Safety element S-1/S-2: Seismic & Geological Issues

Only areas below are considered part of the General Plan.

Seismic & Geologic Issues

The Menifee General Plan area is highly diverse geologically, the result of both the youthful seismic setting of the surrounding region and the effects of climate. No active faults (faults that show evidence of having experienced surface displacement within the last 11,000 years) have been mapped in the Menifee General Plan area; therefore, the hazard of primary surface fault rupture is considered low to none.  However, Menifee is located near several regional active faults —such as the San Jacinto and Elsinore faults—that have the potential to cause strong ground shaking in the area (see Exhibit S-1, Fault Map).

Topographically, the Menifee area encompasses numerous rugged and moderately steep hills and mountains surrounded by a series of broad, nearly flat-bottomed valleys (see Exhibit S-2, Slope Distribution).  Most development in the area occurs in the valleys and low hillside areas, with the prominent hills and ridgelines largely undeveloped. As a result, slope instability, including rockfalls, debris flows, or ridgetop shattering, is a potential hazard only where development has encroached onto the hills or is at the base of the hills. Most slope damage in the region is likely to occur as a result of earthquake-induced shaking or during periods of exceptional and/or prolonged rainfall.

Seismic shaking can also cause various types of ground deformation; liquefaction and slope failure are the most destructive of these. When liquefaction occurs, the soils that liquefy lose their ability to support structures; buildings may sink or tilt, with the potential for extensive structural damage. Three areas in Menifee are thought to have soils that could liquefy during an earthquake: the Salt Creek floodplain, the Warm Springs Creek floodplain, and portions of the Paloma Wash Valley (see Exhibit S-3, Liquefaction and Landslides). The geology of a community also plays a part in determining the significance of its seismic and geologic issues. Sedimentary units in the Menifee area consist mainly of water-transported (alluvial) sand, silt, clay, and gravel derived from erosion of the adjacent hills and mountains (see Exhibit S-4, Geologic Map). See the Safety Element Background Report for further details regarding these geology types.

Goal

• S-1: A community that is minimally impacted by seismic shaking and earthquake-induced or other geologic hazards.

Policies

• S-1.1: Require all new habitable buildings and structures to be designed and built to be seismically resistant in accordance with the most recent California Building Code adopted by the city.
• S-1.2: Encourage owners of old or potentially hazardous buildings- including pre-1952 wood-frame structures, concrete tilt-ups, pre-1971 reinforced masonry, soft-story, and multifamily residential buildings- to assess the seismic vulnerability of their structures and conduct seismic retrofitting as necessary to improve the building's resistance to seismic shaking.
• S-1.3: Encourage the city's utility service providers to identify sections of their distribution networks that are old and/or in areas susceptible to earthquake-induced ground deformation, and to repair, replace, or strengthen the sections as necessary.

Goal

• S-2: A community that has used engineering solutions to reduce or eliminate the potential for injury, loss of life, property damage, and economic and social disruption caused by geologic hazards such as slope instability; compressible, collapsible, expansive or corrosive soils; and subsidence due to groundwater withdrawal.

Policies

• S-2.1: Require all new developments to mitigate the geologic hazards that have the potential to impact habitable structures and other improvements.
• S-2.2: Monitor the losses caused by geologic hazards to existing development and require studies to specifically address these issues, including the implementation of measures designed to mitigate these hazards, in all future developments in these areas.
• S-2.3: Minimize grading and modifications to the natural topography to prevent the potential for man-induced slope failures.
• S-2.4: Manage the groundwater resources in the area to prevent overdrafting of the aquifers, which in turn could result in regional subsidence.

General Plan Exhibits

• Exhibit S-1: Fault Map
• Exhibit S-2: Slope Distribution
• Exhibit S-3: Liquefaction and Landslides
• Exhibit S-4: Geologic Map
• Exhibit S-5: Flood Hazards
• Exhibit S-6: High Fire Hazard Areas
• Exhibit S-7: Critical Facilities
• Exhibit S-8: Very High Fire Hazard Severity Zones and Public Facilities
• Exhibit S-9: Evacuation Routes