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Why LA Has Not Yet Experienced THE BIG ONE

When it comes to California earthquakes, the Los Angeles basin appears to be in a seismic "lull" right now, characterized by relatively smaller and infrequent earthquakes. By contrast, the Mojave Desert is in a seismically active period, suggesting that seismic activity alternates between the two regions. Seismologist James Dolan says, "When we're having earthquakes in L.A., generally we don't have as many earthquakes in the Mojave," meaning that one area's bad luck might predict a stable seismic period for the other area?and vice-versa.

The lull in the Los Angeles basin began 1,000 years ago. "The past 1,000 years has been relatively quiet," Dolan says, referring to what he calls the "urban fault network" under the Los Angeles metropolitan area. This claim will come as news to anyone who has lived through a big quake in Southern California, but Dolan says that even the Northridge earthquake of 1994, the costliest natural disaster in US history at the time, was "a drop in the bucket" compared to the massive jolts that would strike the basin during a period of high seismic activity.

What Dolan and his team call the "urban fault network" does NOT include the more distant San Andreas fault. Though the San Andreas is storing energy at a slower than average rate, a major quake along the fault is always possible. About 10 San Andreas "big ones" have occurred during the current lull on the urban fault network.

During the current lull in Los Angeles, major earthquakes in the eastern California shear zone have included the magnitude 7.1 Hector Mine of 1999, the 7.3 Landers of 1992 and the 7.6 Owens Valley of 1872. Each of these packed four to 20 times the energy of the Northridge quake. While all three quakes occurred in sparsely populated areas, Palm Springs and other desert communities lie close to the eastern California shear zone and could be vulnerable. "These are very large earthquakes," Dolan says.

If this theory is confirmed, detecting the start and end of a lull will become extremely important. Predicting the end of the current lull is impossible at present, according to Dolan, who says, "We're stuck with living here, so we have to understand what we can about this system."

Art credit: freeimages.co.uk

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