Introduction
The Hayward is one of three major fault zones of the San Andreas system that have
produced large historic earthquakes in the San Francisco Bay Area (the others being the
San Andreas and Calaveras). Severe earthquakes were generated by this fault zone in 1836
and in 1868, and several large earthquakes have been recorded since 1868. The Hayward
fault zone is considered to be the most probable source of a major earthquake in the San
Francisco Bay Area, as much as 28% chance for a magnitude 7 earthquake before the year
2021 (Working Group on California Earthquake Probabilities, 1990).
The Hayward fault zone, as described in this work, is a zone of highly deformed
rocks, trending north 30 degrees west and ranging in width from about 2 to 10 kilometers.
The historic earthquake generating activity has been concentrated in the western portion of
the zone, but the zone as a whole reflects deformation derived from oblique right-lateral and
compressive tectonic stress along a significant upper crustal discontinuity for the past 10
million or more years.
The Hayward fault zone is bounded on the east by a series of faults that demarcate
the beginning of one or more structural blocks containing rocks and structures unrelated to
the Hayward fault zone. The eastern bounding faults are, from the south, the Calaveras,
Stonybrook, Palomares, Miller Creek, and Moraga faults. These faults are not considered
to be part of the Hayward fault zone, although they are shown on the map to demarcate its
boundary. The western boundary of the zone is less clearly defined, because the alluvium
of the San Francisco Bay and Santa Clara Valley basins obscures bedrock and structural
relationships. Although several of the westernmost faults in the zone clearly project under
or through the alluvium, the western boundary of the fault is generally considered to be the
westernmost mapped fault, which corresponds more or less with the margin of thick
unconsolidated surficial deposits. The Hayward fault zone is truncated to the south by the
Calaveras fault, which trends about north 10 west, and so forms an oblique east and south
boundary. All of the faults within the southern part of the zone probably splay into the
Calaveras fault. The northern margin of the zone as dealt with herein is San Pablo Bay, but
the zone of deformation undoubtedly continues north of the Bay through the area bounded
by the Rodgers Creek and Tolay faults.
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First posted September 19, 2002
Database files
Geologic explanation pamphlet
Geologic Maps
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