Paleoseismology in the San Francisco Bay Area and Beyond
USGS geologists study active faults in California and beyond. Recent investigations conducted by USGS geologists include studying the Denali-Totschunda Fault in Alaska, the Bear River Fault in Wyoming and Utah, and a wide range of international research projects.
By excavating trenches across active faults, USGS geologists and collaborators are unraveling the history of earthquakes on specific faults. Damaging earthquakes often rupture along a fault up to the ground surface, and, in doing so, offset layered sediments that were deposited by water, wind and down-slope movement. Following an earthquake, new sediment may be deposited across the disturbed land, creating a new undisturbed horizon that is younger than the earthquake.
Geologists use radiocarbon dating and other methods to learn the age of pre-existing layers affected by ancient earthquakes as well as the new layers deposited after the earthquakes, and, by doing so, constrain a fault’s earthquake history. These methods work best at sites on faults that lie near streams, slopes, ponds and other areas that have frequent sediment deposition.
Scientists have successfully pieced together the history of earthquakes over the past several hundred to a few thousand years on many active faults. These histories provide insight into the possibility of future damaging earthquakes. Some faults, such as the Hayward fault in the East Bay, have produced large earthquakes at fairly regular intervals over the past few thousand years. The Hayward fault in particular is thought to be ready for the next damaging earthquake, based on our understanding of the history of past earthquakes exposed by paleoseismic trenching.