Map depicting fault lines
Location of active faults in the greater San Francisco Bay Area, California.

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.