California's vast delta, where the Sacramento and San Joaquin rivers pour their waters toward the sea, could be in serious danger when the next major earthquake strikes along the Hayward Fault -- or on any of the other faults that make up the San Andreas zone, scientists have long warned.
Now a new study is under way to detail just how seismic waves from future quakes would travel through the delta's thick sedimentary earth and whether the waves might grow stronger or weaken as they travel.
The nature of the ground beneath the delta's surface would greatly affect the damage a quake could cause to the waterway's miles of levees, which shelter roads and farms, and to the giant pumps that send water to the state's parched southern regions.
In the study approved by the U.S. Geological Survey, seismologist Donna Eberhart-Phillips of the University of California-Davis and two colleagues at the University of Wisconsin hope to gain insights into the delta's reaction to quakes of any size by recording the earth's movements during the small temblors that occur often along the Hayward Fault as well as along many of Northern California's smaller faults.
"We need to know how the energy from earthquakes travels through the upper two to three miles of the delta's earth -- what the properties of that earth are, how the speed of a quake's seismic waves are attenuated when material in the region absorbs energy and how a quake's effects propagate throughout the delta," Eberhart-Phillips said.
She and her colleagues are basing much of their research on the constant recordings of a dozen "strong-motion sensors" deployed throughout the western portion of the delta by USGS seismologist Jon Fletcher.
A quake with a magnitude of at least 6.7 is most likely to strike along the Hayward Fault -- some 40 miles from the delta's center -- within the next 25 years, according to the most recent probability estimates. But Fletcher and Eberhart-Phillips don't need to wait for one that big to gather their crucial data.
Temblors with magnitudes of 3 or even smaller can yield valuable information on the nature of the compacted soils around the delta and how manmade structures -- particularly the delta's crumbling, century-old levees and the powerful Tracy pumps -- will respond during a larger quake.
"There's always the possibility for a very strong amplification of ground motion, depending on the nature of the ground, which ranges anywhere from two to five miles deep in various areas of the delta," says Fletcher, "but there's very little known about the geologic structures beneath."
Experts have predicted that a magnitude 6.7 quake on the Hayward Fault could cause at least 30 breaches in the delta's 1,100 miles of levees and drown 16 of the delta's 70 islands under 300 billion gallons of salt water flooding in from Suisun Bay. If damage immobilized the pumps, freshwater supplies for more than 20 million people downstream to the Los Angeles area would be contaminated, according to geologist Jeffrey Mount, director of the Center for Watershed Sciences at UC-Davis.
Although no known active seismic faults lie in deep bedrock beneath the peaty earth of the 1,600-square-mile delta region, he said, there's a "difference of opinion" among seismologists whether an active fault exists beneath Sherman Island.
The levees around the island developed widespread cracks some 20 years ago, but whether or not they were caused by tiny quakes remains a puzzle. The real earthquake danger comes from the nearby big faults -- particularly the Hayward, he said.
(E-mail David Perlman at dperlman(at)sfchronicle.com.)
(Distributed by Scripps Howard News Service, www.scrippsnews.com.)
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