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Scientists: Fault still active, but threat is minimal
| Tuesday, Feb 27 2007 10:10 PM
Last Updated: Tuesday, Feb 27 2007 10:16 PM
SACRAMENTO -- The earthquake fault that runs beneath Isabella Lake has produced at least one earthquake as recently as 5,000 years ago and must be considered an active fault, scientists have concluded.
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Handout photograph shows a view to south into Hot Springs Valley along Kern Canyon fault with Auxiliary Dam in forground. Engineer's Point is in the middle foreground. Arrows indicate location of fault.
Jim Vietti, of Bakersfield, fishes for trout at Isabella Lake near the Auxiliary Dam.
Robert Rasey, of Bodfish, walks his dog Ginger on a dirt road along the Auxiliary Dam in Lake Isabella.
The study was commissioned by the U.S. Army Corps of Engineers last year in the wake of growing concerns about seepage under the lake's auxiliary dam and increasing doubts that the Kern Canyon Fault was as inactive as officials believed when the dam was built.
The study made no predictions about when or whether an earthquake could damage the dam or make it collapse, but officials have said they do not believe the threat is serious or imminent.
The completed study, conducted by consultants, was given to the Corps Feb. 13, according to an update on the dam studies. However, officials in the Corps' regional office in Sacramento refused to release it to The Californian without a Freedom of Information Act request, even though it is not rated as classified information.
Jeff Hawk, spokesman for the office, said no one, even local government officials in Kern County, could have access to the report without such a request, which is usually required for more sensitive legal or military documents.
The report contained no real surprises.
Ronn Rose, the geologist in charge of a series of investigations about the safety of the lake's two dams, and other scientists have said repeatedly they believe the fault is active based on evidence they can see on the ground and from aircraft.
The study, which focused heavily on the Rincon section of the fault, which is the portion of the 88-mile long fault that runs beneath the lake, confirms that.
"These findings suggest that the Rincon section of the Kern Canyon Fault has had movement within the past approximately 5,000 to 15,000 years," the report concluded.
The study also gave scientists a better picture of the amount of movement by the two sides of the fault, the slip rate that can build up in periods of inactivity and break loose in an earthquake, Rose said later.
"It appears that this fault has a higher slip rate than we first thought, but it should still be considered a low slip rate fault," he said. "It doesn't generate earthquakes that often, but it generates them more frequently than we thought."
The study is the first of what may be three phases of seismic studies that officials need to do before they will know what they need to know about how to strengthen the dams.
That is a vastly different conclusion than the one reached by scientists who studied the fault in the 1930s, before the dam was built.
Based on the appearance of an ancient lava flow north of the lake, the official conclusion then was that the fault had not ruptured since the lava oozed out onto the ground 3.5 million years ago.
But over the last two years, scientists re-examined the lava flow and decided it indeed had been broken by an earthquake much more recently.
The new study and others have shown that the Kern Canyon Fault is not a strike-slip fault like the San Andreas Fault, where the two sides of massive tectonic plates slip past each other horizontally.
In earthquakes on the Kern Canyon Fault, the west side of the fault rises upward and the east side slips downward.