Earthquakes and Planning for Ground Rupture Hazards

Detailed maps bring a greater resolution to the number and locations of active
faults. Preparing maps at a higher resolution requires extensive field study,
and with a GIS, information, such as tract and parcel data, utility corridors,
and flood hazard zones, can be incorporated to help decision makers in locating
remediation facilities.

After the Sylmar earthquake in 1972, building codes were strengthened, and the
Alquist-Priolo Special Studies Zone Act was passed. Its purpose is to mitigate
the hazard of fault rupture by prohibiting the location of most human occupancy
structures across the traces of active faults. Earthquake fault zones are
regulatory zones that encompass surface traces of active faults with a
potential for future surface fault rupture. The zones are generally established
about 500 feet on either side of the surface trace of active faults.

Active faults and strips of state-mandated zoning along faults (Alquist-Priolo
zones) riddle the Salton Sea Basin. The primary fault, the San Andreas, steps
from the northeast side of the Salton Sea across the southern end, along a
series of poorly understood faults, to the Brawley and Imperial fault systems.
This stepover region has not had a historic ground-rupturing earthquake.
Alquist-Priolo zones could not be defined because the faults are not
well-located. Faults parallel to, and splaying from, the San Andreas are also
capable of major earthquakes.

Initial plans for remediation facilities take into account the generalized
information (at 1:750,000 scale) on active faults, and the fault maps do not
provide information on strong ground shaking. The shaking can damage facilities
that lie far from an earthquake epicenter and far from active faults.
Information on near-surface materials is required to estimate the
ground-shaking hazards.