The potential for earthquakes to cause massive levee failures in the Sacramento-San Joaquin Delta is probably greater than previously estimated, seismic experts say.

In a presentation to the Delta Stewardship Council on Friday, scientists from the U.S. Geological Survey said prior studies made "very, very unrealistic" assumptions about ground motion in the Delta. Those studies assumed soils are relatively uniform across the region, said David Schwartz, USGS earthquake geologist. As a result, the ground shaking assumed in those studies was likely low.

There are 1,100 miles of levees in the Sacramento-San Joaquin Delta, and almost none meet the 100-year-flood standard set by the federal government as minimum protection for urban areas. Delta levees, built to protect farms, are much smaller. The Delta quake threat is a statewide concern, because the estuary is a freshwater supply for 25 million Californians. If several islands flooded at once, salt water would rush in from San Francisco Bay to fill those voids.

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