map depicting paleoseismology sites
Locations at which paleoseismological studies have been completed along principal faults in the San Francisco Bay Area.

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.

graphic depicting Hayward Fault earthquakes
History of ground-rupturing earthquakes on the Hayward fault, as determined from excavation and analysis of sediments affected by repeated earthquakes.
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The San Andreas Fault exposed as a complex pattern of deformed sediments on the floor of a paleoseismological trench.
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Interpretation of deformed sedimentary layers affected by past earthquakes on the San Andreas Fault.
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Three-dimensional perspective model of the Triangle G Ranch site looking southeast along the trend of the Rodgers Creek fault (RCF, in red). Trenches outlined in dark gray show evidence of the most recent earthquake; trench wall marked with blue bar is shown in accompanying trench log. Pressure ridge is area between strands of the fault uplifted during earthquakes.
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Trench log (a) and photograph (b) of exposure on the northeast side of the pressure ridge (trench 3, southeast wall, in perspective view). Faults, in red; ground surface at the time of the most recent earthquake, in gold. Labeled triangles on trench log mark locations of charcoal samples collected for radiocarbon analysis. These radiocarbon dates indicate that the most recent earthquake occurred after A.D. 1690 AD and most likely after A.D. 1715. The absence of accounts of a large, damaging earthquake in the early historical record implies that this earthquake occurred before Mission Dolores was founded in San Francisco in A.D. 1776.