INFORMATION
AND EXPLANATION
OF UBC*
SEISMIC ZONES
(* Universal Building Code)
The
seismic zone factor (or Z factor) corresponds numerically to the effective
horizontal peak bedrock acceleration (or equivalent velocity) that is
estimated as a component of the design base shear calculation.
For instance, the area within seismic Zone 1 (Z-factor of 0.1)
should expect an earthquake-related effective peak bedrock acceleration of
0.1 times the force of gravity. These
values correspond to ground motion values with a 10 percent probability of
being exceeded in 50 years. Seismic Zone 2 is subdivided into two regions.
Seismic Zone 2A has a Z-factor of 0.15 and is not
associated with a particular fault zone: Seismic Zone 2B (not in
this mapping area) has a Z factor of 0.20 and indicates an association with known crustal
faults.
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UBC MAPS HAVE BEEN
SUPERSEDED FOR MOST USES
Building code maps using
numbered zones, 0, 1, 2, 3, 4, are practically obsolete. 1969 was the
last year such a map was put out by this staff. The 1997 Uniform
Building Code (UBC) (published in California) is the only building
code that still uses such zones. Generally, over the past two decades,
building codes have replaced maps having numbered zones with maps
showing contours of design ground motion. These maps, produced by the
U.S. Geological Survey, show earthquake ground motions that have a
specified probability of being exceeded in 50 years. These ground
motion values are used for reference in construction design for
earthquake resistance. The ground motion values can also be used to
assess relative hazard between sites, when making economic and safety
decisions.
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