CHAPTER 6–EARTHQUAKES AND GEOLOGIC HAZARDS CHAPTER 6 – EARTHQUAKES AND GEOLOGIC HAZARDS: RISKS AND MITIGATION CHAPTER CONTENT 6.1 Earthquake Hazards, Vulnerability, and Risk Assessment 6.1.1 Identifying Earthquake Hazards 6.1.2 Profiling Earthquake Hazards 6.1.3 Assessment of State Earthquake Vulnerability and Potential Losses 6.1.4 Assessment of Local Earthquake Vulnerability and Potential Losses 6.1.5 Current Earthquake Hazard Mitigation Efforts 6.1.6 Additional Earthquake Hazard Mitigation Opportunities 6.2 Landslide and Other Earth Movements Hazards, Vulnerability, and Risk Assessment 6.2.1 Identifying Landslide Hazards 6.2.2 Profiling Landslide Hazards 6.2.3 Assessment of Landslide Vulnerability and Potential Losses 6.2.4 Current Landslide Hazard Mitigation Efforts 6.3 Volcano Hazards, Vulnerability, and Risk Assessment 6.3.1 Identifying and Profiling Volcano Hazards 6.3.2 Assessment of Volcano Vulnerability and Potential Losses 6.3.3 Current Volcano Hazard Mitigation Efforts 6.3.4 Additional Volcano Hazard Mitigation Opportunities About Chapter 6 Chapter 6 assesses earthquake and geologic hazards. Earthquake is considered one of the three primary hazards, as explained in the State Hazard Mitigation Plan (SHMP) Chapter 1. This chapter discusses hazards from earthquakes; landslides and other earth movements, and volcanoes. For more information on the criteria and template used for hazard risk assessments and a discussion of the hazard classification system, see Chapter 1: Introduction, Section 1.2.3. EARTHQUAKE HAZARDS, VULNERABILITY, AND RISK ASSESSMENT This section addresses earthquakes as one of three primary hazards noted in the classification system introduced in Chapter 1: Introduction, Section, 1.2 Plan Overview. This risk analysis includes information identifying the following dimensions of this hazard: Overview of earthquake hazard Locations within the state (i.e., geographic areas) that are affected Previous occurrences within the state and the probability of future events (i.e., chances of recurrence) Seismic vulnerability and loss assessment efforts Current and future seismic mitigation efforts IDENTIFYING EARTHQUAKE HAZARDS Overview Earthquakes represent the most destructive source of hazards, risk, and vulnerability, both in terms of recent state history and the probability of future destruction of greater magnitudes than previously recorded. CALIFORNIA STATE HAZARD MITIGATION PLAN │ SEPTEMBER 2018 SECTION 6.1- PAGE 243 CHAPTER 6–EARTHQUAKES AND GEOLOGIC HAZARDS Earthquakes are a significant concern for California for several reasons. First, California has a chronic and destructive earthquake history. Second, California has widespread earthquake vulnerability as indicated by California Geological Survey (CGS) mapping of potential earthquake shaking intensity zones, with these zones commonly located near populated areas. Third, nearly all local governments that have submitted Local Hazard Mitigation Plans (LHMPs) have identified earthquakes as a hazard of importance. In addition to shaking, buildings are also vulnerable to ground displacements associated with primary fault rupture, liquefaction, differential settlement, and landslides. Inundations from tsunamis, seiches, and dam failures can also be major sources of loss to buildings and infrastructure. Causes of Earthquakes: Plate Tectonics California is seismically active because it sits on the boundary between two of the earth’s tectonic plates. Most of the state—everything east of the San Andreas Fault—is on the North American Plate. The cities of Monterey, Santa Barbara, Los Angeles, and San Diego are on the Pacific Plate, which is constantly moving northwest past the North American Plate. The relative rate of movement is about 2 inches (50 millimeters) per year. The San Andreas Fault is considered the boundary between the two plates, although some of the motion (also known as slip) is taken up on faults as far away as central Utah. There are over 15,000 identified faults in California. Over 200 of these identified faults are considered very dangerous based on their slip rates in recent geological time (the last 10,000 years).96 More than 70 percent of the state's 40 million people reside within 30 miles of a known fault where strong ground shaking could occur in the next 30 years. The motion between the Pacific and North American Plates occurs primarily on the faults of the San Andreas system and the eastern California shear zone. Faults are more likely to have future earthquakes on them if they have more rapid rates of movement, have had recent earthquakes along them, experience greater total displacements, and are aligned so that movement can relieve the accumulating tectonic stresses. Nearly all movement between the two plates is on active faults. The San Andreas Fault is not the only significant fault/plate boundary in California. The seismicity north of Cape Mendocino is controlled by faults associated with the Cascadia Subduction Zone, a large offshore fault system that separates the Juan de Fuca Plate to the west and the North American Plate to the east. This area is the most seismically active portion of the state. The Cascadia Subduction Zone is capable of producing great earthquakes (magnitudes greater than 8.0) and last ruptured in the year 1700, causing what was likely an earthquake in the Magnitude 9.0 range. The subduction zone is also capable of generating a large tsunami. Earthquake Hazards: Shaking Overview Damage due to ground shaking accounts for significant amount of all building losses in typical earthquakes. Building damage can be both structural and/or non-structural (i.e., damage to building contents) and both types of damage can cause injury or loss of life. More than 70 percent of the state's 40 million people reside within 30 miles of a known fault where strong ground shaking could occur in the next 30 years. Amplification of Seismic Shaking Although seismic waves radiate from their source like ripples on a pond, the radiation is not uniform due to the complex nature of an earthquake rupture, different paths the waves follow through the earth, and different rock and soil layers near the earth’s surface. Large earthquakes begin to rupture at their hypocenter and the fault ruptures outward from that point. Because the speed of an earthquake rupture on a fault is similar to the speed of seismic waves, waves closer to the epicenter can be compounded by waves from farther along the rupture, creating 96 Uniform Earthquake Rupture Forecast (UCERF 3 – 2014) CALIFORNIA STATE HAZARD MITIGATION PLAN │ SEPTEMBER 2018 SECTION 6.1- PAGE 244 CHAPTER 6–EARTHQUAKES AND GEOLOGIC HAZARDS a pulse of very strong seismic waves that move along the fault in the direction of the fault rupture. Seismic waves may also be modified as they travel through the earth’s crust. Shaking from the 1989 Loma Prieta Earthquake was concentrated to the north, toward San Francisco and Oakland, possibly due to the reflection of seismic waves off the base of the earth’s crust. As seismic waves approach the ground surface, they commonly enter areas of loose soils where the waves travel more slowly. As the waves slow down, their amplitude increases, resulting in larger waves with frequencies that are more likely to damage structures. Waves can also be trapped within soft sediments between the ground surface and deep, hard basement rocks, their destructive energy multiplying as they bounce back and forth producing much greater shaking at the ground surface. The CGS and the U.S. Geological Survey (USGS) recorded large ground waves at many locations during both the Loma Prieta Earthquake and the 1994 Northridge Earthquake. Topography can also affect the local intensity of earthquake shaking. Topographic highs, such as hills and ridges, enhance shaking and topographic lows, such as valleys, lessen shaking. This phenomenon has been observed in many earthquakes, and “topographic amplification” was quite pronounced in the 2003 San Simeon Earthquake where most of the shaking damage to residential structures occurred at the tops of hills. Loma Prieta Earthquake Damage, San Francisco, 1989 Source: U.S .Geological Survey Unexpectedly large seismic ground waves and their resulting damage may be produced from a relatively distant earthquake. Shaking from the 1999 Hector Mine Earthquake in the Mojave Desert produced seismic waves with amplitudes of up to 15 centimeters in the Los Angeles basin, more than 200 kilometers from the epicenter. While there was little damage from the Hector Mine Earthquake, other large earthquakes have caused damage in distant places. For example, Nevada’s 1954 Dixie Valley Earthquake resulted in damage to critical facilities in Sacramento due to water sloshing. CALIFORNIA STATE HAZARD MITIGATION PLAN │ SEPTEMBER 2018 SECTION 6.1- PAGE 245 CHAPTER 6–EARTHQUAKES AND GEOLOGIC HAZARDS Map 6.A: Historic Earthquakes In and Near California by Magnitude (1769-2017) Map 6.A shows the pattern and selected dates of earthquakes in and near California during the past 240 years. CALIFORNIA STATE HAZARD MITIGATION PLAN │ SEPTEMBER 2018 SECTION 6.1- PAGE 246 CHAPTER 6–EARTHQUAKES AND GEOLOGIC HAZARDS Earthquake Hazards: Ground Failure Fissuring, settlement, and permanent horizontal and vertical shifting of the ground often accompany large earthquakes. Although not as pervasive or as costly as the shaking itself, these ground failures can significantly increase damage and under certain circumstances can be the dominant cause of damage. Studies after the 1994 Northridge