Uneven Slip On Hayward Fault Provides Mystery In California

May 28, 1997

Editors: Bilham's poster will be presented May 28 at 1:30 p.m. in Hall E of the Baltimore Convention Center at the spring meeting of the American Geophysical Union. Bilham will be available to answer questions beginning at about 2:30 p.m.

Scientists don't know what is going on beneath Fremont, Calif., and finding the answer could be helpful to tens of thousands of people.

For 130 years, the southern end of the Hayward Fault in Fremont has moved at double the rate of the fault's northern portion, with the west side of the fault moving northwest relative to the east side of the fault.

This higher rate of movement in the southern end suggests that rocks are being "piled up" on the west side of the fault and stretched to the breaking point on the east side, said Roger Bilham, a professor of geological sciences at the University of Colorado at Boulder. On the east side of the fault near Fremont, for example, the stretching has created a topographic "dip," causing a pond to form.

This fault movement should have resulted in an earthquake -- nature's version of stretching a rubber band until it snaps -- but it hasn't happened.

"As a result of the abrupt change in slip rate along the fault, the cumulative strain on each side of the fault is 200 microstrain, a value that typically leads to an earthquake," said Bilham, who has operated instruments in the area for the past four years. "Either an earthquake is overdue or we're missing an important understanding of the mechanics of the fault."

The answer is significant because the Hayward Fault runs along the east side of San Francisco Bay for some 50 miles near an urban population of 2.5 million. A quake estimated at magnitude 7.0 occurred on the fault in 1868.

Bilham presented his findings May 28 at the spring meeting of the American Geophysical Union in Baltimore.

Since the 1868 quake, the northern 80 percent of the Hayward Fault has moved at a rate of about one-fifth inch per year, Bilham said. But the southern end in Fremont has moved about one-third inch per year, a rate almost double that of the remainder of the fault.

When movement on the southern end of the fault is calculated over the 130 years since the last big quake shook the region, the amount of geologic strain is impressive -- perhaps enough for a magnitude 6 earthquake to occur in the near future, he said. The U.S. Geological Survey has estimated a 23 percent to 28 percent probability of a magnitude 7 quake in the area in the next 30 years.

Understanding fault mechanics in an urban area is difficult unless deformation is so concentrated that it causes "visible offsets" to sidewalks and buildings, Bilham said. The area where movement rapidly increases along the fault occurs along a three-quarter mile section in south Fremont.

"If these observations are accurate this means that a lot of elastic energy is stored in a very small region, and we might expect an earthquake soon," Bilham said. "We're not sure whether the strain is concentrated beneath part of Fremont or spread over a wider area."

If the strain is stored more widely, the measurements would support the approach of the magnitude 7 earthquake that has been forecast, he said. Further research is needed to find out the volume of rock involved.

Bilham's work is funded by the U.S. Geological Survey.

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University of Colorado at Boulder

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