The Hayward Fault passes through Santa Clara, Alameda, Contra Costa and Sonoma Counties and is considered one of the most active faults in the Bay area. The Hayward Fault is actually a branch of the San Andreas fault system. The Hayward branches off the Calaveras Fault (also part of the San Andreas system) near the City of San Jose. From San Jose, the Hayward trace runs near the base of the East Bay foothills from Milpitas through Hayward, Berkeley and offshore at San Pablo Bay.

The Hayward fault has been divided into a series of segments by seismologists. The earthquake risk in the East Bay is enhanced because each of these segments may produce earthquakes of Magnitude 7 or greater. The northern segment of the fault begins near San Leandro and ends in San Pablo Bay. The southern active segment begins near Milpitas and ends near San Leandro. The Rodgers Creek fault is also a major segment of the Hayward fault system. It extends from about San Pablo Bay to at least as far north as Santa Rosa. The Rodgers Creek fault has had no known historic fault rupture but has long been recognized as being active based on subsurface data collected from trenches dug across the fault.

Two major historic earthquakes, one in 1836 and the other in 1868, have been attributed to the Hayward fault. Both are thought to have had magnitudes of about 7.0. The 1836 quake is presumed to have had an epicenter on the northern segment of the fault near Berkeley. The 1868 quake is believed to have had an epicenter on the southern segment of the fault near Fremont.

Information on those early Hayward fault earthquakes is dominantly based on how "intense" the earthquakes were felt to be by the inhabitants of the time and on newspaper accounts. We don't have any seismograph readings for these earthquakes. Seismic recording didn't begin in California until 1887 when California's first seismograph was installed in Berkeley. There were some scientific measurements taken of the ground cracking that occurred in the 1868 earthquake; but, unfortunately, most of these records were apparently destroyed for fear that they might ruin the area's reputation!

Certain portions of the Hayward fault exhibit movement called "fault creep." Fault creep is when the two sides of a fault gradually and steadily move past each other. This generally occurs without accompanying earthquakes. Seismologists have measured the amount of creep and found it varies between 0.2 to 0.4 inches per year at certain points along the fault trace. Over the years, this steady movement gradually bends curbs and cracks buildings constructed directly on top of the "creeping" fault. Some geologists believe that the creeping motion serves to increase strain on the remaining portions of the fault that aren't moving, thereby increasing the chance of an earthquake. Others argue that the creeping motion tends to relieve overall strain and increase the time between earthquakes.

Earthquake Forecasting on the Hayward fault