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.
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.
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.