The probability of an earthquake of 6.7 or greater magnitude along the Hayward Fault is currently 31 percent, according to California Geological Survey geologist Chris Wills.
The most recent earthquake of that magnitude to hit the Hayward Fault came on Oct. 21, 1868 and has been described as one of the most destructive earthquakes in California’s history, Wills said.
Measured at 6.8, the historic earthquake has caused geologists to label the Hayward Fault a “tectonic time bomb” that could strike at any moment.
“The average time between significant earthquakes is between 140 and 150 years,” Wills said. “You do the math.“
If and when such an earthquake finally does hit, a scenario planned for under Governor Schwarzenegger’s Delta Emergency Operations Plan, as many as 50 levees might fail – a catastrophe that would flood more than 20 delta islands and “cripple” the state’s water supply systems.
Failure of the more than 100-year-old levees would compromise fresh water supplies for two-thirds of California’s population – over 25 million people. Experts estimate the damage caused by allowing saline-rich water into freshwater pumps would last for weeks or months, and cost billions to repair.
Researchers now hope to track how seismic waves travel through sediment layers beneath the delta and analyze how that energy is conducted.
Using energy data from earthquakes of magnitude two or larger in the area surrounding the Hayward Fault – over 300 of which occurred in the last month alone, according to U.S. Geological Survey data – UC Davis researcher Donna Eberhart-Phillips will create a detailed three-dimensional analysis of what lies beneath the delta.
Her work is just one part of a large-scale multidisciplinary effort by the USGS to gain a full perspective of the delta’s problems.
In addition, engineers are studying the levees‘ structural strength and other researchers at the USGS are developing models of how an earthquake might break on the Hayward Fault – where experts believe the next major earthquake will strike.
“What I’m doing will be very useful to understand if and how the levees might fail,” Eberhart-Phillips said.
She and two colleagues from the University of Wisconsin will use data from the hundreds of small tremors that stem from San Francisco Bay Area faults to help determine how an earthquake might impact delta levees, she said.
“Whether or not there are soft materials two meters down, for example, can make a big difference,” Eberhart-Phillips said. “Different materials below the surface can have a large effect on frequency content and how strongly an earthquake is felt.“
Local scientists have long been concerned with the Delta’s sustainability.
A 2007 report concerning the future of the delta stressed the need for new strategies to manage the delta’s water supply.
Jointly authored by a multidisciplinary team of five UC Davis professors and the Public Policy Institute of California, a nonprofit research institution, the report estimated that one instance of failure to the levee system could cost as much as $40 billion to repair.
“If you wanted to break California’s water supply system, you break it here,” says Jeffrey Mount, director of the Center for Watershed Sciences at UC Davis and coauthor of the 2007 report.
In a November 2008 talk, Mount said the 1,100 miles of naturally-evolved levees in the Sacramento Delta region are unavoidably deteriorating.
Constant farming, increasing force on the levees and sea level rise have all contributed to delta deterioration and caused many of the delta’s islands to subside more than 20 feet below sea level, Mount said.
“The delta of the past is gone,” Mount said. “We’ve exported it into tidal marsh and the atmosphere. Today there is this constant battle of keeping the water out and building the levees bigger and bigger.“
In 2008 Mount coauthored a second report entitled “Comparing Futures of the Sacramento-San Joaquin Delta,” which recommended a peripheral canal as the best way to relieve pressure currently put on the levees.
This second report’s two main goals were to revive a threatened ecosystem in the delta and to ensure a reliable, clean water supply for the state.
The canal plan – though still a controversial and expensive prospective project for the state – would address both issues, even in the event of a large earthquake of magnitude 6.5 or higher.
Eberhart-Phillips said she may continue doing further research after the one-year project she is currently working on is complete.
AARON BRUNER can be reached at firstname.lastname@example.org.