2020 Local Hazard Mitigation Plan

City of Irvine

requires additional study, to determine the location of the fault and the limits of the area prohibited from surface construction on top of the known location of an active fault. Liquefaction Liquefaction occurs when seismic energy shakes an area with low-density, fine grain soil, like sand or silt, that is also saturated with water. When the shaking motion reaches these areas, it can cause these loosely packed soils to suddenly compact, making the waterlogged sediment behave more like a liquid than solid ground. During liquefaction events, the liquified soil can lose most of its stability, which can cause damage to buildings and infrastructure built upon it. In severe cases, some buildings may completely collapse. Pipelines or other utility lines running through a liquefaction zone can be breached during an event, potentially leading to flooding or the release of hazardous materials. Seismic shaking Seismic shaking is the shaking felt on the surface caused by an earthquake. In most cases, earthquakes are not powerful enough for the shaking to be felt. Particularly powerful earthquakes, however, can generate significant shaking, causing widespread destruction resulting in property damage.

Location and Extent

Fault Rupture While no active faults (Alquist-Priolo Special Study Zones) are located within Irvine, there are several regional faults within Alquist-Priolo Special Study Zones near the City that could result in seismic hazards should an earthquake occur along one of them. In addition, numerous earthquake faults have been identified within the City ( Figure 3-9 ); however, they are not considered active (shown movement at the surface in the past 13,000 years) and therefore do not require delineation within a special study zone. Regardless, these faults should be accounted for in future development decisions. Liquefaction Soils must be saturated with water for liquefaction to occur. Areas that have high water tables generally have saturated soil, since the distance between the shallowest aquifer and the surface is minimal. Areas with alluvial soils — soft sands, silts, and clays — are also susceptible to liquefaction as these soils are fine

grain and generally do not bond together well. Liquefaction events do not have a scale of measurement, however other factors can be used to assess the extent of damage associated with a liquefaction event, such as:

Soil type

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• Strength of seismic shaking in the area of liquefaction • Size of the affected area • Degree of destruction as a result of the liquefaction Parts of Irvine are in a liquefaction potential zone ( Figure 3- 10) , according to the California Geological Survey. This is due to the types of soils in this area, the presence of shallow groundwater under the low-lying portions of the City, and proximity to active earthquake faults capable of generating

Liquefaction caused by the 1964 Niigita, Japan earthquake caused these apartment blocks to experience severe leaning. Image from the University of Washington.

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