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Earthquake
Earthquake faults
Map showing the major faults in the Bay Area, the region is also traversed by at
least four major slip-strike fault systems with hundreds of related faults many
of which are "sister faults" of the infamous San Andreas Fault, all of which are
stressed by the relative motion between the Pacific Plate and the North American
Plate or by compressive stresses between these plates. Significant blind thrust
faults (faults with near vertical motion and no surface ruptures are associated
with portions of the Santa Cruz Mountains and the northern reaches of the Diablo
Range and Mount Diablo.
The region is particularly exposed to hazards associated with large earthquakes,
owing to a combination of factors:
Numerous major active faults in the region.
A combined thirty year probability of a major earthquake in excess of seventy
percent.
Poorly responding native soil conditions in many places near the bay and in
inland valleys, soils which amplify shaking as shown in the map to the right.
Large areas of filled marshlands and bay muds that are significantly urbanized,
with most subject to liquefaction, becoming unable to support structures.
A large inventory of older buildings, many of which are expected to perform
poorly in a major earthquake.
Extensive building in areas subject to landslide, mudslide, and in some
locations directly over active fault surface rubble zones.
Most lowrise construction is not fireproof and water systems are likely to be
extensively damaged and so large areas are subject to destruction by fire after
a large earthquake.
The coastal location makes the region vulnerable to Pacific Ocean tsunamis.
Some of these hazards are being addressed by seismic retrofitting, education in
household seismic safety, and even complete replacement of major structures such
as the eastern span of the San Francisco-Oakland Bay Bridge.
For an article concerning a typical fault in the region and its associated
hazards see Hayward Fault Zone.
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