kinds of probability estimates were not possible with earthquake clusters caused by the movement of hot fluid. “We understand them even less than we understand normal earthquakes,” Hardebeck said, adding that scientists do not know why a cluster of earthquakes will occur at one time rather than another. The swarm led to jangled nerves in Brawley, a town of about 25,000 residents 170 miles southeast of Los Angeles near the border with Mexico. “It’s pretty bad. We had to evacuate the hotel just for safety,” Rowena Rapoza, office manager of a local Best Western Hotel, said on Sunday. There were two earthquakes on Sunday afternoon, one with a 5.5 magnitude and one measuring 5.3, Hardebeck said. Those were the largest quakes in the cluster amid hundreds of others, she said. In the past, earthquake clusters have gone on for as long as two weeks, Hardebeck said. Before this recent cluster in Brawley, the last swarm of this size to hit the area was in 1981, she said. –Reuters
Geothermal Region: Gulf of California Rift Zone
The Gulf of California rift zone is a complex transition zone between the dextral (right-lateral) motions of the San Andreas transform fault system and the northwestward progressing spreading ridge complex of the Gulf of California segment of the Eastern Pacific Rise. The Gulf of California and its onshore extension, the Salton Trough (which includes Mexicali, Imperial, and Coachella Valleys), are located over a series of rifts in the Earth’s crust which are filling with sediment from above, chiefly from the Colorado River, and magmatic material from below. The Cerro Prieto geothermal field in Mexico and the Brawley Seismic zone in the U.S. are located above two of these rifts, and young volcanoes in these locations are evidence of intrusion of magma from below. The volcanics in this exploration region are less then 5-million year old and associated with northwest folding, block- and thrust- faulting. Dacite is the most common volcanic rock, with a composition that ranges from basalt to rhyolite. The volcanic activity appears to be related to extension associated with the San Andreas Fault system. The most recent volcanic activity is dated to 10,000 years ago. The heat source for the Geysers geothermal field is provided by a silicic magama chamber. Clear Lake Volcanic Field, California. -GE
Ancient tectonic plates slid under Western North America, thining the crust: The Farallon Plate was an ancient oceanic plate, which began subducting under the west coast of the North American Plate— then located in modern Utah— as Pangaea broke apart during the Jurassic Period. It is named for the Farallon Islands which are located just west of San Francisco, California. Over time the central part of the Farallon Plate was completely subducted under the southwestern part of the North American Plate. The remains of the Farallon Plate are the Juan de Fuca, Explorer and Gorda Plates, subducting under the northern part of the North American Plate; the Cocos Plate subducting under Central America; and the Nazca Plate subducting under the South American Plate. The Farallon Plate is also responsible for transporting old island arcs and various fragments of continental crustal material rifted off from other distant plates and accreting them to the North American Plate. These fragments from elsewhere are called terranes (sometimes, ‘exotic’ terranes). Much of western North America is composed of these accreted terranes. (Left) Baja, California map showing fracture along the Laguna Salada fault during 7.2 magnitude earthquake in 2010. -Wikipedia
