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Posted 5/3/2012

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By Dave Palmer
Los Angeles District

LOS ANGELES, Calif. — Wouldn't it be less menacing if you knew when to expect an earthquake, if they had their own time of year?

San Francisco's "big one" was April 18, 1906 and April is "Earthquake Preparedness Month." Is this earthquake season? How about October? The Loma Prieta quake was Oct. 17, 1989 and the "California Great ShakeOut" is Oct. 18.

Maybe the experts are saying: it's always earthquake season.

During a brown bag luncheon April 24 in the U.S. Army Corps of Engineers Los Angeles District Headquarters conference room, Dr. Swaminathan Krishman spoke about the critical question he and his colleagues at the California Institute of Technology are trying to answer. What is going to happen to modern, tall buildings when an event like a 7.9 magnitude quake happens?

"If you compute an average, it's about every 150 years that you see a big event on the San Andreas Fault," said Krishman.

The data they are using for their modeling is the Fort Tejon earthquake of Jan. 9, 1857. The quake opened up a 220 mile long rupture from Parkfield, Calif., running from north to south. At the time, the Los Angeles Star newspaper quoted witnesses describing the water in the LA River as sloshing from side to side, like shaking a bowl of milk.

Krishman explained that because of the very deep sedimentation in this area (river basin), the earthquake waves slow down and create long-period motion. Simply put, long duration waves cause tall buildings to sway for longer periods of time, increasing their risk of failure.

Today, we have the ability to do very good modeling of earthquakes, added Krishman. He had a captive audience during his presentation.

The luncheons are held for professional development and, on occasions like this, to inform District employees so they are better prepared for the "big one."