Implementation of the Alquist-Priolo Earthquake Fault Zoning Act in Los Angeles County Charles Nestle County of Los Angeles Dept. of Public Works Surface Fault Rupture Workshop May 10, 2013
What Work is Governed by the Alquist-Priolo Earthquake Fault Zoning Act?
California Public Resources Code 2621.6 Project defined (1) Any subdivision of land which contemplates the eventual construction of structures for human occupancy. (2) Structures for human occupancy, with the exception of a single-family wood-frame or steel-frame dwelling not exceeding two stories when that dwelling is not part of a development of four or more dwellings.
What is the intent of the Alquist-Priolo Earthquake Fault Zoning Act?
California Public Resources Code 2621.5(a) Purpose and application of chapter The Legislature declares that this chapter is intended to provide policies and criteria to assist cities, counties, and state agencies in the exercise of their responsibility to prohibit the location of developments and structures for human occupancy across the trace of active faults.
California Code of Regulations 3600. Purpose It is the purpose of this subchapter to set forth the policies and criteria of the State Mining and Geology Board, to prohibit the location of developments and structures for human occupancy across the trace of active faults. The policies and criteria set forth herein shall be limited to potential hazards resulting from surface faulting or fault creep within earthquake fault zones delineated on maps officially issued by the State Geologist.
California Code of Regulations 3603(a). Specific Criteria No structure for human occupancy, identified as a project under Section 2621.6 of the Act, shall be permitted to be placed across the trace of an active fault.
Definitions and interpretation
California Code of Regulations 3601(e). Definitions A "structure for human occupancy" is any structure used or intended for supporting or sheltering any use or occupancy, which is expected to have a human occupancy rate of more than 2,000 person-hours per year.
California Code of Regulations 3601(a). Definitions An "active fault" is a fault that has had surface displacement within Holocene time (about the last 11,000 years), hence constituting a potential hazard to structures that might be located across it
California Code of Regulations 3601(b). Definitions A "fault trace" is that line formed by the intersection of a fault and the earth's surface, and is the representation of a fault as depicted on a map, including maps of earthquake fault zones
Active Fault This definition does not, of course, mean that faults having no evidence for surface displacement within Holocene time are necessarily inactive. A fault may be presumed to be inactive based on satisfactory geologic evidence; however, the evidence necessary to prove inactivity sometimes is difficult to obtain and locally may not exist. California Geological Survey Special Publication 42 (rev. 2007), pg. 5.
Active Fault If appropriate data cannot be provided, then the presence of an active fault trace within the area of investigation must be assumed. County of Los Angeles Manual for the Preparation of Geotechnical Reports December 2010, pg. 13.
Does The A-P Act Specify a Setback?
California Code of Regulations 3603(a). Specific Criteria Furthermore, as the area within fifty (50) feet of such active faults shall be presumed to be underlain by active branches of that fault unless proven otherwise by an appropriate geologic investigation no such structures shall be permitted in this area.
Other Issues
A-P Fault Zone Boundaries Zone boundaries on early maps were positioned about 660 feet away from the fault traces to accommodate imprecise locations of the faults and possible existence of active branches. The policy since 1977 is to position the boundary about 500 feet away from major active faults and about 200 to 300 feet away from well-defined, minor faults. California Geological Survey Special Publication 42 (rev. 2007), pg. 6.
California Code of Regulations 3603. Specific Criteria (f) One (1) copy of all such geologic reports shall be filed with the State Geologist by the lead agency within thirty (30) days following the reports acceptance. The State Geologist shall place such reports on open file.
Questions Is there any concern that supporting infrastructure is permitted to be placed within a fault zone? Does the County view denial of a project based on an 11,000+ year old hazard as extreme? Is the County concerned that the avoidance of an 11,000+ year old hazard does not balance against the 475 year return period for structural design?
Questions?