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< UBC Zones

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.

 

UBC MAPS HAVE BEEN SUPERSEDED FOR MOST USES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.