Residents of the San Francisco Bay Area have reason to fear "the big one" because the area is home to a variety of fault lines. Now researchers have reason to believe another earthquake hazard should be added to the map: Results presented today at the annual meeting of the American Geophysical Union in San Francisco suggest that the earth beneath Marin County may contain a hidden fault line.

The known faults in the San Francisco area include the San Andreas, the Hayward, the Rodgers Creek and the San Gregorio. Measurements of so-called slip rates on these faults, which range from three to 24 millimeters a year, are inconsistent. The movement of the northern San Andreas line and that of the southern portion are off by as much as four millimeters annually. "In the past, the thought was that there must be something wrong with the calculations on the San Gregorio fault," remarks study leader Kevin P. Furlong of Penn State University. Instead, Furlong and his colleagues hypothesized that there could be another fault whose movement would explain the measured discrepancies. "We think we have evidence that there is an additional earthquake hazard in the San Francisco area due to a blind thrust fault," he explains. "Blind thrust faults are notorious because they are hard to find until an earthquake occurs on them."

Upward movement of the area's Mt. Tamalpais could provide indirect evidence of the thrust fault but the uplift is too small to be measured accurately with current Global Positioning System (GPS) measurements and there are not enough historical data for comparison. Instead, the scientists calculated the steepness of rivers on the nearby Bolinas Ridge and determined that their gradually rising slopes were consistent with the uplift of a blind fault. If their calculations are correct and the thrust fault's movement accounts for the entire discrepancy between the other faults' rate of movement, the researchers calculate that it could give rise to an earthquake of magnitude 6 to 6.5 on a timescale of several hundred years. The team is planning experiments to find direct evidence of the fault, but standard seismic imaging does not work on the type of rock in the area, so alternative methods must be employed.

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