4.6 Geology and Soils

Redwood City New General Plan 4.6 Geology and Soils

4.6 GEOLOGY AND SOILS This section describes the geologic conditions of the plan area based on readily available geotechnical and geological literature, previous geotechnical investigation reports, and geologic maps. A discussion of relevant geology and soils plans, policies, and regulations and the New General Plan’s consistency with such plans, policies, and regulations is also provided. This section also assesses potential impacts from seismic and geologic hazards that may occur as a result of adoption of the New General Plan. Information in this section is derived from research on the New General Plan prepared by BASELINE Environmental Consulting.

4.6.1 ENVIRONMENTAL SETTING

Seismic and Geological Conditions

Topography Redwood City (City) is located within California’s Coast Ranges Geomorphic Province, a geologically young and seismically active region. Northwest-southeast trending ranges of low mountains and intervening valleys dominate this region.1

The northeastern portion of the plan area is located in existing and former tidal marshes at elevations near sea level. The central portion, located southwest of U.S. 101 and including El Camino Real, is a gently sloping plain draining northeast to the tidal marsh, with elevations up to about 20 feet National Geodetic Vertical Datum of 1929 (NGVD).2 The City’s urban center with commercial and mixed uses is located in this central portion of the plan area. The southwestern portion forms the eastern foothills of the Santa Cruz Mountains, and has elevations up to about 600 feet NGVD sloping northeast. Most of this portion of the City is developed as residences and parks.

Faults and Geologic Units

The plan area is located on the San Francisco Peninsula. The Peninsula is traversed by three large faults of the San Andreas Fault System: the San Andreas, the Pilarcitos, and the San Gregorio Faults. These faults have divided the Peninsula into geologic units. The plan area is located east of the San Andreas Fault in the San Francisco Bay Block (Block) or

1 California Geological Survey (CGS), 2002, California Geomorphic Provinces, Note 36, California Dept. of Conservation. 2 National Geographic Holdings, Inc., 2001, Seamless USGS Topographic Maps on CD-ROM, California.

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4.6-1 Redwood City New General Plan 4.6 Geology and Soils

geologic unit. This Block is characterized by Franciscan basement rocks and rocks sheared by fault movement.3

Geologic units underlying the plan area vary with distance from the San Andreas Fault. The southwestern portion of the plan area is located adjacent to the San Andreas Fault Zone, and includes serpentinite, greenstone, and sheared rock including greywacke, siltstone, and shale of Jurassic and Cretaceous age. The central portion of the plan area is underlain by coarse-grained older Quaternary alluvial fan and stream terrace deposits, and younger Quaternary finer-grained alluvial fan deposits and basin deposits of silt and clay closer to the San Francisco Bay. The northern and eastern portions of the plan area, beneath and/or north/east of U.S. 101, is underlain by Holocene Bay Mud. Bay Mud consists of organic fine clay and silt. Some of the former tidal flats are covered with manmade fill.4

Soils

Soil is generally defined as the unconsolidated mixture of mineral grains and organic material that mantles the land surface. Soils can develop on unconsolidated sediments and weathered bedrock. The characteristics of soil reflect the five major influences on their development: topography, climate, biological activity, parent (source) material, and time. The plan area is mantled by soils that reflect the characteristics of the underlying materials on which the soil is developed.

Three basic categories of soils are present in the plan area. Soils in the current and former tidal flat areas and coastline of the plan area are classified as Urban Land Orthents, and include the Novato and Reyes Series soils. These tidal flat and coastline soils are nearly level, poorly drained, clay and silty clay on reclaimed tidal flats with high shrink-swell potential. Soils in the central portion of the plan area including the downtown area are also classified as Urban Land Orthents on nearly level to gently sloping land. These soils can be poorly drained to well-drained, and are present on alluvial fans, flood plains, and stream terraces. Soils in the upland areas of the plan area are Urban Land Orthents including the Los Gatos and Fagan Series soils. These soils are present on gently rolling to very steep terrain, and are well-drained and underlain by sandstone.5

Seismicity

The plan area is located in the seismically active San Francisco Bay Area. The main feature generating the seismic activity in the region is the tectonic plate boundary between the North American and Pacific plates. Locally, this boundary is referred to as the San

3 Sloan, Dorothy, 2006, Geology of the San Francisco Bay Region, University of California Press. 4 United States Geological Survey (USGS), 1983, Geologic Map of San Mateo County, California, Map I-1257- A. 5 USDA, 1991, Soil Survey of San Mateo County, Eastern Part, and San Francisco County, California, May.

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Andreas Fault Zone (SAFZ), which includes the San Andreas Fault and numerous other active faults.

The SAFZ includes active faults found by the California Geological Survey under the Alquist-Priolo Earthquake Fault Zoning Act (APEFZA) to be “active” (i.e., to have evidence of fault rupture in the past 11,000 years).6 Some of the major regional active faults within the SAFZ include the San Andreas, Hayward, Rodgers Creek, Calaveras, San Gregorio-Seal Cove, Maacama, West Napa, Green Valley, Concord, Greenville, and Calaveras faults. The closest active fault to the plan area is the San Andreas Fault, located about 2,000 feet southwest of the southwestern plan area boundary. The inactive Pilarcitos Fault runs almost parallel to the San Andreas Fault about two miles west of the plan area. The San Gregorio-Seal Cove, an active fault, is located about 9.5 miles west of the plan area. Regional active faults within the vicinity of the plan area are shown on Figure 4.6-1.

In a fact sheet published in 2003, the U.S. Geological Survey estimated that there was a 62 percent probability that between 2003 and 2032, a 6.7 or greater magnitude earthquake will occur in the San Francisco Bay Region. The probability of a 6.7 magnitude or greater earthquake occurring along individual faults was estimated to be 21 percent along the San Andreas Fault, ten percent along the San Gregorio Fault, 27 percent along the Hayward- Rodgers Creek Fault, and 11 percent along the Calaveras Fault. 7

Seismic Hazards Surface Rupture

Surface rupture occurs when the ground surface is broken due to fault movement during an earthquake. The location of surface rupture generally can be assumed to be along an active major fault trace. The active San Andreas Fault is oriented roughly parallel to the southwestern plan area boundary, with a local splay, known as the Cañada Fault, just southwest of the plan area. The easternmost edge of the Alquist-Priolo Earthquake Fault Zone for this fault is located approximately 2,000 feet southwest of the plan area, near Cañada College and I-280, as shown on Figure 4.6-2.8 Areas within the Alquist-Priolo Earthquake Fault Zone require special studies to evaluate the potential for surface rupture to ensure that no structures intended for human occupancy are constructed across an active fault.9

6 Alquist-Priolo Earthquake Fault Zoning Act of 1972, California Public Resources Code, Division 2, Chapter 7.5. 7 USGS, 2003, Earthquake Probabilities in the San Francisco Bay Region: 2002 to 2032 – A Summary of Findings, Open File Report 03-214. 8 California Division of Mines and Geology (CDMG), 1974, State of California Special Studies Zones, Woodside Quadrangle, Official Map, July 1. 9 Alquist-Priolo Earthquake Fault Zoning Act of 1972, op cit.

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4.6-3 Plan Area P ila F rc au i lt to s

Santa Cruz Co.

Not to Scale

Source: ABAG 2005, Taming Natural Disasters: Multi-Jurisdicational Local Government Hazard Mitigation Plan for the San Francisco Bay Area, March. G

Redwood City I General Plan EIR Regional Fault Locations F 4.6-1 Geografika Consulting 06.09.09 H G U O L S

ER RG BU EIN ST

SLOU EW GH R C S K R CO

e u n e v A n o t r e h t A

RTH NO

Legend 1 inch equals 5,500 feet Feet 0 2,600 5,200

Source: CDMG, 1974, Special Studies Zones, Woodside Quadrangle, July 1 G

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Multiple potentially-active Quaternary faults cross the plan area. These faults have evidence of activity between 11,000 years and 1.6 million years ago, and are shown on Figure 4.6-2. The faults are shown as concealed or buried,10 and are not classified under the APEFZA to be active faults. The current version of the Alquist-Priolo mapping indicates that an APEFZA zone does not cross the plan area.11

Ground Shaking

Ground shaking is a general term referring to all aspects of motion of the earth’s surface resulting from an earthquake, and is normally the major cause of damage in seismic events. The extent of ground shaking is controlled by the magnitude and intensity of the earthquake, distance from the epicenter, and local geologic conditions. Magnitude is a measure of the energy released by an earthquake; it is assessed by seismographs.12

Intensity is a subjective measure of the perceptible effects of seismic energy at a given point and varies with distance from the epicenter and local geologic conditions. The Modified Mercalli Intensity Scale (MMI) is the most commonly used scale for measurement of the subjective effects of earthquake intensity (Table 4.6-1). Intensity can also be quantitatively measured using accelerometers (strong motion seismographs) that record ground acceleration at a specific location, a measure of force applied to a structure under seismic shaking. Acceleration is measured as a fraction or percentage of the acceleration under gravity (g).

The San Andreas Fault is considered capable of generating a magnitude 7.9 (MW) earthquake, which is similar to the 1906 San Francisco earthquake. A 7.2 (MW) magnitude event on the Peninsula portion of the San Andreas Fault or a 7.9 (MW) event on the entire San Andreas Fault could be capable of generating violent (MMI IX) to very strong (MMI VIII) seismic shaking in the plan area. Located across the Bay, the Hayward fault could produce a 6.5 (MW) event that could result in very strong (MMI VIII) to moderate (MMI VI) seismic shaking in the plan area.13

10 CGS, 2003, Digital Database of Faults from the Fault Activity Map of California and Adjacent Areas, September 29. 11 CGS, Interim Revision 2007, Fault-Rupture Hazards Zones in California, Special Publication 42, California Department of Conservation.

12 In the past, the common standard for measurement of magnitude (ML) by geologists and earthquake seismologists was the Richter Scale. However, due to limitations of the instrumentation used to measure Richter magnitude, moment magnitude (MW) is now commonly used to characterize seismic events. Moment magnitude is determined from the physical size (area) of the rupture of the fault plane, the amount of horizontal and/or vertical displacement along the fault plane, and the resistance of the rock type along the fault to rupture. The moment magnitude can be calculated following an earthquake or estimated for an expected earthquake if the fault rupture area and displacement and rock properties can be estimated accurately. Therefore, the magnitudes of expected earthquakes in the San Francisco Bay Area are reported as moment magnitudes. 13 Association of Bay Area Governments (ABAG), 2004, Earthquake Shaking Scenario Maps, accessed 4-29- 08 at: http://quake.abag.ca.gov.

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4.6-6 Redwood City New General Plan 4.6 Geology and Soils

Table 4.6-1 Modified Mercalli Intensity (MMI) Scale

MMI Scale Description

I Not felt except by a very few under especially favorable circumstances.

II Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.

III Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration like passing of truck. Duration estimated.

IV During the day felt indoors by many, outdoors by few. At night some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.

V Felt by nearly everyone, many awakened. Some dishes, windows, etc., broken; a few instances of cracked plaster; unstable objects overturned. Disturbances of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop.

VI Felt by all, many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight.

VII Everybody runs outdoors. Damage negligible in building of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures; some chimneys broken. Noticed by persons driving motor cars.

VIII Damage slight in specially designed structures; considerable in ordinary substantial buildings, with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Persons driving motor cars disturbed.

IX Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken.

X Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed (slopped) over banks.

XI Few, if any, (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly.

XII Damage total. Practically all works of construction are damaged greatly or destroyed. Waves seen on ground surface. Lines of sight and level are distorted.

Source: California Geological Survey, 2002, How Earthquakes and Their Effects are Measured, Note 32.

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4.6-7 Redwood City New General Plan 4.6 Geology and Soils

Estimates of the peak ground acceleration have been made for the plan area based on probabilistic models that account for multiple seismic sources. Under these models, consideration of the probability of expected seismic events is incorporated into the calculated prediction of the level of ground shaking at a particular location. The expected peak horizontal acceleration (with a ten percent chance of being exceeded in the next 50 years) generated by any of the seismic sources potentially affecting the plan area is estimated by the California Geological Survey at 60 percent to 80 percent of the acceleration of gravity (g), with greater acceleration closer to the San Andreas fault (ie, in western portions of the plan area).14

Liquefaction and Lateral Spreading

Liquefaction is the temporary transformation of loose, saturated granular sediments from a solid state to a liquefied state as a result of seismic ground shaking. In the process, the soil undergoes temporary loss of strength, which commonly causes ground displacement or ground failure to occur. Since saturated soils are a necessary condition for liquefaction, soil layers in areas where the groundwater table is near the surface have higher liquefaction potential than those in which the water table is located at greater depths.

The San Mateo County Hazards Mitigations maps indicate that the lowland areas of the City have a moderate to high potential for liquefaction.15 Regional liquefaction hazard mapping indicates that a 7.2 (MW) magnitude event on the Peninsula portion of the nearby active San Andreas Fault or a 7.9 (MW) event on the entire San Andreas Fault could result in moderate to high liquefaction hazard in the lowland portions of the City.16 High to very high liquefaction susceptibility was mapped in 1998 in the tidal flat and coastline areas of the City, where a sand boil, formed by liquefaction during the 1989 Loma Prieta earthquake, was reported in Corkscrew Slough.17 Recent mapping shows moderate to very high liquefaction susceptibility in the tidal flat and coastline areas based on soil type.18

14 CGS, 2003, Probabilistic Seismic Hazards in California, based on USGS/CGS Probabilistic Seismic Hazards Assessment (PHSA) Model, accessed 4/30/08 at: http://redirect.conservation.ca.gov/cgs/rghm/pshamap/psha12237.html. 15 San Mateo County Hazards Mitigation Maps, 2005, Earthquake Liquefaction: San Andreas Fault – Peninsula accessed 4-30-08 at http://www.co.sanmateo.ca.us/smc/department/home/0,,555771_5558929_436489912,00.html. 16 ABAG, 2001, Liquefaction Hazard Map, interactive mapping tool accessed 4/30/08 at http//: quake.abag.ca.gov. 17 USGS, 2000, Preliminary Maps of Quaternary Deposits and Liquefaction Susceptibility, Nine-County San Francisco Bay Region, California, accessed 4/29/08 at http://pubs.usgs.gov/of/2000/of00-444/, and USGS, 1998, The Loma Prieta, California, Earthquake of October 17, 1989 –Liquefaction, USGS Professional Paper 1551-B. 18 USGS, 2006, Maps of Quaternary Deposits and Liquefaction Susceptibility in the Central San Francisco Bay Region, California, Open-File Report 06-1037, accessed 4/30/08 at http://earthquake.usgs.gov/regional/nca/qmap/.

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4.6-8 Redwood City New General Plan 4.6 Geology and Soils

The Seismic Hazards Mapping Act (SHMA) requires site-specific geotechnical investigations within the Zones of Required Investigation to identify the seismic hazard and formulate mitigation measures prior to permitting developments designed for human occupancy.19 While Seismic Hazard Zone Maps have not been developed for the majority of the City, a Seismic Hazard Zone Map has been prepared for the eastern portion of the City. This map for the eastern portion of the City shows a liquefaction investigation zone at the intersection of Woodside Road and El Camino Real, with the Seismic Hazard Zone extending along stream beds and northerly into the tidal flats and salt crystallization ponds. For the remainder of the City, the U.S. Geological Survey is currently preparing Seismic Hazard Zone maps for the Redwood Point and San Mateo quadrangle. Mapping of a portion of the Woodside quadrangle in also planned.20

Lateral spreading is a form of horizontal displacement of soil toward an open channel or other “free” face, such as an excavation boundary. Lateral spreading can result from either the slump of low cohesion and unconsolidated material or more commonly by liquefaction of either the soil layer or a subsurface layer underlying soil material on a slope, resulting in gravitationally-driven movement.21 Earthquake shaking leading to liquefaction of saturated soil can result in lateral spreading where the soil undergoes a temporary loss of strength. Portions of the plan area are highly susceptible to liquefaction hazards, indicating that lateral movement to an open face is possible.

Geologic Hazards Soil Erosion

Soil erosion is a natural process that can be caused by wind or water. Eroded soils can be entrained in storm water runoff and be discharged to surface waters, thereby affecting the water quality of receiving waters. Stormwater runoff quality both during and after construction is regulated by the National Pollutant Discharge Elimination System (NPDES) program (established through the Federal Clean Water Act); the NPDES program objective is to control and reduce pollutants discharges to surface water bodies. In California, the NPDES program is administered by the State Water Resources Control Board (State Board), with local oversight provided by the Regional Water Quality Control Boards (RWQCBs). Redwood City is a co-permittee under the San Mateo Countywide Water Pollution Prevention Program under the San Francisco Bay Area RWQCB.

19 Seismic Hazards Mapping Act (SHMA) of 1990, California Public Resources Code, Chapter 7.8, Section 2690-2699.6. 20 CGS Seismic Hazard Mapping Act information reviewed online at http://gmw.consrv.ca.gov/shmp/html/pdf_maps_no.html on 27 May 2008. 21 Rauch, Alan F., 1997, EPOLLS: An Empirical Method for Predicting Surface Displacements due to Liquefaction-Induced Lateral Spreading in Earthquakes, Ph. D. Dissertation, Virginia Tech, Blacksburg, VA.

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4.6-9 Redwood City New General Plan 4.6 Geology and Soils

Expansive Soils

Expansion and contraction of volume can occur when expansive soils undergo alternating cycles of wetting (swelling) and drying (shrinking). During these cycles, the volume of the soil changes markedly. As a consequence of such volume changes, structural damage to building and infrastructure may occur if the potentially expansive soils were not considered in project design and during construction.

The Novato and Reyes Series soils in the lowland portions of Redwood City are predominately clays and silty clays with high shrink-swell potential. The Fagan Series soils in the upland areas also have a high percentage of clay and moderate to high shrink- swell potential. Clay and associated materials can result in weak, compressible, or expansive soils. These soils are classified as expansive soils.22

Slope Stability

Slope failure can occur as either rapid movement of large masses of soil (“landslide”) or slow, continuous movement (“creep”). The primary factors influencing the stability of a slope are: 1) the nature of the underlying soil or bedrock; 2) the geometry of the slope (height and steepness); 3) rainfall; and 4) the presence of previous landslide deposits. A few areas with slides and earthflows are mapped in the southwest hills of Redwood City. Most of the hills in the southwest portion of Redwood City are mapped as “few or very few landslides”. The lower elevations in the eastern and northeastern portion of Redwood City are mapped as “flatland” with no landslides.23

The Seismic Hazard Zone map completed for a portion of east central Redwood City shows a few Zones of Required Investigation due to landslides near Woodside Road and I-280.24

Settlement and Differential Settlement

Differential settlement or subsidence could occur if buildings or other improvements were built on low-strength foundation materials (including imported fill) or if improvements straddle the boundary between different types of subsurface materials (e.g., a boundary between native material and fill). Although differential settlement generally occurs slowly enough that its effects are not dangerous to inhabitants, it can cause significant building damage over time. Portions of the plan area that contain loose or uncontrolled (non- engineered) fill may be susceptible to differential settlement. Portions of the plan area located in former tidal flats are expected to be susceptible to settlement due to low- strength native soils and potential unconsolidated fill, and differential settlement in areas where fill abuts native soil.

22 USDA, 1991, op cit. 23 ABAG, Summary Distribution of Slides and Earth Flows in the San Francisco Bay Region, based on USGS, 1997, Open File Report 97-745 E, accessed 4/28/08 at www.quake.abag.ca.gov. 24 CGS, 2006, op cit.

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4.6-10 Redwood City New General Plan 4.6 Geology and Soils

Subsidence and Collapse

Subsidence can occur in areas where the subsurface materials such as limestone rock or salt deposits are dissolved by fluid flow, creating subsurface voids that can collapse. Subsidence can occur where groundwater or natural gas is extracted, and soil grains compact. Surface rupture of faults with vertical offset will result in relative subsidence on one side of the fault. Mining operations create subsurface tunnels which could collapse, possibly resulting in ground surface subsidence.

Decomposition of highly organic soils and seasonal drying of expansive clay soils can result in subsidence which could damage buildings. Organic and expansive soils in the plan area are subject to subsidence.

Tsunami

Large earthquakes can generate seismic sea waves, or tsunamis, which can cause damage along the coastline. Redwood City is located about 10 miles east of the Pacific Ocean shoreline, and is not within the County of San Mateo Tsunami Evacuation Planning area.25

4.6.2 REGULATORY SETTING

Seismic Hazards Alquist-Priolo Earthquake Fault Zoning Act

The Alquist-Priolo Earthquake Fault Zoning Act regulates development in California near known active faults due to hazards associated with surface fault ruptures. The Earthquake Fault Zones indicate areas with potential surface fault-rupture hazards, and specific geological investigations are required prior to development.

Seismic Hazard Mapping Act

The Seismic Hazard Mapping Act was passed in 1990 following the Loma Prieta earthquake to reduce threats to public health and safety and to minimize property damage caused by earthquakes. The act directs the U.S. Department of Conservation to identify and map areas prone to the earthquake hazards of liquefaction, earthquake-induced landslides, and amplified ground shaking. The act requires site-specific geotechnical investigations to identify potential seismic hazards and formulate mitigation measures prior to permitting most developments designed for human occupancy within the Zones of Required Investigation.

25San Mateo County Hazards Mitigations Maps, 2005, Tsunami Evacuation Planning, accessed online 27 May 2008 at http://www.co.sanmateo.ca.us/vgn/images/portal/cit_609/30/38/436489908tsunami.pdf

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4.6-11 Redwood City New General Plan 4.6 Geology and Soils

The Seismic Hazard Map for the Palo Alto Quadrangle has been completed, which covers the southern and central portions of Redwood City; maps for other portions of the City are currently under preparation. Areas where the potential for liquefaction and earthquake- induced landslides have been identified are shown on Figure 4.6-3.

California Building Codes

Redwood City enforces the 2007 California Building Codes (CBC) and requires all development within the City to comply with the most current CBC standards. The CBC covers grading and other geotechnical issues, building specifications, and non-building structures. The CBC requires that a foundation and soil investigations report be prepared by a registered design professional for seismic design categories C, D, E, and F as defined by the CBC. The site-specific geotechnical report shall provide measures to reduce potentially significant seismic hazards such as surface fault ruptures, ground shaking, liquefaction, and seismically-induced slope failures and settlement, regardless of the proposed grading on slopes. The geotechnical report would be reviewed by City staff prior to issuance of building permits to ensure compliance.

Geologic Hazards California Building Code

The CBC requires that any required foundation and soils investigation report(s) be prepared by a registered professional to evaluate geologic and seismic hazards on proposed developments, as discussed above. The site-specific geotechnical report shall provide measures to reduce potentially significant geologic hazards, such as expansive and corrosive soils, differential settlement, and slope stability. The geotechnical report would be reviewed by City staff prior to issuance of building permits.

Project Consistency Analysis The New General Plan will require proponents of future development projects within the plan area to provide project-specific assessments performed by state-licensed geologists and specialists to identify potential seismic and geologic hazards and incorporate recommended mitigation measures into proposed projects, per Programs PS-23 and PS- 24 of the New General Plan. Additionally, Program PS-25 would require the City to implement the International Building Code seismic safety standards, consistent with CBC. Thus, all future permitted developments within the plan area will be designed based on the most recent state seismic requirements and building codes. These measures will ensure the reduction of potential risks to people and property resulting from seismic and geologic hazards.

With regard to consistency with the Alquist-Priolo Earthquake Fault Zoning Act and the Seismic Hazard Mapping Act, the New General Plan includes several policies and implementation programs that would identify areas sensitive to seismic and related

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4.6-12 Legend Source: California Department of Conservation, California Geological Survey, October 18, 2006, Seismic Hazard Zones, Palo Alto Quad. Zones of Required Investigation: Mapped by Hogle Ireland Inc., 2009. Liquefaction Areas where historic occurrence of liquefaction, or local geological, City Boundary geotechnial and groundwater conditions indicate a potential for Sphere of Influence permanent ground displacements such that mitigation as defined Freeway/Highway in Public Resources Code Section 2593(c) would be required. Major Roads Notes: Railroad This map is intended for general and use use planning only. ORTH Waterways Information on this map is not sufficient to serve as a N Areas where previous occurrence of landslide movement, or local substitute for detailed geologic investigations of individual topographic, geological, geotechnical and subsurface water USGS Quadrangle sites, nor does it satisfy the evaluation requirements set conditions indicate a potential for permanent ground displacements Index Boundary forth in geologic hazards regulations. such that mitigations as defined in Public Resources Code Section 2693(c) would be required. 1 inch equals 4,800 feet G

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hazards and abate or modify such hazards, including Policies PS-6.1 through PS-6.3 and PS-9.1. The New General Plan would therefore be consistent with the applicable policies relevant to geology, soils, and seismicity.

4.6.3 THRESHOLDS OF SIGNIFICANCE

Redwood City has not established local CEQA significance thresholds as described in Section 15064.7 of the State CEQA Guidelines. Therefore, significance determinations utilized in this Section are from Appendix G of the CEQA Guidelines. A significant geologic impact could occur if development allowed by the New General Plan would:

a) Expose people or structures to potential substantial adverse effects, including the risk of loss, injury, or death involving:

i. Rupture of a known earthquake fault, as delineated on the most recent Alquist-Priolo Earthquake Fault Zoning Map issued by the State Geologist for the area or based on other substantial evidence of a known fault. Refer to Division of Mines and Geology Special Publication 42.

ii. Strong seismic ground shaking.

iii. Seismic-related ground failure, including liquefaction.

iv. Landslides.

b) Result in substantial soil erosion or the loss of topsoil.

c) Be located on a geologic unit or soil that is unstable, or that would become unstable as a result of the project, and potentially result in on- or off-site landslide, lateral spreading, subsidence, liquefaction or collapse.

d) Be located on expansive soil, as defined in Table 18-1-B of the Uniform Building Code (1994), creating substantial risks to life or property.

e) Have soils incapable of adequately supporting the use of septic tanks or alternative waste water disposal systems where sewers are not available for the disposal of waste water.

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4.6-14 Redwood City New General Plan 4.6 Geology and Soils

4.6.4 ENVIRONMENTAL IMPACTS AND MITIGATION MEASURES

Issues Not Discussed Further Fault Rupture

The plan area is not located within an Alquist-Priolo Earthquake Fault Zone. An Alquist- Priolo zone is located about ½ mile west of the plan area, roughly paralleling and west of I-280. The plan area is subject to a number of other potential seismic hazards, as discussed under Impact 4.6-1, but the likelihood of damages resulting from fault rupture within the plan area are considered remote and are therefore not discussed further.

Septic Systems

The plan area is served by a sanitary sewer system (described in Section 4.15, Utilities and Service Systems). No septic systems would be anticipated to be developed under the New General Plan, so this issue is not discussed further.

Project Impacts Impact 4.6-1: The plan area is subject to a number of potential seismic hazards, including ground shaking, landslides, liquefaction, and lateral spreading that would affect future development allowed under the New General Plan. (Less than Significant with Mitigation)

The plan area is within a seismically active region, where a number of seismic hazards may occur. The New General Plan would allow for an incremental increase in the amount of residential and commercial development in the plan area. Specifically, under the New General Plan, a total of approximately 9,103 residential units and 7.3 million square feet of non-residential development could occur by the horizon year of 2030. Adoption of the New General Plan could therefore result in additional people and structures being exposed to seismic risks and related hazards such as liquefaction and slope instability. Particularly in regards to liquefaction, implementation of the Mixed Use – Waterfront Neighborhood in the Bayfront area along the Redwood Creek channel would introduce new structures and individuals (residents, employees, and visitors) to an area with high liquefaction susceptibility due to the types of soils and presence of water. Some of these geohazards, particularly those related to seismic shaking, could result in injuries and/or fatalities. All of the geohazards discussed could result in damage to structures and property. Furthermore, the potential for sea level rise could increase the risk of liquefaction in the Bayfront areas, which are most vulnerable to inundation or water table increase in the event of global sea level rise. Please refer to Section 4.16, Global Climate Change for a discussion of sea level rise and its potential environmental effects to the plan area.

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4.6-15 Redwood City New General Plan 4.6 Geology and Soils

Existing federal and state programs, described in Section 4.6.2, are designed to provide accurate and timely information detailing seismic hazards, impose regulatory requirements regarding geotechnical and soils investigations, provide limitations on the locations of structures for human habitation, impose requirements for hazard notices to potential users, and establish structural standards for requirements for buildings and grading projects.

In addition, the Public Safety Chapter of the New General Plan includes policies and implementation programs that would identify areas particularly sensitive to seismic and related hazards and abate or modify such hazards. See Policies PS-6.1, PS-6.2, PS-6.3, and PS-9.1 of the New General Plan, all of which are listed in Appendix A. These policies would work to provide public information regarding seismic safety, and improve inter- jurisdictional cooperation and communication regarding seismic safety upgrades of public facilities and infrastructure.

Programs PS-23 through PS-26, PS-35, PS-36, PS-44, and PS-50 would also include implementation programs that require CEQA analyses for future developments in the plan area and to provide mitigation for seismic hazards; ensure that appropriate emergency strategies are in place in the event of an earthquake, including the preparation of geologic hazard abatement district maps for soils and seismic safety, as well as making information regarding seismic events readily available to City and County emergency responders.

Compliance with the regulatory requirements and New General Plan policies and implementation programs would ensure that future development projects are appropriately investigated in terms of potential seismic hazards and that any new buildings and structures are constructed to withstand the anticipated range of seismic events. At the programmatic level, seismic impacts would be reduced to a less than significant level. Consistent with the New General Plan policies and implementation programs, individual development projects would be required to undergo project-specific environmental review, which may generate additional mitigation measures to reduce any significant seismic impacts.

Notwithstanding the above, there is the possibility that project-specific geotechnical studies indicate that compliance with and application of these measures could not adequately mitigate potential seismic hazards, in which case additional mitigation would be required, as per Mitigation Measure 4.6-1.

Mitigation Measure 4.6-1: On sites where threats from seismic hazards cannot be adequately mitigated through the application of existing regulatory requirements and New General Plan policies and implementation programs, the City shall use open space easements and/or other related regulatory measures to limit development and thus avoid public safety hazards.

Significance After Mitigation: Implementation of Mitigation Measure 4.6-1 would reduce impacts from seismic activities to a less than significant level.

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4.6-16 Redwood City New General Plan 4.6 Geology and Soils

Impact 4.6-2: The plan area is subject to a number of geologic and soils- related hazards, including soil erosion, expansive soils, slope stability, differential settlement, and subsidence that would affect future development allowed under the New General Plan. (Less than Significant)

Future development allowed under the New General Plan would likely be subject to a variety of geologic hazards, including expansive soils, slope stability, differential settlement, and subsidence. The City’s continued application of the CBC, which requires a site-specific geotechnical report for proposed developments, and implementation of measures identified in the report to address these hazards, would mitigate these potential hazards.

In addition, Programs PS-24 and PS-25, as detailed in Appendix A, would require a geotechnical analysis and would commit the City to implementing and updating building codes as needed. Other New General Plan implementation programs, including Programs PS-29, PS-35, PS-36, and PS-44, would establish Geologic Hazard Abatement Districts, implement the mapping of geohazards, maintain and update an emergency response preparedness plan, and maintaining funding for emergency response teams would further reduce the severity of potential geologic impacts.

Soils that are disturbed as part of redevelopment under the New General Plan would be susceptible to erosion. Compliance with erosion control measures, as required by the NPDES program described in Section 4.6.1, would reduce the potential impacts related to soil erosion to a less than significant level. Refer to Section 4.8, Hydrology and Water Quality, for further discussion regarding erosion control requirements.

Adherence to the above listed New General Plan policies and implementation programs would effectively reduce program level impacts related to geologic and soils-related hazards to a less than significant level. No additional mitigation is required. Individual developments within the plan area will be required to undergo a project-specific environmental review. If project-level impacts are identified and are found to be significant, CEQA requires that these impacts be mitigated.

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4.6-17 Redwood City New General Plan 4.6 Geology and Soils

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4.6-18