Chapter 82.15 — GEOLOGIC HAZARD (GH) OVERLAY

The Geologic Hazard (GH) Overlay established by §§ 82.01.020 (Land Use Plan and Land Use Zoning Districts) and 82.01.030 (Overlays) is created to provide greater public safety by establishing investigation requirements for areas that are subject to potential geologic problems, including active faulting, landsliding, debris flow/mud flow, rockfall, liquefaction, seiche, and adverse soil conditions.

(Ord. 4011, passed - -2007)

The GH Overlay shall be designated:

(a) In areas that are adjacent to active earthquake fault traces as depicted on the Alquist-Priolo Earthquake Fault Zone Maps;

• (b) In areas that are adjacent to potentially active earthquake fault traces as designated by the County Geologist.

• (c) In areas where landslides, debris flows/mud flows, rockfall or other slope instabilities occur;

• (d) In areas where liquefaction of the soil or seiche is associated with earthquake activity; and

(e) In areas of adverse soil conditions, such as those underlain by hydrocollapsible, expansive, corrosive soils.

(Ord. 4011, passed - -2007)

Except as provided for areas of adverse soil conditions, a detailed geologic study prepared by a California Registered Professional Geologist shall be submitted with all land use applications and development permits proposed within the GH Overlay, that would lead to the construction of roads or structures or the subdivision of land.

(a) Report Contents.

(1) Areas of Faulting. In areas of the GH Overlay where faulting is a concern, the geologic report shall confirm the presence or absence of active faults and, if applicable, shall establish appropriate construction setbacks from active faulting.

(2) Areas of Slope Stability. In areas of the GH Overlay where slope stability is a concern, the geologic report shall evaluate landslides and other slope instabilities that could affect the project and, if applicable, shall include recommendations for mitigation.

(3) Areas of Liquefaction Susceptibility. In areas of the GH Overlay where liquefaction is a concern, the geologic report shall evaluate the potential for liquefaction based upon anticipated ground shaking, historic groundwater levels and character of the alluvial materials. If the investigation determines that a potential for liquefaction exists, a geotechnical investigation may be required.

(4) Areas of Potential Seiche. In areas of the GH Overlay where seiching is a concern, the geologic report shall evaluate the potential for seiching based upon anticipated ground shaking, the character of the water body and the elevation of the proposed development with respect to the high water line. The investigation shall determine if the freeboard requirements will be met.

(5) Areas of Adverse Soil Conditions. In areas of the GH Overlay where adverse soil conditions are known to exist, a geotechnical investigation report shall be prepared by a California Registered Professional Engineer. The

investigation shall sample and conduct laboratory analysis of subsurface soils and, if applicable, shall include recommendations for mitigation.

(b) Exemptions from Report Requirements. Exemptions to the requirement for a geologic study include:

(1) A single-family residential wood or steel frame dwelling not exceeding two stories in height provided the proposed structure is not located within the boundaries of a mapped landslide or within that portion of the GH Overlay designated as having adverse soil conditions.

(2) Single-family residential wood frame or steel dwellings located within a subdivision of land for which a geologic report was prepared and approved;

(3) A non-habitable structure that is accessory to a residential use that is not physically connected to the principal structure; and

(4) Alterations or additions to any structure where the value or area does not exceed 50 percent of the structure. (Ord. 4011, passed - -2007)

Development and land uses proposed within the GH Overlay shall comply with the following standards. (a) A structure used for human occupancy shall be located 50 feet or farther from any active earthquake fault traces. Lesser setbacks may be applicable in certain situations as determined by an appropriate geologic investigation and approved by the County Geologist or other engineering geologist designated by the Building Official.

(b) A structure used for critical facilities shall be located 150 feet or farther from any active earthquake fault trace as indicated by General Plan. Critical facilities shall include dams, reservoirs, fuel storage facilities, power plants, nuclear reactors, police and fire stations, schools, hospitals, rest homes, nursing homes and emergency communication facilities.

(c) Utility lines and streets shall not be placed within the construction setback area of a hazardous fault except for crossing which can be made perpendicular to the fault trace or as recommended by the project geologist and approved by the County Geologist or individual designated by the Building Official.

(d) The use of development restricted areas as recreation and common open spaces is encouraged. (Ord. 4011, passed - -2007)