2020 Local Hazard Mitigation Plan

City of Irvine

exceptionally low. Refer to Table 3-14 for the probability of a major earthquake occurring in faults close to Irvine. Regional faults, like the San Andreas or San Jacinto, are more likely to experience a significant earthquake within the next quarter-century but may be too distant from Irvine to generate significant shaking intensity to trigger a liquefaction event. As a result, it is only possible to say that liquefaction could occur in the City, but it is not possible to say with certainty when and or where a future liquefaction may occur in Irvine. Seismic shaking Irvine is in a seismically active area with many faults in the surrounding area and region-at-large. The only known faults that run through Irvine is the San Joaquin Hills blind thrust fault, traveling in an east-west/ southeasterly direction. The fault runs just north of the San Joaquin Hills, with the Irvine Civic Center sitting on top of the inferred location (since it is buried). If an earthquake were to occur on this blind- thrust fault, the rupture would most likely occur underground, reducing the risk to surface structures. There would still be a danger, of course, posed by any seismic shaking which could damage buildings or infrastructure. It is almost inevitable that an earthquake will occur along one of the adjacent or regional fault lines and cause a major seismic event. The Third Uniform California Earthquake Rupture Forecast (UCERF3) was released in 2015 and is the most recent assessment of the probability of a major earthquake on various faults between 2015 to 2044. Table 3-14 shows the results for nearby and regional fault lines for Irvine. In addition to UCERF3 forecasts, which project the odds of a major earthquake on local and regional faults, the U.S. Geological Survey forecasts the severity of seismic shaking in different locations for various plausible earthquake scenarios. Table 3-15 shows the anticipated shaking in Irvine from some of these scenarios. The U.S. Geological Survey scenarios show that the Newport-Inglewood and San Joaquin Hills faults could cause the strongest seismic shaking in Irvine. However, the largest magnitude events are anticipated to come from the more distant San Jacinto and San Andreas faults, which could cause earthquakes that have an overall higher magnitude than the Newport-Inglewood or San Joaquin Hills faults but, due to the former faults’ distance from Irvine, the shaking intensity felt in Irvine would be redu ced compared to the shaking that would be felt nearer the earthquakes’ epicenters. The overall magnitude of potential earthquake scenarios occurring along the Newport-Inglewood and San Joaquin Hills faults is lower than some of the more regional faults, but their proximity to Irvine means that the City would be subjected to high intensity shaking from these earthquakes. In other words, these lower magnitude earthquake scenarios may overall be more destructive in Irvine than higher magnitude earthquake scenarios that are more distant. As noted in Table 3-14 , however, the likelihood of a powerful earthquake occurring along these faults within the next 25 years is exceptionally low.

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