Geologic Hazards. Geologic Setting

Transcription

1 Geologic Hazards Geologic Setting Geology for Planning: Central and Southeastern Marin County, California, published in 1976 by the California Geological Survey (formerly the California Division of Mines & Geology) provided the basis for the geologic discussion and seismic and landslide hazard analysis for the Environmental Safety Element produced for the Town of Fairfax in While there have been few notable changes in the geologic units underlying the Town of Fairfax since that report was written, significant advances have been made in scientific understanding of how various geologic units interact with seismic and other earthmoving geologic processes, and their potential impact on man-made structures. Various technical reports have been consulted in preparing this discussion including the Technical Background Report prepared for the Update of the Marin Countywide Plan in 2002, and publications prepared by the United States Geological Survey and the California Geological Survey. A list of sources is provided at the end of this discussion. Fairfax is located in the central portion of the Coast Range geomorphic province which is dominated by ridges and valleys. The northwest-southeast directional trend is a result of plate tectonics. Local geology, and that of California in general, has been formed by the interaction of the Pacific and North American plates. In Marin County that manifests itself as the San Andreas Fault Zone that separates the Point Reyes Peninsula from the rest of the County, including the Town of Fairfax. Geology: The predominant geologic units underlying the Town of Fairfax are: Bedrock (Sandstone and Shale; Greenstone and Chert); and Surficial Deposits (Alluvium). (See Figure 1) The characteristic and behavior of the bedrock deposits are dependent on several variables, including degree of weathering, bedding, fracturing, etc.; therefore it is essential that site specific studies be conducted to determine the relative strength of geologic materials when considering a development proposal, particularly relative to earthquake induced or rainfall induced landslides. All the bedrock units are considered part of the Franciscan Complex, and comprised of metamorphosed rock, which is considered to have low to moderate slope and earthquake stability, particularly on steep slopes. The surficial deposits are alluvium and primarily loose and soft sediments and debris deposited along streambeds with the last 10,000 years. These deposits are typically those that are the most susceptible to seismic shaking, liquefaction and differential settlement. (For a more detailed discussion of geology, see excerpts from the Geology, Mineral Resources and Hazardous Materials Technical Background Report appended to this section.)

2 Reserved for Figure 1 East Marin County Geology Map

3 Earthquake Faults: Fairfax lies nearly equidistant from the San Andreas Fault Zone and the Hayward- Rodgers Creek Fault Zones. (See Figure 2) Either of these fault systems is capable of generating a large earthquake that could cause damage to the Town of Fairfax, and greater damage to extensive portions of the San Francisco Bay Region. Ground shaking: The greatest risk to life and property in an earthquake is from ground shaking. During an earthquake, the ground can shake for a few seconds or over a minute. The strength and duration of ground shaking is affected by many factors. Distance from the fault is the most significant factor; however, geologic conditions, direction of the fault rupture, magnitude and depth are also critical. Shaking, particularly horizontal shaking, causes most earthquake damage because structures often have inadequate resistance to this type of motion. The strongest shaking is typically close to the fault where the earthquake occurs. Weak soils, such as valley alluvium or soils along river and stream beds, also experience strong shaking in earthquakes, even from distant quakes. Figures 3 and 4 show projected groundshaking for two scenario earthquakes affecting Fairfax. There are a number of different scales and terms used to describe the amount of shaking that occurs in an earthquake, including Moment Magnitude, Richter Magnitude, Modified Mercalli Intensity (MMI) and Peak Ground Acceleration (PGA). In this Plan, the term magnitude refers to the Moment Magnitude, which represents both the strength and duration of shaking and is commonly used. This is similar to the well-known Richter Magnitude scale, but it generally correlates better to an earthquake s damage potential. Earthquake shaking intensity, expressed using the MMI scale, classifies shaking by the amount of damage observed. For example, VI on the MMI scale means that everyone felt the earthquake, people had trouble walking, but there is not structural damage even in poorly built structures. Intensity of shaking is a subjective measure, meaning it can vary based on where it is applied and by whom. PGA is an alternate, objective way to express the level of ground shaking. It refers to the highest ground acceleration measured in a particular location during an earthquake. It is often reported using the unit g, which relates to the force of gravity. All of these terms capture different aspects of earthquake shaking and are important to accurately communicate risk. Table 1 details how the MMI scale correlates with PGA in terms of perceived shaking and potential damage. Table 1 Comparison of MMI to PGA Perceived Shaking Potential Damage PGA (%g) MMI Intensity Not felt Weak Light to Moderate Strong to Very Strong Moderate to heavy Severe to Violent Extreme none none None to heavy Very very light heavy < >124 I II-III IV-V VI-VII VIII-IX X or greater

4 Surface Rupture: Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. After an earthquake, one side of a fault can shift from its previous location, causing splits in any structures or pipelines crossing the area. During the 1906 earthquake horizontal displacement along the San Andreas fault occurred Marin County. Although there are no known earthquake faults in Fairfax that could result in surface rupture, the incidence of surface rupture in other locations of the San Francisco Bay Region in a major earthquake could cause disruption of services in Fairfax. Liquefaction: Liquefaction is a phenomenon that occurs in wet, sandy soils. When shaken, the soil grains consolidate, pushing water towards the surface and causing a loss of strength in the soil. The soil surface may sink or spread laterally. Structures located on liquefiable soils can sink, tip unevenly, or even collapse. Pipelines and paving can tear apart. The potential for liquefaction in Fairfax exists primarily in the creek beds and adjacent floodplains which are underlain with alluvium. Three ingredients are necessary for liquefaction to occur: a high water table, layers of loose sand, and moderate or greater earthquake shaking. Figure 5 shows areas susceptible to liquefaction. Landslide: Seismically triggered landslides are a concern in areas with steep and unstable slopes. Two types of landslides can cause damage to the built environment. The first, disrupted slides and falls, includes rock falls, soil falls, disrupted soils slides, and rock slides, and generally impacts buildings or infrastructure from above. The second type, called coherent slides, includes rock slumps, soil slumps, rock block slides, and slow earth flows, and generally affects property and infrastructure from below. Falls can occur on slopes greater than seventy percent, and slides can occur on slopes of greater than thirty percent, when exposed to ground shaking intensity of more than MMI VIII. i Earthquakeinduced landslides can also be exacerbated during periods of high rainfall, where the ground is saturated and even normally stable materials can fail. These slides could result in significant property and infrastructure damage, and potential injury in the Town of Fairfax. Figure 6 shows areas of potential landslide risk. Post-Earthquake Fire: Fire often accompanies earthquakes, caused by breaks in natural gas lines, damaged electrical systems, or toppled appliances with pilot lights. Fire following an earthquake is particularly difficult to suppress because of the likelihood of numerous simultaneous ignitions, broken water mains, blocked or damaged routes for evacuation and firefighter access, and other demands on fire personnel. This threat was tragically demonstrated in the 1906 earthquake in both San Francisco and Santa Rosa, the 1989 Loma Prieta earthquake in the San Francisco Marina District, and the 1995 Kobe, Japan earthquake.

5 Densely populated neighborhoods with wooden homes, such as the residential areas in Fairfax, particularly those on steep slopes in the wildland-urban interface areas, are most at risk, along with utility systems, and other infrastructure. Earthquake Probabilities: The Working Group on California Earthquake Probabilities (WG02) found that there is a 62% probability of at least one magnitude 6.7 or greater earthquake before 2032 within the San Francisco Bay Region (Figure 7). This earthquake is likely to occur on one of the seven major fault systems in the bay area. It was determined that the Hayward-Rodgers Creek, San Andreas and Calavares fault systems have the highest probabilities of generating a M>6.7 earthquake before The San Andreas and the Hayward-Rodgers Creek fault systems could have the greatest impacts on Fairfax because of their proximity to population centers within the region and the fact that they have the highest probability of rupture in the San Francisco Bay Region. The WG02 found a 21% probability for the San Andreas fault system and a 27% probability on the Hayward-Rodgers Creek fault system for a M>6.7 earthquake before It was also found that an estimated probability of 80% exists for a M6.0 to M6.7 earthquake event in the San Francisco Bay Region. The 2007 Working Group on California Earthquake Probabilities updated their previous work by reporting earthquake probability for the entire state of California instead of by subregions. The result is that the overall probability for an earthquake of M6.7 or greater in the next 30 years is 99.7%. Larger earthquake events are less likely with the probability of a M7 or greater at 94%, a M7.5 or greater at 46%, and a M8.0 or greater at 4.5%. When dividing the state into approximately equal areas, the northern California portion of the State has a 94% probability of a M6.7 or greater, and a 37% chance of a M7.5 or greater event occurring within the next thirty years.

6 Reserved for Figure 2 Earthquake Faults

7 Reserved for Figure 3 San Andreas Ground Shaking Map

8 Reserved for Figure 4 Hayward/Rodgers Creek Ground Shaking Map

9 Reserved for Figure 5 Liquefaction Suseptibility Map

10 Reserved for Figure 6 Landslide Susceptibility Map

11 Reserved for Figure 7 Earthquake Probabilities Map

12 Potential Impacts The following is a general list of some building types and a brief description of some issues related to their earthquake performance (portions of the following are taken from Bertero (2000) and CSSC (1999)): Wood-Frame Structures: Among the materials that are used for construction, wood is considered the most efficient earthquake resistant material for low-rise buildings. Based on past earthquake experiences the greatest considerations for wood-frame structures are that they should be carefully designed and constructed, provided with lateral bracing and all of their components should be tied together from the roof down to the foundation. A major cause of failure in older wood-frame structures is failure at the framing/foundation junction in which the framing is not properly connected to the foundation or the lower portions of the framing are not adequately braced. Unreinforced Masonry Structures (URMs): Unreinforced masonry buildings have caused deaths and significant property damage, including damage to historic structures in past California earthquakes, because of their susceptibility to ground shaking. Solid brick masonry is very heavy and its tensile strength is low. Old unreinforced masonry buildings, whose walls are not properly connected to floors, roof, and interior and exterior transverse walls, are an extreme seismic hazard. However, if masonry is properly reinforced it can be used in seismicresistant construction. In compliance with the California URM Law, the Town of Fairfax identified four unreinforced masonry buildings and established a voluntary program for retrofit. All four buildings have been retrofit to life safety standards. Additional URM s that did not meet the criteria under the URM law may still exist in the Town of Fairfax. Concrete Structures: Concrete is a relatively heavy material and it has a low tensile strength. It is usually reinforced with steel and when done properly reinforced concrete can be used in seismicresistant construction. It is very important that beam/column connections be designed, detailed and constructed with the proper amount and type of reinforcing steel to provide ductility. If not constructed properly, drastic failure of a structure may occur during earthquake ground shaking. Common types of damage during earthquakes include shearing of concrete columns that results from the lack of adequate steel reinforcement and severe cracking of concrete walls, which is common in older, lightly reinforced structures. Multi-story concrete frame buildings built from the 1950s to early 1970s often have inadequate reinforcing in their columns. Consequently, these buildings have the potential for a pancake type collapse (CSSC, 1999).

13 It is not known how many concrete buildings constructed without adequate steel reinforcement exist today in the Town of Fairfax. Steel Structures: The strength, ductility and toughness per unit weight are significantly higher than concrete and masonry materials. This makes it a useful construction material. However, because of its high strength per unit weight, the slenderness of steel structural members could result in failure during seismic shaking. Buckling failure of steel members is a common phenomenon during earthquake shaking. Another issue in steel design is the connection of the structural member, the most common being welds. If steel members are not connected properly to each other then failure may occur. Steel frames should be designed with strong column beams such that the ultimate failure mode would be in beams yielding and not columns. Damage to steel structures in past earthquakes has typically been observed in high-rise buildings. Therefore, there does not appear to be a risk to the Town of Fairfax from existing buildings. Any new development, if constructed to current code, should be seismically resistant.

14 State and local controls: Alquist-Priolo Earthquake Fault Zoning Act: The Alquist-Priolo Earthquake Fault Zoning Act was signed into law December 22, 1972, and went into effect March 7, The Act was enacted to regulate development near active faults in order to mitigate the hazard of surface fault rupture. In general, the Act has two requirements: prohibiting the location of developments and structures for human occupancy across the trace of active faults; and, establishing Earthquake Fault Zones as defined by the State Geologist, within which affected cities and counties must establish special procedures for reviewing and approving applications for new building permits within the Zones. The Town of Fairfax does not lie within an Alquist-Priolo Earthquake Fault Zone. Seismic Hazard Mapping Act: The State Legislature passed the Seismic Hazards mapping Act in 1990, which was codified in the Public Resources Code as Division 2, Chapter 7.8, which became operative on April 1, The purpose of the Act is to identify areas where earthquakes are likely to cause shaking, liquefaction, landslides, or other ground failure, and to regulate development so as to reduce future earthquake losses. The California Geological Survey has responsibility for developing the hazard maps, and has incrementally focused their efforts on the highest risk areas and areas undergoing significant development. Therefore, the mapping quadrangles covering the Town of Fairfax have not yet been mapped and are unlikely to be completed in the near future. Geologic Hazard Abatement Districts: Geologic Hazard Abatement Districts (GHADs) were enacted by the Beverly Act of 1979 (SB1195) and allow local residents to collectively mitigate geological hazards that pose a threat to their properties. They are enabled by Division 17 of the Public Resources Code, Sections GHADs may be formed for the following purposes: prevention, mitigation, abatement, or control of a geologic hazard; and, mitigation or abatement of structural hazards that are partly or wholly caused by geologic hazards. A geologic hazard is defined by the Code as an actual or threatened landslide, land subsidence, soil erosion, earthquake, fault movement, or any other natural or unnatural movement of land or earth. (Check with Ann Welsh regarding any prior or existing GHAD s in Fairfax) Local Ordinances: The Town of Fairfax has adopted the 2007 edition of the California Building Code (Title 24 Part 2) based upon the 2006 International Building Code (IBC). The code includes the most current standards for seismically resistant construction. The building code sets minimum criteria for the structural design of buildings. The earthquake design provisions contained in the Uniform Building Code (UBC) have

15 traditionally been based on recommendations developed by the Structural Engineers Association of California (SEAOC). These recommendations have adopted a seismic design philosophy intended to protect life safety, but allow for some structural and potentially significant nonstructural damage from earthquake levels as severe as can be expected in active seismic regions. Buildings designed in accordance with the UBC are anticipated to experience significant damage loss, when affected by a major earthquake. Further, the design provisions of the UBC primarily address damage caused by ground shaking. They do not address the effects of other site hazards, such as liquefaction, ground lurching, landslides, ground surface rupture, etc. Any of these types of ground failure can result in excessive damage and potentially, even collapse of buildings meeting the code criteria. The Town also has an Unreinforced Masonry Retrofit Ordinance (Section ), designed to promote public safety and welfare by reducing the risk of death or injury that may result from the effects of earthquakes on unreinforced masonry buildings. Section of the Town Code requires subsurface geotechnical reports, as required by the Town Engineer. The reports may include the following: 1. Subsurface investigations. Subsurface investigations (including drilling to establish bedrock) to consider the potential, on the entire slope face, both on and adjacent to the subject property, for ground failure, erosion subsidence, differential settlement, liquefaction and any other adverse geologic conditions. 2. Development recommendations. Recommendations for restrictions on development where development poses a hazard and proposed mitigation measures for hazardous conditions. 3. General plan conformance. Reference to all geologic safety concerns and events discussed in the safety element of the general plan as they relate to the subject property.

16 Considerations for Planning 1. The Town of Fairfax does not contain any active faults as designated by the Alquist - Priolo Earthquake Fault Zoning Act; however it is subject to moderate to high levels of groundshaking (40-50%g) which could cause significant damage and disruption to infrastructure, Town facilities, businesses and residences. 2. Liquefaction areas are limited to creek side areas built in alluvium deposits. 3. Hillside construction is vulnerable to earthquake induced landslides. This vulnerability is increased during periods of intense or prolonged rainfall when soils become saturated. 4. Aging Town infrastructure, such as bridges and pipelines, may suffer damage and result in local transportation, water and sanitation disruptions. 5. Fairfax, even if not significantly damaged, could experience impacts caused by damage at a distance; e.g., damage to Bay Area transportation, communication, power, water and sanitation systems. 6. Greatest risk may be from isolation due to transportation disruption which could impact the delivery of essential supplies and disrupt commute patterns for a period of years. 7. Fairfax residents must be prepared to be self-sufficient for periods of three to seven days. 8. Communication and coordination with external agencies is critical to Fairfax preparedness. 9. Fairfax has adopted the most current building codes to guide new development and substantial improvements to existing development. 10. Fairfax is in compliance with state legislation (SB 547). Four unreinforced masonry buildings have been identified and retrofitted for life safety. 11. There are an unknown number of other types of potentially hazardous buildings, such as soft first story apartment buildings which have been shown to be vulnerable in previous earthquakes in California. 12. Many Fairfax homes were built prior to current codes which require seismic resistant foundations. Due to their age and location, many of these homes may have wood rot problems that will weaken their performance in a strong earthquake. 13. Risk to new development can be minimized by conducting thorough geotechnical investigations, incorporating findings into the design and construction, and strict compliance with current building codes. 14. It is unlikely that Seismic Hazard Zone Maps will be developed for the Fairfax area by the California Geological Survey; however, it is beneficial for the Town of Fairfax to follow local government guidelines required by the Seismic Hazard Mapping Act in reviewing development plans.

17 Goal and Policies: Goal: Protect people and property from risks associated with seismic activity and geologic conditions. Policies: 1. Require development to avoid or minimize potential hazards from earthquakes and unstable ground by requiring geotechnical studies, and through rigorous enforcement of all relevant codes and construction standards. 2. Identify potential hazardous high occupancy buildings. 3. Conduct an inventory of existing or suspected soft-story residential structures. 4. Preserve the Fairfax housing stock by encouraging home owners to seismically retrofit their property, including installation of a seismically resistant foundation, bolting the sill plate to the foundation, installing shear wall strengthening to cripple walls, etc. 5. Seek funding through Caltrans Local Highway Bridge Program to retrofit bridges identified by Caltrans as seismically deficient. Determine the seismic stability of Meadow Way, Manor and Creek Road Bridges. 6. Evaluate Town owned critical facilities and infrastructure to identify those elements that are seismically deficient or weakened due to age or lack of maintenance, and would result in significant disruption of service in a major earthquake. 7. Create accurate localized maps of geotechnical hazard areas based on state-of-the-art knowledge, historical knowledge, and geotechnical studies performed for development permits. 8. Provide technical guidance and financial incentives for residential and commercial property owners to undertake seismic retrofit of wood frame structures. 9. Continue to support post earthquake self-sufficiency through the CERT and Get Ready Programs. 10. Provide multiple sources of public education materials, including Town Hall, Fairfax Library, Town website, special mailings, etc., to ensure all residents have access to the most current preparedness information.

18 Sources: Geology, Mineral Resources and Hazardous Materials Technical Background Report The Marin County Community Development Agency, Planning Division Developed for the Marin Countywide Plan 2002, Updated November The Uniform California Earthquake Rupture Forecast by the 2007 Working Group on California Earthquake Probabilities; CGS Special Report 203 and USGS Open File Report , and as SCEC Contribution Geology for Planning: Central and Southeastern Marin County, California, California Geological Survey, Open File Report Geologic Map and Map Database of Parts of Marin, San Francisco, Alameda, Contra Costa, and Sonoma Counties, California; M. C. Blake Jr., R.W. Graymer, and D.L. Jones U.S. Geological Survey Miscellaneous Field Studies Map MF-2337, Version Preliminary Maps of Quaternary Deposits and Liquefaction Susceptibility. Nine-County San Francisco Bay Region, California: 3 Digital Databases. United States Geological Survey Open File Report 00-44, On-Line Version Summary Distribution of Slides and Earth Flows in the San Francisco Bay Region. United States Geological Survey Open-File Report C

19 Excerpts from Geology, Mineral Resources and Hazardous Materials Technical Background Report, Developed for the Marin Countywide Plan 2002, Updated November 2005

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