San Francisco Bay Region Geology and Geologic Hazards

About Quaternary Faults > How Do Geologists Find Quaternary-Active Faults?

How Do Geologists Find Quaternary-Active Faults?

Geologists trace faults by following the characteristic effects that young faults have on the landscape.

Some faults, called creeping faults, move very slowly all the time. Structures such as bridges, sidewalks, and buildings on top of these faults will be offset a small amount each year as the faults move. You can try to find a creeping fault by looking for bent or offset curbs and sidewalks. Not every offset curb is a fault, but if you find several offsets that line up, you may have found a creeping fault.

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This curb in Hayward is being offset by creep on the Hayward Fault. Notice the change between 1974 and 1993.

Most faults don't creep, however, so geologists must look for other ways that faults affect the landscape. Usually the evidence is easiest to spot from the air. For example, natural features such as streams, valleys, and ridges may be offset where they cross faults as movement from many earthquakes is accumulated. Active faults also create their own landscape features. For example, if one side of a fault moves up or down, a straight, low ridge called a scarp is created. As faults accumulate offset, the rock along the fault is broken and ground down, and the resulting shattered zone is more easily eroded than the surrounding rock. This type of erosion produces other common fault-related landforms, such as benches, saddles, and linear valleys along the fault. Faults also can disrupt the movement of underground water, forcing it to the surface to form springs and ponds. Finally, faults can be recognized by the offset they produce in the rocks that underlie the landscape, which can be recognized by careful study.

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This figure shows some of the types of landforms associated with active faults. Examples of fault-related landforms in the San Francisco Bay region.

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Crystal Springs Reservoir fills the linear valley of the San Andreas Fault in San Mateo County.

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A small scarp deforms the fairway of this golf course along the Hayward Fault in Contra Costa County (also notice the trench across the fault).

The newest tool in the effort to find active faults is Laser Imaging Detection And Ranging (LIDAR), which uses laser light projected from an airplane to make a detailed image of the ground surface, even through trees in a forest.

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This figure shows how LIDAR can help reveal active faults. (Left) A regular aerial photograph of an area of trees obscuring part of the San Andreas Fault Zone in Sonoma County. (Center) The same area in a computer rendering of LIDAR data to "virtually" remove the trees and other vegetation. Scarps and other landforms associated with the Quaternary-active fault are now much easier to see. (Right) Fault strands traced onto the LIDAR image.



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Page Last Modified: 8/18/2006