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SACRAMENTO – Fourteen
new or revised Earthquake Fault Zone
maps that direct construction away from
areas where earthquake faults have
broken through to the ground’s surface
were released today by the California
Department of Conservation.
The maps affect
Riverside, San Bernardino and San Diego
counties and the cities of San Diego,
Coronado, Corona and Chino Hills.
The first new
“Alquist-Priolo” maps in three years
from DOC’s California Geological Survey
are screening tools. They determine
whether extra care should be taken to
ensure new construction does not take
place on top of active faults, those
that have displaced the ground surface
in the last 11,000 years. Locating new
buildings away from active faults can
result in fewer injuries and lower
repair costs in the event of an
earthquake. Surface rupture generally is
the most easily avoidable seismic
hazard.
The maps primarily
affect developments of four or more
homes or commercial building projects.
The maps are scheduled to become
official on May 1, 2003, following
public review. By state law, local
planners and developers must refer to
the official maps before beginning new
construction. The law also requires
sellers and real estate agents to inform
buyers whether real property being sold
is within an Earthquake Fault Zone.
The maps show
regulatory earthquake fault zones about
1,000 feet wide. If a proposed
development is within that zone, the
developer must perform a geologic
investigation, typically by hiring a
geologist registered in California, to
determine whether the construction
project area is underlain by active
earthquake faults. If an active fault is
found, new buildings must be set back
from the fault. Setback widths generally
are 50 feet from either side of the
fault.
The Point Loma
quadrangle, including downtown San
Diego, was initially zoned in 1991, but
recent investigations both onshore and
offshore have revealed additional active
faults near the southern end of the Rose
Canyon Fault Zone. The new zoning could
impact new and infill construction in
San Diego. New and revised zoning also
impacts the cities of Chino Hills,
Corona and Coronado. There is a great
deal of new development in the Chino
Hills area, near the Chino Fault, and
Alquist-Priolo zoning there should be
timely in preventing structures from
being built across active faults.
Most of the new
mapping is for sparsely populated areas
affected by the 1999 magnitude 7.1
Hector Mine earthquake, which caused
observable surface displacement
extending for a distance of 30 miles.
The earthquake caused some ground to
move as much as 18 feet relative to its
former position.
The Alquist-Priolo
Earthquake Fault Zoning Act was passed
in 1972, a direct result of the 1971 San
Fernando Earthquake, in which surface
fault rupture damaged numerous homes,
commercial buildings, and other
structures.
There have been 27
earthquakes associated with surface
faulting in California since the first
Alquist-Priolo maps were issued in 1974.
The last six out of seven earthquakes in
California that generated ground rupture
occurred mainly within established AP
zones.
In the past 15 years
there have been several earthquakes
generated by movement along blind thrust
faults, which can’t be seen at the
Earth’s surface. These faults generally
lie at least 3 miles below the Earth’s
surface. The 1994 Northridge Earthquake
caused major damage in the Los Angeles
and San Fernando Valley areas, but the
blind thrust fault that produced the
earthquake did not reach the ground
surface.
Copies of the
Preliminary Review Maps may be examined
at the offices of affected cities and
counties, and at the California
Geological Survey office in Los Angeles,
(213) 239-0878. The maps also can be
viewed at
www.conservation.ca.gov/cgs. Copies
may be purchased from BPS Reprographic
Services, 149 Second Street, San
Francisco, California, 94105, (415)
512-6550.
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