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2021 Multijurisdictional Local Hazard Mitigation Plan 10.2.3 Frequency Historical records of earthquake occurrences give some indication of future probabilities. Seismic activity was more frequent from 1830 to 1930 than it has been since. This leads some scientists to suspect that pressure is building up along the faults in the Bay Area that can result in a large quake. Such a quake could have dramatic and devastating effects throughout the Bay Area. The USGS reports the following earthquake probabilities for the Bay Area over next 30 years (U.S. Geological Survey, n.d.):  72 percent probability of an earthquake measuring magnitude 6.7  51 percent probability of an earthquake measuring magnitude 7  20 percent probability of an earthquake measuring magnitude 7.5 The Third Uniform California Earthquake Rupture Forecast identified recurrence intervals for four deterministic scenarios applicable to San Mateo County (Working Group on California Earthquake Probabilities, 2021):  San Andreas Fault scenario, recurrence interval = 160 years  San Gregorio Fault Scenario = 481 years  Butano Fault Scenario = 2,881 years  Monte Vista Fault Scenario = 1,894 years 10.2.4 Severity The severity of an earthquake can be expressed in terms of intensity or magnitude (see Section 10.1.3). The State of California Department of Conservation probabilistic ground shaking maps, based on current information about fault zones, show the PGA that has a certain probability of being exceeded in a 50-year period. San Mateo County is in a high-risk area, with a 10-percent probability in a 50-year period of ground shaking from a seismic event exceeding 60 percent of gravity in some parts of the County. Figure 10-5 shows the expected peak horizontal ground accelerations for this probability. 10.2.5 Warning Time There is no current reliable way to predict the day or month that an earthquake will occur at any given location. Research is being done with warning systems that detect the lower energy compressional waves (P waves) that precede the secondary waves (S waves) experienced as an earthquake. Earthquake early warning systems may provide a few seconds’ or a few minutes’ notice that a major earthquake is about to occur. The warning time is very short, but it could allow for someone to get under a desk, pause hazardous or high-risk work, or initiate protective automated systems in critical facilities. 10.3 EXPOSURE 10.3.1 Population The entire population of the planning area is potentially exposed to direct damage from earthquakes or indirect impacts such as business interruption, road closures, and loss of function of utilities. 10-12