City of Irvine
2020 Local Hazard Mitigation Plan
Seismic Shaking Seniors, pregnant women, and persons with disabilities are more threatened by seismic shaking since they may have limited mobility and may be unable to reach shelter in time. Even if these groups reach shelter in time, they may find themselves trapped if furniture or building components have fallen around them. Renters and low-income persons are also more threatened by seismic shaking since these groups may live in homes that are not properly retrofitted to survive the stresses of a seismic event. These groups may not be able to absorb the costs associated with repairing their homes or looking for new housing should their existing housing be too damaged for occupancy. In terms of geography, seismic shaking effects are anticipated to reduce in areas of higher elevation, which tend to have lower population densities and high household incomes.
Other Threats
Fault Rupture Seismic events that cause surface fault rupture tend to damage roads and structures in the areas of impact. The length of rupture is typically a component of the magnitude of the seismic event. The stronger the event, the greater distance that rupture can occur. Within Irvine, most of the fault segments identified are not considered active; therefore, they have a lower likelihood of rupturing. In addition, most of the fault movement occurring in the region is associated with strike-slip faulting along the San Andreas, San Jacinto, Whittier Elsinore, and Newport Inglewood fault zones. The faults within the City of Irvine are less likely to rupture in comparison to these faults, and if a rupture were to occur, it would most likely impact small areas. Liquefaction Services and mobility may be disrupted during and following a liquefaction event. Sidewalks, roadways, and pipelines may become fractured and disjointed as a result of the liquefying soils. Roads and sidewalks may be usable in some form, but a severe liquefaction event may render them impassible until they are repaired. Broken gas and water pipelines would result in utilities outages in Irvine homes and businesses. Since these are underground, the duration of the outage could likely be extended until the pipelines are excavated and replaced by utility operators. Damage to power lines is unlikely since the power lines themselves are not rigid structures and can move if any of the transmission towers experience slight leaning. Homes may be damaged, and mid-rise office buildings in the Centerpointe Complex would likely be rendered unsafe for occupancy if they experience any leaning or structural damage resulting from the liquefaction. This would curtail the City’s and region’s economic activity. Seismic Shaking As early earthquake warnings systems become operational soon, it can be expected that utilities will take advantage of these advance warnings to shut off gas, water, and power transmission to control any potential leaks following the seismic shaking. Authorities may have enough time to halt the use of bridges or move workers to a safe distance away from hazardous materials. Workers will cease their activity and take shelter until they can be safely evacuated. Therefore, all services will be non-operational during the seismic shaking and remain inactive until authorities are confident that it is safe to reactivate utilities and to return employees to their workplaces. The length of this time would vary depending on the magnitude of the event. A significant earthquake would likely put utilities out of commission and halt any employment activity in the City for a few hours or several days. The City and the region would lose
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