2020 Local Hazard Mitigation Plan

City of Irvine

as the fires burn up many of the trees, brush, and other vegetation that help stabilize the earth. Earthquakes may also be a source of landslides as the shaking can destabilize already loosened soils. Subsidence Subsidence occurs when the level of the ground decreases as if the surface is sinking. Subsidence can either be sudden (as in a sinkhole) or happen gradually over time. It can be caused by mining, groundwater pumping, or fossil fuel extraction, creating empty underground spaces that can collapse and cause the soil above to drop. Erosion, natural cave collapses, and seismic activity can also cause subsidence.

Location and Extent

Landslide There is the potential for landslides in the steeper portions of the foothills of the Santa Ana Mountains to the northeast of the City and the San Joaquin Hills to the southwest of the City. These areas are characterized by steep topography and geologic units that can become unstable. Figure 3-5 identifies the areas of the City that are considered vulnerable to seismic induced landslides. Even these areas, however, are designated as having a moderately low risk of landslides due to seismic conditions, and a low likelihood of a landslide under other conditions (Dept. of Conservation 1976). While no definitive scale for measuring landslides exists, landslide events are usually measured using the amount of material that is displaced (i.e., the cubic feet of earth that moved). In addition, to these landslide hazards, the California Geological Survey has mapped deep-seated landslide hazards, which uses a scale of landslide susceptibility that is based on slope steepness and the strength of the underlying rock, with 0 being no susceptibility and 10 being the highest susceptibility. Figure 3-6 identifies the deep-seated landslide susceptibility for Irvine. Areas in the foothills of the Santa Ana Mountains and San Joaquin Hills show the greatest susceptibility within the City. Subsidence The City identified that the most likely locations for subsidence in Irvine are the low-lying areas that sit on top of the OCWD groundwater basin. Other sections of the city are potentially subject to subsidence in the event of a major earthquake (M w 5.0 or greater), although Irvine does not have a history of seismically induced subsidence. In terms of extent, subsidence is typically measured by the distance that the ground has sunk from its original elevation (i.e., in feet or inches) or by using the rate of subsidence (i.e., inches or centimeters per year). There is evidence of subsidence in most of Orange County as a result of excessive groundwater pumping in the first half of the 20th century, prior to the development of the California State Water Project, which siphons water from the Owens Valley. 67 Currently, it is estimated that the greater LA Basin (including Orange County) experiences approximately 20 mm of net subsidence seasonally due to groundwater pumping and artificial recharge. 68

Past Events

Landslide Major landslides have occurred throughout the Southern California region. For example, landslides were set off by the 1971 San Fernando and 1994 Northridge earthquakes. 69 As a master-planned community,

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