Utah scientific: Age of fault probed by geologists

Published: Monday, Nov. 6 2006 12:00 a.m. MST

A fault zone about 20 miles west of California's infamous San Andreas Fault may be younger and moving faster than earlier believed, according to research in which a Utah State University geologist is participating.

The latest study on the subject is "Stratigraphic Record of Pleistocene Faulting and Basin Evolution in the Borrego Badlands, San Jacinto Fault Zone, Southern California," published in the November issue of the Geological Society of America Bulletin.

Authors are Andrew T. Lutz and Rebecca J. Dorsey of the University of Oregon, Eugene; Bernard A. Housen of Western Washington University, Bellingham, and Susanne U. Janecke, USU, Logan.

The paper was based on a master's thesis by Lutz, said Janecke, associate professor in the university's geology department.

New dating of the arrival of gravel material in the Borrego Badlands showed the material flowed in a little before 1 million years ago. Meanwhile, another study by Janecke and S.M. Kirby found that 30 miles to the east, about the same time, "coarse sediment comes into this basin."

She added, "Then all of a sudden, just a little bit before a million years ago, the whole area was flooded by sand and gravel. And up to half a kilometer (about 0.3 mile) of sand and gravel accumulated after that." The new sediments were not three-tenths of a mile wide, but that deep.

Another study dating to 1991 came up with the same date. "Three places, widely separated," had a "big, abrupt change," she said.

The dating was possible because of a change in Earth's magnetic field, with the fluctuations preserved in the alignment of magnetic particles in the sediments. "We were lucky in being able to very precisely date the first arrival of the coarse sand and gravel," Janecke said.

The top of the gravel was about 600,000 years old, the bottom a little over 1 million years old.

Earlier work estimated the age of this San Jacinto fault zone at about 2 million or 2.5 million years. Another study from 1993 estimated the northeast end of the San Jacinto fault zone was 1.2 million to 1.5 million years old, and "that was not accepted by our whole (geologist) community," she said.

"There was still a number of geologists who suspected the fault had initiated earlier."

But the new indications are that the fault is even newer.

The age of a fault plays a role in estimating how fast the ground continues to move in the region. If it has been moving a long time, it may be a slow slip rate; if for a shorter time, the fault may be moving faster.

An earlier estimate of the slip rates was "significantly slower" than what the new research may indicate. In fact, recent studies using Global Positioning System satellite data also apparently show the slip "is quite rapid."

That's not to say it's always moved at the same rate. "Maybe the fault was faster at some times and slower at some times," she said. "We don't know. Or maybe it had a steady state."

It's important to resolve the question because the slip rate will give a better idea of when to expect future earthquakes, she said.

The new study will help scientists understand how faults work in the earthquake cycle, Janecke said. "We need more and more data" to interpret the findings.


E-mail: bau@desnews.com

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