The Hayward fault is part of the complex plate boundary system in central California and is a major branch of the San Andreas Fault System. Near Hollister, the Calaveras fault branches off from the San Andreas fault toward a more northerly direction, and further north, the Hayward fault branches off from the Calaveras toward the northwest. At a finer scale, numerous thrust faults parallel and cross the San Andreas fault in the greater San Francisco Bay area.
In the figure above (plotted in an oblique Mercator projection), the fault segments which have ruptured in historic time are denoted in red. Holocene ruptures (within the past 10,000 years) are drawn in orange and Quaternery ruptures (within the past 1.5-2 million years) are drawn in black. The faults are drawn with solid lines where known and dashed and dotted lines where inferred.
The Hayward fault ruptured in 1868 and is a prime candidate in Northern California for a magnitude 7 earthquake within the next 30 years. This constitutes a significant seismic hazard for the 6-7 million people at risk in the area and is the primary motivation for the installation of the seismometers, GPS receivers, creepmeters, and strainmeters in the San Francisco Bay Area. Disaster planners and emergency response managers are using earthquake scenarios for the Hayward fault to help prepare for future events. And many cities, counties, and companies are working to retrofit facilities in anticipation of the Next Big One!
