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Scientists study activity of Kern Canyon Fault under Isabella dams
| Saturday, Nov 25 2006 8:15 PM
Last Updated: Saturday, Nov 25 2006 8:19 PM
Elisabeth Nadin was a Cal Tech graduate student in geology about six years ago when she began studying the Kern Canyon Fault for her doctoral degree.
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Elisabeth Nadin spent several summers the Sierra back country, camping and hiking and studying the rocks for her doctoral thesis on the Kern Canyon Fault.
By then, a growing number of scientists had acquired nagging doubts about the official conclusion that the fault, which runs directly under Lake Isabella, was inactive.
Nadin's research helped government scientists and dam safety officials reach a consensus last year that the potential for an earthquake on the fault is a serious threat to the two Isabella dams that protect Bakersfield -- and the nearby town of Lake Isabella -- from flooding by the Kern River.
It also helped write a dramatic new chapter in the geologic record of the Southern Sierra Nevada, proving that the mountains in east Kern are slowly stretching apart from east to west. That was something scientists did not think possible until recently and had only begun to speculate about.
But earthquakes are not the only threat to the dams. The other is a worrisome discovery that seepage of water under one of the dams is heavier than it should be, an indication that the soil below may not be stable enough to hold the water when the reservoir is full, especially it it's shaken by an earthquake.
With Isabella suddenly labeled the No. 1 safety concern among dams operated by the U.S. Army Corps of Engineers, the Corps has launched a major series of studies that could eventually result in a repair program at a cost of hundreds of millions of dollars.
Officials hasten to add there is no reason for people who live and work below Isabella to panic.
"The good news is that while we think the Kern Canyon Fault is active, it probably is not a very active fault," said Ronn Rose, a Corps of Engineers geologist and dam safety program manager.
Until the safety worries became public this year, few Kern County residents even knew the Kern Canyon Fault existed.
Everyone knows about the San Andreas Fault, the boundary between two tectonic plates of the earth's surface that runs along the county's western boundary and is capable of devastating damage. And the worst earthquake damage in the county's modern history was caused by the 1952 magnitude 7.5 quake on the White Wolf Fault south of Arvin.
But scientists know the 93-mile-long Kern Canyon Fault well. It is the longest fault in the Southern Sierra, a major rift in the earth's crust that has been relieving the stress of the region with countless earthquakes as it reacted to the pushing and shoving of tectonic plates for at least the last 100 million years.
By contrast, Nadin said the San Andreas Fault in Southern California is only about 5 million years old.
Nadin was encouraged to study the Kern Canyon Fault for her doctoral thesis by her faculty adviser, Jason Saleeby. He had been studying the Southern Sierra for decades, and was one of a growing number of scientists, including Rose, who felt the fault was not an extinct geologic dinosaur.
That was the conclusion reached chiefly by Bob Webb, another Cal Tech geologist, in 1936.
Rose said Webb examined an ancient lava flow across the fault about 30 miles north of the dam site and decided it had not been ruptured by an earthquake in the last 3.5 million years.
Nadin's primary scientific interest was the ancient origin of the fault, 100 million years ago or more.
"But I had told her to be alert for more recent activity," Saleeby said.
Saleeby and other scientists had seen what they thought were flat planes, or small cliffs, of exposed bedrock in many places on the west side of the fault line, with crumbled rock debris piled at the bottom of the little cliffs, known as scarps.
That is a telltale sign of earthquakes caused when one side of a fault -- the west side in this case -- rises upward and the other side slips down. It is the result of the earth's crust pulling apart. That is known to scientists as "normal" faulting. It is different from the better known "strike slip" faulting of the San Andreas and most other faults in California, where two pieces of the crust slide past each other horizontally.
Nadin eventually concluded that the Kern Canyon Fault has had several periods of activity and inactivity in its many centuries of existence, and is currently in an active stage. While the 5,000 years or so since the last known earthquake is a long time for humans, it is a tiny blip in geologic time.
There is disagreement among scientists about why the earth is pulling apart in the Southern Sierra, but there is no longer much question that it is happening. They are familiar with such stretching in vast parts of the Western United States, including eastern California, but they were surprised to find it in the Sierra.
An early paper on the fault prepared for an American Geophysical Union conference a couple of years ago caught Rose's attention and he contacted Saleeby and Nadin. They began to compare notes.
By late last summer, concern over the safety of the dams had grown to the point that the Corps of Engineers had hired consultants to begin serious studies.
Despite Nadin's research, the agency didn't have a smoking gun to prove the fault was active. That came quickly and dramatically.
In September of last year, Rose convinced his superiors to fund a helicopter trip for himself and the consultants to survey the whole fault visually.
They invited Nadin to accompany them, a major coup for a graduate student. The most important stop was at the lava flow, Rose said.
"We took the helicopter in and hiked in and, sure enough, it's faulted," he said.
Saleeby said later he believes the faulting was missed by Webb and other scientists earlier because they were looking for strike-slip motion on the fault and did not recognize signs of west-up movement in the often-rubbly lava flow.
Rose and others said the dam can be strengthened to withstand the earthquake and seepage threats. They also said knowledge that the earthquake fault is active probably wouldn't even prevent its construction in the same place today.
"As a geologist," Nadin said, "you see a dam there and you think that's really stupid because there's an earthquake fault. But you see that all over this country. Lots of dams are built on earthquake faults."