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Future Quake Would Threaten Delta Levees - Warnings from USGS are more dire than previously thought

February 2011

A major earthquake could have a greater impact on the Sacramento-San Joaquin Delta than previously estimated and could happen at any time, according to experts at the US Geological Survey.

“You can run, but you can’t hide,” said Dr. David Schwartz in a presentation to the Delta Stewardship Council during its January meeting, noting that significant impacts can occur at considerable distance from an earthquake’s epicenter. Schwartz is a senior earthquake geologist with the federal agency.

The USGS said the Delta region is most vulnerable to, and will be most impacted by, the large-plate boundary faults in the East Bay.

The Council requested the USGS presentation because it must use best available science in making policy recommendations in the Delta Plan. Section 85305, subsection (g), of the Delta Reform Act requires the Council to “reduce risks to people, property, and state interests in the Delta by effective emergency preparedness, appropriate land uses, and investments in flood protection.”

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Scientists at the USGS say that in the next 100 years there will be strong shaking in the Delta because of an East Bay earthquake and that could lead to wide-scale levee failure. The 1906 earthquake was so strong it released all of the stress in the entire region, and over the last 100 years that stress has returned, Schwartz said. The USGS believes we are entering a period similar to pre-1906 conditions, when the region was hit with more frequent medium and large earthquakes.

“Somewhere, somebody has got to bite the bullet,” Schwartz told the Council. “Something has to be put in place to keep the whole thing from falling apart.”

For years, experts agreed that many Delta levees would fail during a significant earthquake, flooding the region and threatening the state’s water supply, but reliable information was hard to come by.

Previously, scientists had believed the peat soils of the Delta would “dampen” seismic waves, but the USGS has now discovered that the make up of the Delta is so complex and varied that an earthquake could actually amplify shaking in some cases. This means levees would shake harder and could result in multiple failures.

The Delta Risk Management Study had predicted that as many as 20 islands could flood simultaneously in a major quake. Not only is this a concern for the Delta itself, but a massive levee failure would allow salt water to invade the Delta from the San Francisco Bay and shut off the fresh water supply for millions of Californians.

But there are some who question whether concerns about levees aren’t masking other motives.

“All the hype about levee strength is just another thing to throw out there to prove the Delta is unsustainable and in danger,” said Delta farmer Mike Robinson. “I don’t agree with that. And it’s another thing to justify a canal or a pipeline.” Whichever direction the Delta Plan takes regarding levees, the Council will depend on science and facts to respect the coequal goals of ecosystem restoration and water supply preservation.

“A group of people will soon be saying the state should be protecting every levee every place,” said Council Chair Phil Isenberg. “Then there will be those who say [an earthquake] hasn’t happened in 100 years so it probably won’t happen. Everyone will have an opinion about what should or shouldn’t be done and we need to figure it Estimated earthquake

Council member Felicia Marcus laid the groundwork for future discussions by asking what the Council should recommend. “One scenario you can take from this presentation is ‘oh, my God, we should abandon the Delta or armor the Delta’ but what recommendations and suggestions should we be looking at?”

The Council will use the science provided by the USGS and others to help develop policy recommendations in the upcoming Delta Plan. The Plan will suggest an organization or prioritization of where to concentrate investments in levee repair and public health, construction, and safety protection in the Delta.

To view the USGS PowerPoint presentation, please visit here.

Coequal goals

The Delta Stewardship Council was created in legislation to achieve the state mandated coequal goals for the Delta. "'Coequal goals' means the two goals of providing a more reliable water supply for California and protecting, restoring, and enhancing the Delta ecosystem. The coequal goals shall be achieved in a manner that protects and enhances the unique cultural, recreational, natural resource, and agricultural values of the Delta as an evolving place." (CA Water Code §85054)