San Francisco Bay Region Geology and Geologic Hazards
About Quaternary Faults > How Do Geologists Know When a Fault Last Broke?
How Do Geologists Know When a Fault Last Broke?
Geologists find the age of prehistoric fault rupture where faults cut young deposits.
Although the evidence in the landscape that a fault leaves behind can guide geologists to Quaternary-active faults, it cannot tell them just when the fault last lurched in an earthquake. In the San Francisco Bay region, geologists have mapped the extent of ground rupture for only three earthquakes: 1868 (Hayward Fault), 1906 (San Andreas Fault), and 1980 (Greenville/Las Positas Faults). Everywhere else, they have to determine the age of the most recent fault rupture in other ways. Earthquakes can offset and disrupt the sedimentary layers that naturally accumulate over time from deposition by rivers, streams, wind, and waves. If a layer is not cut or bent by a fault, then the last earthquake on that fault must have occurred before the layer was deposited.
It commonly is difficult to determine if layers are cut or bent by looking at the ground surface because the surface usually is smoothed out soon after an earthquake by rain, wind, and animal activity. To get a clearer view of young layers, geologists dig trenches across faults where they know that young deposits have accumulated. In some places, the subsurface view of the fault and the young layers reveals distinct evidence of one or more past earthquakes. Trench studies also give geologists a better chance to find the fossils and bits of carbon (for radiometric dating) that might tell them how old the layers of young deposits are.