A major earthquake in the Bay Area could flood numerous islands in the Sacramento-San Joaquin River Delta, cripple the state's water system and cost billions of dollars, according to a state report released Friday.
The report, from the Department of Water Resources, found there is a 40% probability of an earthquake of magnitude 6.7 or higher causing 27 or more islands to flood at the same time in the next 25 years.
If 20 islands were flooded, the flow of fresh water through the delta could be interrupted for a year and a half, the report found. Emergency repairs on 20 islands could cost up to $2.3 billion and take about three years.
"Not everybody agreed an earthquake in the delta would be a problem, so we had to have a good team of well-respected scientists and engineers take a look at it," said Dave Mraz, chief of the department's Delta-Suisun Marsh office, which oversaw the report. "As it turns out, it's a very significant one."
U.S. Geological Survey seismologist Lucy Jones said she was very happy the state has taken the step of trying to quantify the scope of the problem. "This is the type of thing needed by decision makers to act," she said.
Former state Assemblyman John Laird began asking for a study of the effect of natural disasters on the levees in early 2005. The disastrous consequences of levee breaks during Hurricane Katrina later that year added urgency.
Mraz said his office worked with consulting groups URS Corp. and Jack R. Benjamin and Associates on the assessment as quickly as possible to synthesize data from earthquake scientists, engineers and economists. The report released Friday about the scope of the risk was the first phase. A second report will look at ways to address the risks.
Friday's report cited U.S. Geological Survey research finding that there is a 67% probability of an earthquake registering magnitude 6.7 or greater in the Bay Area in the next 25 years.
An earthquake of that size in a delta would probably cause liquefaction, in which shaking causes loose, water-soaked soil to turn into quicksand, Mraz said. The earthen levees that help channel water would sink, leading to flooding on the islands and salt water intrusion into the freshwater delivery system.
The ground movements "required to cause the delta to liquefy are very low," Mraz said. "That's kind of an Achilles heel in the delta."