Earthquake Geology and Paleoseismology

Science Center Objects

USGS scientists study active fault zones by mapping faults, excavating trenches, studying landforms offset by earthquakes, and measuring past and current motion of active faults using alignment arrays, global positioning systems (GPS), and airborne, terrestrial and mobile laser scanning technology.

By excavating trenches across active faults, USGS geologists and collaborators are unraveling the history of earthquakes on specific faults.

Overview  |  Earthquake Geology and Paleoseismology  |  Tectonic Geomorphology and Near-Field Geodesy

Earthquake Response  |  Salton Seismic Imaging  |  Special Earthquakes, Earthquake Sequences, and Fault Zones

 

Earthquake Geology

USGS scientists study active fault zones by mapping faults, excavating trenches in fault zones, describing and dating sedimentary layers affected by earthquakes, mapping and dating landforms offset by earthquakes, and measuring past and current motion of active faults using alignment arrays, global positioning systems (GPS), and airborne, terrestrial and mobile laser scanning technology. The USGS works in active tectonic areas around the world and provides scientific response to damaging earthquakes.

The greater San Francisco Bay region is an active tectonic zone that sits at the boundary of two tectonic plates. The generally northward motion of the Pacific Plate relative to the North American Plate is accommodated across a large number of active faults, which are prone to damaging earthquakes. In order to better understand the hazard posed by these faults, USGS scientists document the history of earthquakes on the faults, the rate at which the opposite sides of the faults are slipping past one another, and the manner in which energy of plate motion is released on a given fault.

Most faults store energy that may be released during potentially damaging earthquakes. Some faults, however, slip nearly constantly, releasing energy slowly as “creep,” and some faults exhibit a complex mix of “locked” and creeping behavior. Further, some faults are known to be short and segmented and therefore produce only small to moderate earthquakes, while others are long and continuous or may participate in earthquakes with nearby faults, thereby generating very large earthquakes. For example, the magnitude 7.9 San Francisco Earthquake in 1906 ruptured almost 300 miles along the San Andreas Fault.

Understanding a fault’s slip behavior, as well as its length and connectivity, is important for constraining the magnitude range and frequency of earthquakes that a particular fault is likely to produce. Some faults that pose significant earthquake hazard may not have a clear expression on the Earth’s surface, but may have vertical motion that over time leads to creation of mountains and valleys.

Paleoseismology

Locations at Which Paleoseismological Studies Have Been Completed Along Principal Faults in the San Francisco Bay Area

Locations at which paleoseismological studies have been completed along principal faults in the San Francisco Bay Area. (Public domain.)

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.