Seismic Zones are a vestige of the Uniform Building Code or UBC and were introduced in the 1949 edition when the USA seismic hazard map, published in 1948 by the US Coast and Geodetic Survey was adopted into the UBC. The first edition of the U.S. Uniform Building Code (UBC); however, was published in 1927 by the Pacific Coast Building Officials (PCBO), contained an optional seismic appendix, that used a coefficient C’ which ranged from 7.5% to 10% g (% of gravity applied as a lateral inertial load). The 1949 maps were intended to be a better representation of the potential seismic accelerations of various regions or Zones throughout the US and ranged from 0 to 3. The Zone number correlated to a level of acceleration expressed as a % of gravity or g.
The maps were intended to represent the likely levels of earthquake ground shaking and, therefore, the potential for structural damage. The maps evolved over time including the addition of a Seismic Zone 4 in 1976 and the division of Zone 2 into 2A and 2B in 1988. The ground accelerations associated with the Zones were probabilistic based and correlated to prescribed levels of ground accelerations with Zone 4 being the highest and 0 being negligible. However, they were also somewhat political in that the Zones in many cases follow the state boundaries and in California ultimately the whole state was defined as either Zone 3 or 4 so that they didn’t have any Zone 2 areas within the states jurisdiction. Of course the reality is the potential intensity of seismic ground shaking does not recognize state boundaries and the jurisdictional convenience of Seismic Zones evolved as the US Geodetic Survey (USGS) developed a new generation of seismic maps.