Quake-Exacerbating Soil Mapped

At least a third of Orange County lies atop soil susceptible either to earthquake-induced landslides or liquefaction, according to the latest version of an ongoing seismic mapping study by the state Department of Conservation.

Landslides, as well as liquefaction--in which loosely compacted, moist earth gives way after jarring from an earthquake--typically account for a substantial portion of the secondary damage from earthquakes.

Builders who plan to develop in high-risk areas designated by the study will now be required to submit a geological evaluation of their proposed sites. Land sellers will have to fill out hazard disclosure statements for potential buyers.

The department released 24 new hazard maps Thursday, covering about 2,600 square miles and 83 cities in Los Angeles and Orange counties.

About 260 square miles of Orange County had been mapped earlier for susceptibility to liquefaction and landslides. The new maps designate as high risk an additional 90 square miles from Seal Beach to Los Alamitos.

The addition brings the total mapped area to 60% of the developed portions of the county, said senior geologist Al Barrows.

The department plans to complete mapping the rest of the county, including the El Toro area and San Juan Capistrano, over the next year, he said.

The primary affected areas are the coastal plains, as well as the low-lying regions between the Santa Ana and San Gabriel rivers, Barrows said.

The low-lying areas fall on prehistoric river beds and feature sandy soil with underground water levels that come within 40 feet of the surface. The soil underneath can literally liquefy in an earthquake.

Geologists cautioned against panic. They said the maps are intended simply as a guide for local governments to use in assessing proposed developments in the zones.

“It doesn’t mean that every area in the zones is hazardous,” said Chuck Real, supervising geologist for the project. “But based on the data we’ve collected, liquefaction and landslides have a higher likelihood in the zones.”

If it’s found that a particular site might be subject to liquefaction or slides, state law requires developers of commercial buildings or housing projects of more than four dwellings to take steps to cut down on the danger.

That could include laying thicker foundation slabs or replacing the earth with a more cohesive soil, like clay or gravel.

The last time liquefaction struck the county was in the 1933 Long Beach quake, which actually had its epicenter in Huntington Beach. Portions of Pacific Coast Highway sank into the ground and water shot out from fields in Westminster and Fountain Valley, Barrows said.

During the 1994 Northridge quake, liquefaction took a significant toll on houses in the San Fernando Valley. Homes damaged by liquefaction required an average of $67,000 in repairs, compared with $12,000 for those damaged simply by shaking, geologists said.

Spending an additional $4,000 or $5,000 to shore up each house could have easily prevented the damage, Real said.

To examine the maps, check your local city hall, call the Department of Conservation at (213) 620-3560 or go to the Internet at https://www.consrv.ca.gov/.

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High-Risk Areas

The state Department of Conservation has released an updated map detailing areas in Los Angeles and Orange Counties that are highly vulnerable to liquefaction and landslides after an earthquake. Most of the high-risk areas are along the county’s coastal plains, as well as low-lying regions that fall between the Santa Ana and San Gabriel rivers. But geologists are quick to point out that risk designation does not automatically mean liquefaction or landslides will occur. During a quake, some designated regions will encounter serious problems, while others may escape negative effects altogether. Liquefaction and landslides typically account for a substantial portion of the secondary damage from earthquakes. Geologists, hoping to minimize such damage, have so far mapped about 60 percent of the developed portions of Orange County.

What is Liqefaction?

Liquefaction occurs when a seismic shock sends groundwater up through loosely compacted soil, saturating it and causing the earth to give way. It can severely damage building foundations and cause roadways to sink into the ground.