July 31, 2008

Some Areas More Sensitive to Quake

By Andy Franks

LONG BEACH - Your description of Tuesday's earthquake might be different from your friend's across town, but for a good reason: Geologic surveys show that some parts of Long Beach are more sensitive to ground motion.

A survey conducted by the California Department of Conservation shows that large ground areas of Long Beach - including Belmont Shore and Naples Island - are more susceptible to liquefaction, or the sudden transformation of loose soils from a solid to a liquid state.

While a number of factors contribute to the degree of liquefaction, including the origin and size of the earthquake, the onset of seismic activity will greatly reduce the strength of liquefaction-prone soils.

Assistant Professor Nate Onderdonk with the Department of Geological Sciences at Cal State Long Beach described liquefaction- prone soil as a material that will amplify earthquake motions. When shockwaves hit these sediments, they tend to slow down, and it causes them to amplify.

"The types of geologic deposits where this is common are stream or river deposits, especially if a water plane is high," Onderdonk said. "How that pertains to Long Beach is that the lands that border the west end are more prone to liquefaction due to deposits from the LA River; and on the other end \, you have the same problems."

Onderdonk also cites reclaimed swamp, wetland or bay areas where loose soil has been developed, and may be prone to a stronger or prolonged shake.

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