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Ridgecrest earthquake mystery: Why so little destruction from huge temblors?

Rong-Gong Lin II, Los Angeles Times on

Published in News & Features

Repeating the July 4 and 5 quakes in the Bay Area and Southern California would result "in a lot of homes off their foundations," Maffei said. "Without retrofits, the Bay Area and Los Angeles do not have resilient housing."

There are at least 1 million of these vulnerable homes in California, but Ridgecrest has very few.

The more obvious signs of damage in Ridgecrest did not make many structures uninhabitable -- cracked concrete walls surrounding yards or a broken decorative brick facade on a home, said Southern California structural engineer Wayne Chang, who visited the region Sunday and shared his observations with the Earthquake Engineering Research Institute.

Some of the worst damage was to mobile homes, which often are not secured to their foundations, engineers said.

The happenstances of geology and geography also worked in the town's favor.

The magnitude 7.1 earthquake started at an epicenter 10 miles northeast of central Ridgecrest. But it occurred on a fault that focused the worst shaking waves away from Ridgecrest and Trona, to the northwest and southeast respectively of the epicenter, and into sparsely populated areas, California Institute of Technology seismologist Egill Hauksson said.

 

On the Modified Mercalli Intensity Scale, Ridgecrest endured "very strong," or level 7 shaking, enough to break chimneys and damage badly designed structures but keep damage negligible in well-designed buildings. Trona got a level 6 "strong" shaking.

By contrast, much of the San Fernando and Santa Clarita valleys saw at least level 8, or "severe," shaking during the Northridge quake -- an intensity that can greatly damage poorly built structures. (The shaking nearly caused a new steel frame Auto Club building in Santa Clarita to collapse and seriously damaged or destroyed 200 apartment buildings.

Even though the Northridge earthquake produced much less total energy than the July 5th temblor, its location caused shaking to be worse directly underneath a highly populated area.

Trona, an older city, was more heavily hit than Ridgecrest.

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