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<br /> <br />2021 Multijurisdictional Local Hazard Mitigation Plan <br /> <br />Table 10-2. NEHRP Soil Classification System <br />NEHRP Soil <br />Type <br /> <br />Description <br />Mean Shear Velocity to 30 <br />m (m/s) <br />A Hard Rock 1,500 <br />B Firm to Hard Rock 760-1,500 <br />C Dense Soil/Soft Rock 360-760 <br />D Stiff Soil 180-360 <br />E Soft Clays < 180 <br />F Special Study Soils (liquefiable soils, sensitive clays, organic soils, soft clays >36 m thick) <br /> <br />10.1.8 Secondary Hazards <br />Earthquakes can cause disastrous landslides. River valleys are vulnerable to slope failure, often as a result of loss <br />of cohesion in clay-rich soils. Earthen dams and levees are highly susceptible to seismic events, and the impacts <br />of their eventual failures can be considered secondary risk exposure to earthquakes. Depending on the location, <br />earthquakes can also trigger tsunamis. Additionally, fires can result from gas lines or power lines that are broken <br />or downed during the earthquake. It may be difficult to control a fire, particularly if the water lines feeding fire <br />hydrants are also broken. <br /> <br />10.2 HAZARD PROFILE <br />10.2.1 Past Events <br />Table 10-3 lists recent earthquakes with a magnitude of 5.0 or greater within 100 miles of San Mateo County. The <br />last significant (greater than magnitude 6.0) seismic event in the San Mateo vicinity was the 7.1 magnitude San <br />Andreas Loma Prieta Earthquake in 1989, which originated 10 miles northeast of Santa Cruz. Other significant <br />local earthquakes include the 1906 earthquake in San Francisco and the 2014 Napa earthquake. Although the <br />1906 earthquake is most associated with the City of San Francisco, San Mateo County was also greatly affected. <br /> <br />Table 10-3. Recent Earthquakes Magnitude 5.0 or Larger Within 100-Mile radius <br />Date Magnitude Epicenter Location <br />8/24/2014 6.0 6 miles southwest of Napa, CA <br />10/31/2007 5.6 10 miles northeast of San Jose, CA <br />8/10/2001 5.50 9 miles west of Portola, CA <br />9/3/2000 5.17 8 miles northwest of Napa, CA <br />10/17/1989 7.1 10 miles northeast of Santa Cruz, CA <br />3/31/1986 5.70 12 miles east-northeast of Milpitas, CA <br />Source: USGS, 2021a <br /> <br />10.2.2 Location <br />Fault Locations <br />San Mateo County is in a region of high seismicity because of the presence of the San Andreas Fault that bisects <br />the county, the Hayward Fault across the bay to the east, and the San Gregorio Fault to the west. The primary <br /> <br /> <br /> <br /> <br />10-6