A stream channel offset by the San Andreas fault, Carrizo Plain, central California (photo by Robert E. Wallace)

Earthquake geologists study the surface expression of earthquake faults and ground failure caused by strong ground shaking. An important facet of earthquake geology is paleoseismology, the study of the timing, location, and size of prehistoric earthquakes. Paleoseismologists typically excavate trenches across known active faults to expose and examine the evidence left by large, surface-rupturing earthquakes. On any given fault, the interval between large damaging earthquakes is typically hundreds to thousands of years, usually much longer than the historical record of earthquakes. Knowledge of when, where, how often, and with what magnitude large earthquakes occur is crucial for understanding and characterizing the seismic hazard of a region. The amount that geologic features are offset across a fault can provide an important gauge of the rate of fault movement, and thus of earthquake activity, as averaged over multiple earthquake cycles. Ground deformation phenomena, such as tectonic subsidence and uplift, liquefaction, and shaking-induced landslides, can be used to identify earthquakes in the geologic record and to evaluate the scope of the earthquake hazard faced by society.

Quaternary-Active Fault Map
Shows the distribution of faults in the San Francisco Bay region most likely to generate future earthquakes, with information about active faults and how geologists find them and study them.

The San Andreas Fault
Good general introduction to the fault, its geologic setting and the earthquakes on it, with nice photos. By Sandra S. Schulz and Robert E. Wallace.

The Hayward Fault

Virtual Helicopter Tour of the fault
Take a 3D tour above the Hayward fault using Google Earthª mapping service to view the fault traces and various evidence for activity. The fault mapping is based on a newly released USGS digital database now available online. Other interactive map viewers are also available for older computers.

Photographic Tour of the Hayward fault
A visual introduction to the kinds of fault creep evidence seen along the Hayward fault, by Dr. Sue Hirschfield, California State University, Hayward.

Paleoseismology, Hayward Fault, California
Join USGS and Japanese scientists in a recent 3-day field effort to uncover traces of ancient earthquakes on the Hayward fault.

Timing of Paleoearthquakes on the Northern Hayward Fault
Scientists excavated two trenches along the northern Hayward fault to help constrain the timing of paleoearthquakes for the 1999 Bay Area earthquake probability study.

Portfolio of Landscape, Seascape and Faults of the San Francisco Bay Area
Developed by Robert E. Crippen (JPL) and Ross Stein (USGS), these images drape Landsat TM satellite imagery over Digital Elevation Models to show the bay as it has never been seen before.

San Francisco Bay Region Geologic Map
Shows the distribution of geologic materials and geologic structures that are visible at the Earth's surface, with information about geologic materials, geologic time, fossils, and the geologic history of the San Francisco Bay region.

USGS Scientific Expedition: Earthquake in Turkey - 1999
Scientists and engineers from the U.S. Geological Survey departed for the epicentral region on August 19, 1999, less than 2 days after the quake occurred, to join in an international scientific effort that will deploy seismic and geodetic instruments, make geologic field measurements and assess and record the structural damage.

1998 USGS Expedition to Mongolia
In September 1998, USGS and other scientists investigated surface ruptures associated with the 1905 Bulnay earthquakes. During the field expedition, they sent updates of their observations and progress in near real-time. These reports summarize their geologic findings and approaches, along with their experiences and observations from Mongolia (along with a few photographs, too).

Paleoseismic investigations of the Gurvan Bulag thrust fault in Mongolia
Scientists studied a total of five exposures (hand-dug trenches and cleaned stream cuts) along the main Gurvan Bulag thrust, and one exposure across a subsidiary fault.

Faulting in the August 17, 1999 Izmut, Turkey Earthquake (Mw=7.4)

• Set 1
• Set 2

Faulting in the September 21, 1999 Taiwan Earthquake

Faulting in the October 16, 1999 Hector Mine, California Earthquake (M=7.1)