A proposed mid-Holocene age definition for hazardous faults in California
A California Technical Advisory Committee is now reviewing the relative hazard of potential surface-fault rupture for single-family structures. Passage of the Alquist-Priolo (AP) Act in 1972 initially defined an active fault as one with surface or near-surface rupture within approximately the past 10 ka. This deterministic number evolved to approximately 11 ka (Pleistocene-Holocene boundary) and now, locally, to approximately 13 ka when expressed in calibrated radiocarbon years. Based on lessons learned since 1972 and on the recurrence considered acceptable for mitigation of other geologic processes, the potential for surface-fault rupture should stem from science and the professional judgment of qualified geological practitioners. Ideally, no specific deterministic number should define a hazardous fault, which may be practically mitigated based on professional judgment, cost, available technology, and societal constraints. But this concept is generally not presently acceptable to many California geological consultants and regulators. More pragmatic, therefore, is changing the existing active fault definition from approximately 11 ka to a mid-Holocene ( approximately 4-ka to 6-ka) range. This range is non-rigid and should be periodically revised based on technological advances in the geologic and engineering sciences. The proposed "mid-Holocene" age recognizes uncertainty and closes the gap between the present 11-ka active-fault criterion and the approximately 100-500-year recurrence of other natural hazards typically mitigated by engineering design. Change in regulation does not come easily and may be detrimental to the economic livelihood of some geologists. Nevertheless, to sensibly ensure public health, safety, and welfare, we must learn from new science and experience and then adjust accordingly.