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Environmental and Engineering Geoscience

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Implementing the California Earthquake Fault Zoning Act: A Proposal for Change

¹ GeoDynamics, Inc., 558 St. Charles Drive, Suite 116, Thousand Oaks, CA 91306

The California Alquist-Priolo (AP) Earthquake Fault Zoning Act of 1972 prohibits building structures for human occupancy over active faults. This directive seems simple but in practice has proven difficult to follow. The language of the Act and the related policies and criteria allow for different interpretations. Some believe that the language provides a degree of latitude for the professional geologist to make judgments regarding the hazard associated with minor faults. Others believe the language requires stringent criteria with no such latitude and that all faults within the AP zones are potential hazards that must be avoided in the absence of clear evidence precluding Holocene ground rupture. The AP Act has been implemented for many years using the less stringent criteria. The GeoHazards Committee of the California State Mining and Geology Board recently recommended that the language be interpreted to mean that all faults within an AP zone are presumed to be active until demonstrated otherwise. This interpretation presents difficult challenges to consultants, reviewers, city officials, and developers. The stringent criteria probably cannot be met where tract developments are proposed in complex fault zones. Public safety and welfare is better served by a mitigation program that combines avoidance of well-defined faults with engineered mitigation applied to all other areas where construction is proposed within an AP Zone.

Key Words: Alquist-Priolo Earthquake Fault Zoning Act • Engineering Geology • Ground Rupture Mitigation • Active Fault