Seismic Hazard Maps Show Some O.C. Areas With Liquefaction Risk

State geologists released the first seismic hazard maps for parts of Orange County on Tuesday under a new program requiring geologic investigations and mitigation measures for new building projects in zones subject to ground liquefaction in big quakes.

Once the maps become official in about six months, homeowners in a seismically hazardous zone who sell their properties will be required to disclose the potential danger to buyers.

The preliminary maps issued Tuesday were for a 120-square-mile area from Buena Park and Fullerton south through Anaheim to Newport Beach.

Public comment will be accepted by the Division of Mines and Geology in the state Department of Conservation until Jan. 7, and the official maps will be released in March, state officials said. Preliminary maps for much of the rest of the county will be issued in early 1997.

Liquefaction is a process by which water-saturated soils turn to jelly under heavy shaking, jeopardizing building foundations. It was a particular cause of heavy damage in the 1985 Mexico City quake and in the Marina District of San Francisco during the 1989 Loma Prieta earthquake.

Orange County has escaped major quake damage in the 20th century. The largest quake close to the county, the magnitude 6.3 Long Beach quake of 1933, occurred on a segment of the Newport-Inglewood fault that lies outside the county.

But a major rupture of the southern segment of this fault lying within the county, or of an offshore thrust fault, is certainly conceivable, scientists say.

The maps released Tuesday show that more than half of the area detailed falls in what scientists believe to be liquefaction zones, a proportion much higher than is the case in the city of San Francisco.

Charles Real, the supervising geologist for the project, explained that much of the area lies within the varying historic and prehistoric beds of the Santa Ana River, where sand and silt is common and water tables come within 40 feet of the surface.

Also, certain coastal areas, such as the Balboa Peninsula and other areas adjacent to Newport harbor, are in the liquefaction zones.

The geologists decided that all such zones should be considered subject to liquefaction, although they caution on each map that not all areas subject to the process may be shown.

They also caution, “A single earthquake capable of causing liquefaction . . . will not uniformly affect the entire area.”

Not shown on any of the maps released Tuesday were major fault lines, such as the Newport-Inglewood fault, where heavy shaking could cause extensive damage in areas outside the designated liquefaction zones.

Real said that seismologists and engineers have not yet been able to agree on criteria for bringing these zones under the designation as seismic hazard zones.

But, he noted, for the most part shaking damage is regulated under building codes, while these hazardous zones are designed more to reduce dangers from underlying special geological conditions. Real said work has yet to be completed in the Orange County areas mapped thus far on designating localities subject to quake-caused landslides. These areas too will ultimately be designated hazardous zones.

Being designated a hazardous zone triggers the state mandate that local governments require both geotechnical investigations and mitigation measures for any commercial project or any housing project containing more than four dwelling units.

Construction of just one housing unit will not be subject to the requirement, but single units will fall under the disclosure requirements at time of sale.

Real said that a common mitigation step in an area subject to liquefaction may be strengthening of foundations.

He said that putting in $1,000 to $2,000 more steel in the foundation could save money later when a quake comes.

“You don’t have to restrict development under this program,” he said. “You just have to know what to do to make things safe.”

He acknowledged, as is often the case with state mandates, the investigations will have to be done at local expense, although he said that in most cases the cost probably would be passed on by the municipalities to developers.

There are no penalties written into the law for municipalities that do not adhere to the new requirements, but so far most local officials have shown a willingness to cooperate, he said.

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Another Hazard

Liquefaction, the process by which water-saturated soil can turn to jelly during an earthquake and threaten building foundations, is possible in a large swath of Orange County. Here’s a look at the areas at risk, where much of the soil is sand and silt, and the water table is within 40 feet of the surface. Only this 120-square-mile area of the county was studied:

Source: California Department of Conservation