4.7 GEOLOGY AND SOILS
This section describes the geology of the Planning Area and analyzes issues such as potential
exposure of people and property to geologic and soil hazards. In addition, potential seismic
hazards such as ground shaking and ground failure from earthquakes, as well as differential
settlement on bay fill, are discussed.
4.7.1. EXISTING SETTING
TOPOGRAPHY AND LOCAL GEOLOGY
The City of San Mateo encompasses a variety of upland, hillside, valley, and alluvial fan land
forms. The city is situated along the northeasterly flank of the central Santa Cruz Mountains but is
separated from the range both geologically and topographically by the San Andreas fault and
its associated rift valley. The bedrock types that underlie the City are different from most of those
found to the southwest across the San Andreas fault.
GEOTECHNICAL CONDITIONS
Bedrock types in San Mateo comprise the sedimentary-volcanic-metamorphic rocks of the
Fransican Formation and the conglomerate, sandstone and minor mudstone of the Santa Clara
formation. The rocks exist in masses having the form of blocks, slices, and wedges, which are, for
the most part, separated by long inactive faults. The broad, rolling upland area which
characterizes much of the western hills of San Mateo is part of an extensive uplifted surface
which is scarred by the drainage canyons of Laurel and San Mateo Creeks. To the northeast, soil
conditions are that of bay mud and imported fill material. These soil conditions roughly extend
from San Francisco Bay to areas near the Bayshore Freeway (US 101), representing the
approximate location of the shoreline in the San Mateo area before the Bay was filled.
GEOLOGIC HAZARDS, FAULTS, AND SEISMICITY
The severity of an earthquake can be expressed in terms of both intensity and magnitude. For
detailed descriptions of these terms, please see Table 4.7-1.
TABLE 4.7-1
MAGNITUDE AND INTENSITY
Magnitude
Intensity
1.0 – 3.0
I
Description
Not felt except by a very few under especially favorable conditions.
Felt only by a few persons at rest, especially on upper floors of buildings.
3.0 – 3.9
4.0 – 4.9
II – III
IV – V
Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many
people do not recognize it as an earthquake. Standing motor cars may rock slightly.
Vibrations similar to the passing of a truck. Duration estimated.
Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes,
windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking
building. Standing motor cars rocked noticeably.
Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects
overturned. Pendulum clocks may stop.
Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster.
Damage slight.
5.0 – 5.9
VI – VII
Damage negligible in buildings of good design and construction; slight to moderate in wellbuilt ordinary structures; considerable damage in poorly built or badly designed structures;
some chimneys broken.
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Magnitude
6.0 – 6.9
7.0 and
higher
Intensity
VIII – IX
VIII or
higher
Description
Damage slight in specially designed structures; considerable damage in ordinary substantial
buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys,
factory stacks, columns, monuments, walls. Heavy furniture overturned.
Damage considerable in specially designed structures; well-designed frame structures thrown
out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted
off foundations.
Some well-built wooden structures destroyed; most masonry and frame structures destroyed
with foundations. Rails bent.
Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.
Damage total. Lines of sight and level are distorted. Objects thrown into the air.
The San Francisco Bay Area is recognized by geologists and seismologists as one of the most
seismically active regions in the United States. The significant earthquakes that occur in the Bay
Area are generally associated with crustal movement along well-defined, active, fault zones of
the San Andreas fault system, which regionally trend in a northwesterly direction. The San
Andreas fault, which generated the great San Francisco earthquake of 1906, is located
approximately 2 miles to the west of the City of San Mateo. The Hayward fault is located
approximately 14 miles northeast of the project area.
There are no known active faults or Alquist-Priolo earthquake hazard zones in the City of San
Mateo. The inactive faults which are present are older features which do not exhibit indications
of recent motion. There is no reason to expect a recurrence of movement along these fault
traces. Many of the geologic processes active in San Mateo are related to earthquake-induced
ground shaking, common to the area. The entire San Francisco Bay Area is occasionally
subjected to strong earthquakes that originate on the San Andreas, Hayward, and Calaveras
faults.
Ground Shaking
In populated areas, the greatest potential for loss of life and property damage is a result of
ground shaking from a nearby earthquake. The degree of damage depends on many
interrelated factors. Among these factors are the Richter magnitude, focal depth, distance from
the causative fault, source mechanism, duration of shaking, high rock accelerations, type of
surficial deposits or bedrock, degree of consolidation of surficial deposits, presence of high
groundwater, topography, and design, type, and quality of building construction.
The City of San Mateo could be affected by strong ground shaking caused by a major
earthquake during the next 30 years. This hazard is common to all development in the San
Francisco Bay Area. Ground shaking may affect areas hundreds of miles distant from the
earthquake’s epicenter. Historic earthquakes have caused strong ground shaking and damage
in the San Francisco Bay Area, the most recent being the 6.9 (moment magnitude) Loma Prieta
earthquake in October 1989. The epicenter for this event was approximately 40 miles southeast
of the project area; the earthquake caused strong ground shaking for about 20 seconds and
resulted in varying degrees of structural damage throughout the Bay Area. The areas that
experienced the worst structural damage due to the Loma Prieta earthquake were not those
closest to the fault, but rather those with soils that magnified the effects of ground shaking. Areas
that are underlain by bedrock tend to experience less ground shaking than those underlain by
unconsolidated sediments such as artificial fill. Alluvial-type soils have a moderate to high
potential of amplifying ground shaking during an earthquake. Damage to buildings and utilities
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is likely to be greatest on those sites underlain by deep, loose, compressible deposits such as bay
mud. These areas include the residential neighborhoods east of US 101 and the Mariner’s Island
Area.
No active or potentially active faults underlie the City of San Mateo based on published
geologic maps. The Planning Area is not located within an Alquist-Priolo Fault Study Zone and
surface evidence of faulting has not been observed. However, due to the proximity to the San
Andreas Fault Zone, Hayward Fault Zone, and other active faults, the Planning Area may
experience severe ground shaking during a seismic event.
Six dams affect the City of San Mateo: Crystal Springs, San Andreas, Laurel Creek and East Laurel
Creek (2), and Tobin Creek in Hillsborough.
Lower Crystal Springs Dam retains water supply for San Francisco and most cities within San
Mateo County. In 1977, the seismic safety of the dam was studied, finding that the risk of
structural damage to the dam with a maximum magnitude of 8.3 on the Richter scale
earthquake is low and that landslides which might be triggered by such an earthquake would
not generate waves capable of overtopping the dam. Although the probability of Lower Crystal
Spring Dam’s failure is remote, should such an event occur, the San Mateo Area Office of
Emergency Services (OES) estimates that a population of 70,000 would be affected, with
inundation occurring from the downtown area north to the Burlingame Recreation Lagoon and
south to the Ralston Avenue/US 101 interchange.
San Andreas Dam is located on San Andreas Creek in Burlingame and is also used to impound
water for San Francisco and much of San Mateo County. Seismic safety studies in 1979 and 1983
indicated that the dam would probably remain stable during strong seismic shaking.
Laurel Creek Dam is located at the end of Laurelwood Drive and reduces the peak stormwater
runoff of 600 cubic feet per second (cfs) in half. The most recent reports by the California Division
of Safety of Dams (DSOD) indicate that the dam is structurally safe and will perform without
failure during a major seismic event.
East Laurel Creek Dam is located at the end of East Laurel Creek Drive and is also used to
control peak storm runoff. The dam is too small to be regulated by DSOD, and its seismic stability
is unknown. Two other small dams are located in Belmont (East Laurel Creek) and in Hillsborough
(Tobin Creek).
Ground Failure
Ground failure is a secondary effect of ground shaking and can include landslides, liquefaction,
lurching, and differential settlement. Buildings can tilt or sink, utility lines can rise to the surface,
and levees can fail. If soils are poorly consolidated, the ground can subside.
Landslides
Problems of slope instability are most prevalent in the hillside areas where landsliding has
occurred previously and where landslide deposits can be found. Landslide failures have also
occurred in areas where slopes were modified by grading. Excavating too steeply, undercutting
slopes, or placing fills or structures on unstable slopes can potentially cause a landslide. This can
be seen along State Route (SR) 92 in the western hills and in the area north of East Laurel Creek
Road.
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Although City records verify past slope failures in areas of the western hills, instability is not
widespread in San Mateo. Generally, natural slope areas have demonstrated good stability in
the past. However, numerous slopes exhibiting instability due to overgrading may become
subjected to failure in the event of prolonged periods of heavy rainfall or during major
earthquakes. While landslides have caused property damage, no loss of life or of dwellings has
occurred. Generally, natural slope areas have demonstrated good stability in the past.
Liquefaction
Liquefaction is the loss of soil strength due to seismic forces generating various types of ground
failure. Liquefaction occurs when saturated and poorly consolidated granular material is shaken
during an earthquake and is transformed into a fluid-like state. The potential for liquefaction
must account for soil types and density, the groundwater table, and the duration and intensity
of ground shaking. If soils are poorly consolidated, the ground can subside. San Mateo faces the
potential for liquefaction of man-placed fill, bay mud sediments, and alluvium in areas built on
filled bay mud.
Lateral Spreading
Lateral spreading occurs as a response to earthquake-induced ground shaking and is the
movement of ground material toward a free face (i.e., a cliff or stream bank). Lateral spreading
typically occurs as a form of horizontal displacement of relatively flat-lying material toward an
open face such as an excavation, channel, or body of water. Generally in soils, this movement is
due to failure along a weak plane and may often be associated with liquefaction. As cracks
develop within the weakened material, blocks of soil displace laterally toward the open face.
Cracking and lateral movement may gradually propagate away from the face as blocks
continue to break free. Lateral spreading potential is highest in areas underlain by soft, saturated
materials, especially where bordered by steep banks or adjacent hard ground. Lateral
spreading is possible along the banks of water drainage courses that are not constrained in
concrete channels and/or by other protective measures.
Settlement and Expansive Soils
Settlement is the drop in elevation of a ground surface caused by settling or compacting of soils
under the weight of existing fill and any new fills or building loads. This settlement would generally
be gradual and can continue beyond the life of a project’s development. The amount of
settlement is primarily influenced by the thickness of the bay mud, the site history, the thickness
of the existing and proposed fills, and the layout and magnitude of any building loads.
Differential settlement is uneven settlement, where one part of a structure settles more or at a
different rate than another part. Differential settlement could be expected to occur in areas
where building loads supported on fill vary substantially.
Shrink and swell movements occur in fine-grained sediments containing expansive clays. Soils
containing high clay content often exhibit a moderate to high potential to expand when
saturated and contract when dried out. This shrink/swell movement can adversely affect
building foundations, often causing them to crack or shift, with resulting damage to the buildings
they support. However, there are relatively minor economic losses from these conditions, and
there are no risks to human life associated with the shrink/swell condition of clayey soils. Proper
foundation engineering can usually overcome this problem.
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Tsunamis and Seiches
Tsunamis are ocean waves generated by certain undersea earthquakes, volcanic eruptions, or
landslides, and seiches are waves created in closed bodies of water by geologic instability.
Potential for damage caused by tsunamis is considered low given the City is not directly
exposed to the open ocean.
4.7.2 REGULATORY FRAMEWORK
STATE
California Division of Mines & Geology
The Alquist-Priolo Earthquake Fault Zoning Act of 1972 (prior to January 1, 1994, called the
Alquist-Priolo Special Studies Zones Act – CCR, Title 14, Section 3600) sets forth the policies and
criteria of the State Mining and Geology Board that governs the exercise of governments’
responsibilities to prohibit the location of developments and structures for human occupancy
cross the trace of active faults. The policies and criteria are limited to potential hazards resulting
from surface faulting or fault creep within Earthquake Fault Zones delineated of maps officially
issued by the State Geologist. Working definitions include:
•
Fault – A fracture or zone of closely associated fractures along which rocks on one side
have been displaced with respect to those on the other side.
•
Fault Zone – A zone of related faults, which commonly are braided and sub parallel, but
may be branching and divergent. A fault zone has a significant (with respect to the scale
at which the fault is being considered, portrayed, or investigated), ranging from a few
feet to several miles.
•
Potentially Active Fault – A fault that showed evidence of surface displacement during
Quaternary time (last 1.6 million years) for the purpose of evaluation for possible zonation.
No longer used.
•
Sufficiently Active Fault – A fault that has evidence of Holocene surface displacement
along one or more of its segments or branches.
•
Well-Defined Fault – A fault whose trace is clearly detectable by a trained geologist as a
physical feature at or just below the ground surface. The geologist should be able to
locate the fault in the field with sufficient precision and confidence to indicate that the
required site-specific investigations would meet with some success.
“Sufficiently Active” and “Well Defined” are the two criteria used by the state to determine if a
fault should be zoned under the Alquist-Priolo Act.
California Building Code
The California Building Code (CBC) is another name for the body of regulations known as the
California Code of Regulations (C.C.R.), Title 24, Part 2, which is a portion of the California
Building Standards Code. Title 24 is assigned to the California Building Standards Commission,
which, by law, is responsible for coordinating all building standards. Under state law, all building
standards must be centralized in Title 24 or they are not enforceable.
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Published by the International Code Council (ICC), the International Building Code is a widely
adopted model building code in the United States. The California Building Code incorporates by
reference the International Building Code with necessary California amendments. About onethird of the text within the California Building Code has been tailored for California earthquake
conditions.
LOCAL
Association of Bay Area Governments
The Association of Bay Area Governments (ABAG) has adopted a Manual of Standards (May
1995) for Erosion & Sediment Control Measures, a comprehensive field guide for controlling soil
erosion in the area.
City of San Mateo Site Development Code
The City’s Site Development Code (Chapter 23.40 of the City of San Mateo Municipal Code)
establishes administrative procedures, regulations, required approvals, and performance
standards for site grading, construction on slopes, and removal of major vegetation. In general,
a planning application and a subsequent site development permit are required for
development where grading exceeds 5,000 square feet in area; grading exceeds a volume of
550 cubic yards; removal of major vegetation (trees over 6 inches in diameter) is proposed; and
construction is proposed on a slope of 15 percent or greater. The intent of the ordinance is to
protect public and private lands from erosion and earth movement, minimize the risk of injury to
persons and damage to property, and ensure that each development relates to adjacent lands
to minimize physical problems.
4.7.3 IMPACTS AND MITIGATION MEASURES
STANDARDS OF SIGNIFICANCE
The California Environmental Quality Act (CEQA) Guidelines (Appendix G) indicate that a
proposed project may have potentially significant geologic impacts if it results in any of the
following:
1) 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;
ii)
Strong seismic ground shaking;
iii) Seismic-related ground failure, including liquefaction;
iv) Landslides.
2) Result in substantial soil erosion or the loss of topsoil.
3) 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.
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4) 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.
5) Have soils incapable of adequately supporting the use of septic tanks or alternative
wastewater disposal systems where sewers are not available for the disposal of
wastewater.
It should be noted that development in the City of San Mateo is required to utilize the sanitary
sewer system and does not provide waste disposal through a septic system. The City of San
Mateo is not exposed to the open Pacific Ocean, and tsunamis historically have resulted in little
damage around the San Francisco Bay. Additionally, the only areas in the Bay Area that have
risk analysis for tsunamis are the Pacific Ocean side of San Francisco and San Mateo County.
METHODOLOGY
This impact analysis focuses on potential effects to geologic resources and soils associated with
seismic events in the City of San Mateo area. The evaluation was conducted based on current
information regarding the geologic, soil, and seismic characteristics of the City, and the review
of regional reports prepared by the City of San Mateo, the State of California, and various
consultants. The analysis is focused on the potential for new development and redevelopment
from the implementation of the General Plan Update to be exposed to geological and seismic
hazards.
PROJECT IMPACTS AND MITIGATION MEASURES
Seismic Ground Shaking
Impact 4.7.1
Land uses and development under the proposed City of San Mateo General
Plan Update may expose additional people, structures, and development to
ground shaking as a result of earthquakes resulting in the risk of loss, injury, or
death. This is considered a less than significant impact.
The hazards related to ground shaking include the risk of loss, injury, or death. Buildings that were
constructed in the City prior to modern seismic design standards, including unreinforced
masonry (URM) buildings that have not been seismically retrofitted, are most likely to have some
structural failure or may potentially collapse in a seismic event. In 1989, the City's Building
Inspection Division identified 25 unreinforced masonry structures which may be seismically
hazardous. As of August 2008, seismic retrofitting has been completed on 23 of the 25
unreinforced masonry buildings to increase stability in an earthquake. The remaining two
unreinforced masonry buildings are expected to be seismically retrofitted by the end of 2009.
Buildings that have been seismically retrofitted would have a decreased chance of failure.
However, even structurally enhanced buildings and newer buildings could still experience
significant damage and present a hazard to occupants.
The Working Group on California Earthquake Probabilities (WG99) has estimated that there is a
70 percent probability of at least one magnitude 6.7 or greater earthquake before 2030 within
the San Francisco Bay Region (USGS, 1999). Smaller magnitude earthquakes (between
magnitudes 6.0 and 6.7), capable of considerable damage depending on proximity to urban
areas, have about an 80 percent chance of occurring in the San Francisco Bay Area by 2032. A
large earthquake in the San Francisco Bay Area would have a regional effect and could impact
the future development and land uses that would occur in San Mateo regardless of the
adoption of the proposed General Plan Update. The largest areas where greater shaking
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damage is anticipated are near the Bay and the Mariner’s Island area. Deeper, unconsolidated
alluvial deposits occupy these areas, which is underlain by saturated, estuarine deposits,
including the very weak compressible bay muds. Deep, unconsolidated deposits associated
with fill are subject to higher amplitude, longer duration shaking motions (ground shaking
amplification), which can cause more damage to improvements than those sited on firmer,
shallower deposits.
Deep unconsolidated deposits have greater potential for stronger earthquake shaking. The CBC
provides for more stringent earthquake-resistant design parameters for such areas. Thus, while
these shaking impacts are potentially more damaging, they also will tend to be reduced in their
structural effects due to CBC criteria that recognize this potential. This includes provisions for
buildings to structurally survive an earthquake without collapsing, including such measures as
anchoring to the foundation and structural frame design.
Based on geologic maps, historic records, and aerial photographs of the City of San Mateo, no
active or potentially active faults underlie the area and the City is not located within an AlquistPriolo Fault Study zone. However, the City is relatively close to known active faults, such as the
San Andreas Fault Zone, and the Hayward Fault Zone. The intensity from ground shaking has the
potential to cause significant damage. Additionally, ground failure, including landslides,
liquefaction, and differential settlement, are all potential hazards in the project area, especially
those areas built on fill.
Geotechnical investigations are required for all new development as required in the CBC,
including multi-family residential and non-residential developments, and single-family dwellings
over two stories in height. Geotechnical investigations are also required on sites that are
identified in the General Plan Update Safety Element as having moderate or high potential for
ground shaking and/or ground failure. Mitigation for new construction often includes installation
of deep foundation support piers (anchored to bedrock), installation of appropriate drainage
improvements around a structure, and seismic design pursuant to the CBC.
General Plan Policies
The following policy provisions are proposed in the General Plan Update to address seismic
hazards:
S 1.1:
Geologic Hazards. Require site specific geotechnical and engineering studies,
S 1.4:
Unreinforced Masonry Buildings. Maintain the program which requires mandatory
subject to the review and approval of the City Engineer and Building Official, for
development proposed on sites identified in Figure S-1 as having moderate or high
potential for ground failure. Permit development in areas of potential geologic
hazards only where it can be demonstrated that the project will not be endangered
by, nor contribute to, the hazardous condition on the site or on adjacent properties.
modifications of existing unreinforced masonry buildings identified as being
potentially hazardous, and similar unsafe building conditions, to reduce the
associated life safety hazards. The mandatory structural modifications should be
designed to be in character with the existing architectural style.
Implementation of the above policy provisions and continued implementation of City
requirements regarding geotechnical investigations and compliance with CBC standards would
reduce the potential hazards associated with seismic ground shaking and geological stability to
an acceptable level and result in a less than significant impact.
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Mitigation Measures
None required.
Ground Failure
Impact 4.7.2
Implementation of the proposed General Plan Update may expose
additional people, structures, and development to seismic-related ground
failures, including lateral spreading, lurching, and liquefaction, as well as
potential failure of flood protection features resulting in the risk of loss, injury, or
death. This is considered to be a less than significant impact.
The topography of San Mateo is varied, from lowland areas to steeply sloping hillsides. Most of
San Mateo’s western hills were subdivided during the 1950s and 1960s, some including slopes
over 25 percent grade. During this subdivision process, numerous lots were created which far
exceeded the minimum lot size standards and therefore could have been further subdivided.
However, most of these “remainder” parcels were left unsubdivided due to their steep slopes,
poor accessibility, and/or poor soil conditions. With the increasing demand for housing,
subdivision and development of the remainder parcels in the western hills has become feasible.
Development of steeply sloped property poses greater risks than relatively flat land. It typically
involves substantial grading and alteration of the existing topography, which may affect the
stability of adjoining property and alter local drainage patterns. If not properly engineered, a
new structure may crack and separate in the event of seismic activity or landslide.
Although City records verify past slope failures in areas of the western hills, instability is not
widespread in San Mateo. Generally, natural slope areas have demonstrated good stability in
the past. However, numerous slopes exhibiting instability due to overgrading may become
subjected to failure in the event of prolonged periods of heavy rainfall or during major
earthquakes. While landslides have caused property damage, no loss of life or of dwellings has
occurred.
Lateral spreading and liquefaction are all potential hazards within the City of San Mateo, as
discussed previously, due to development on weaker surficial deposits including fill materials and
bay mud. The seismically induced failure of levees and other embankments can occur due to
the direct failure of the embankment itself or due to seismic failure of the natural foundation
materials beneath the embankment, leading to failure of the overlying embankment structure.
The risk of flood protection features failure resulting from seismic shaking could be moderate or
higher. As identified in Section 4.8, Hydrology and Water Quality, the City has specific
requirements regarding the protection of development from flood hazards, consistent with
Federal Emergency Management Agency (FEMA) provisions.
Soil erosion and the resulting sedimentation of creeks and storm drains are natural processes
which can be greatly accelerated by human activities such as grading, vegetation clearing,
and poorly engineered drainage systems. The impacts of erosion and altered drainage patterns
can be most critical on land which has shown a tendency toward landsliding, where existing
erosion gullies are apparent, or on very steep slopes that have a 15 percent or greater grade.
This problem is most critical in the western hills and can be controlled through development
restrictions and engineering techniques.
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General Plan Policies
The following policy provisions are proposed in the General Plan Update to address ground
failure, including landslides and erosion.
C/OS 3.1: Hillside Development Principles. Minimize the impact of hillside development through
conformance with the City's Zoning and Site Development Codes, and by utilizing the
following principles:
a. Limit the development of steep slopes through conformance with City regulations
which consider slope in the determination of appropriate minimum lot area for
subdivisions and parcel maps, permitted floor area ratio (FAR), and density of
multi-family development.
b. Cluster development to preserve steep slopes as private or common open
spaces.
c. Preserve the form of the existing topography by limiting cuts and fills, and the
height and visibility of new development.
d. Comply with Safety Element Policies S 1.1, S 1.2, and S 1.3 regarding site stability.
C/OS 3.2: Low-Impact Development. Regulate the location, density, and design of
development throughout the City in order to preserve topographic forms and to
minimize adverse impacts on vegetation, water, and wildlife resources.
S 1.1:
Geologic Hazards. Require site specific geotechnical and engineering studies,
subject to the review and approval of the City Engineer and Building Official, for
development proposed on sites identified in Figure S-1 as having moderate or high
potential for ground failure. Permit development in areas of potential geologic
hazards only where it can be demonstrated that the project will not be endangered
by, nor contribute to, the hazardous condition on the site or on adjacent properties.
S 1.2:
Hillside Development Standards. Regulate hillside development consistent with the
City's Site Development Code and Open Space/Conservation Policy 3.1.
S 1.3:
Erosion Control. Require erosion control measures for all development sites where
grading activities are occurring, including those having landslide deposits, past
erosion problems, the potential for storm water quality impacts, or slopes of 15% or
greater which are to be altered. Control measures shall retain natural topographic
and physical features of the site if feasible.
City of San Mateo Municipal Code
The Site Development Code (Municipal Code Chapter 23.40) seeks to minimize risk of harm to
persons or property by requiring appropriate engineering and/or hydrologic studies and
recommendations for development located on slopes 15 percent or greater, or within a slope
setback area as defined in the Site Development Code. These slope setback areas also apply to
slopes adjacent to water features.
Implementation of the above policy provisions, in addition to the provisions of the CBC and City
development standards, would reduce the potential impacts associated with ground failure to
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less than significant. These codes and policies require site-specific geologic, hydrologic, or
seismic evaluations for development proposals that establish appropriate recommendations
and requirements to reduce potential hazards.
Mitigation Measures
None required.
Settlement and Subsidence
Impact 4.7.3
Land uses and new development or redevelopment under the proposed
General Plan Update may expose people, structures, and development to
the effects of ground settlement resulting in the risk of loss, injury, or death. This
impact is less than significant.
Subsidence and settlement result from the same physical processes. Settlement is usually
considered to occur within a relatively short time frame and within a small area, for instance on
the project scale. Subsidence takes place over a longer time frame and a broader regional
area. Subsidence/settlement can occur differentially; that is, one area or location subsides or
settles more than another. The results of subsidence/settlement, especially when it occurs
differentially, can be quite damaging.
Ground subsidence/settlement has two basic mechanisms: elastic settlement and consolidation.
Elastic settlement occurs from structures and other loads that cause deformation of the
subsurface soils. Elastic settlement from structures is usually minor and usually occurs during
construction or within the first few weeks after construction.
Longer-term ground subsidence requiring months to decades also occurs as a result of the
consolidation of natural surficial materials that are compressible. The bay mud that underlies the
eastern portion of the City near San Francisco Bay may experience subsidence. When fill or
structure loads are placed on these muds for development, flood control, or other purposes,
settlement can result. It is expected that fills previously placed on these deposits are likely
undergoing consolidation and settlement of the ground surface. Any new fill or structure loads
will induce new settlement in addition to any ongoing settlement. The time required to complete
consolidation of the bay mud depends on the thickness of the bay mud and distance to a
drainage layer (underlying sand lenses). The time required to complete settlement can range
from a few months to many decades.
Subsidence may result in flooding as ground levels are lowered, including the freeboard of flood
control levees. Subsidence can also cause damage to structures, utilities, and roadways from
differential settlement. Foundation and walls can crack and the structures tilt out of level.
Gravity-based utilities and storm drains can become inoperable due to differential settlement
that causes sag in the lines or slope reversal.
Detailed geotechnical investigation is required in order to reduce the amount of settlement to
acceptable levels. Geotechnical investigations are required for all new development as
required in the CBC, including multi-family residential and non-residential developments, and
single-family dwellings over two stories in height. In addition, geotechnical investigations are
required on sites that are identified in the General Plan Update Safety Element as having
moderate or high potential for ground shaking and/or ground failure such as areas underlain by
bay mud. Mitigation for new construction often includes installation of deep foundation support
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piers, installation of appropriate drainage improvements around a structure, and seismic design
pursuant to the California Building Code.
General Plan Policies
The following policy provisions are proposed in the General Plan Update to address
subsidence and settlement.
S 1.1:
Geologic Hazards. Require site specific geotechnical and engineering studies,
subject to the review and approval of the City Engineer and Building Official, for
development proposed on sites identified in Figure S-1 as having moderate or high
potential for ground failure. Permit development in areas of potential geologic
hazards only where it can be demonstrated that the project will not be endangered
by, nor contribute to, the hazardous condition on the site or on adjacent properties.
Implementation of the above policy provisions, as well as compliance with current City of San
Mateo Code provisions previously described, would reduce the potential impacts associated
with subsidence and settling for seismic and rainfall events to less than significant.
Mitigation Measures
None required.
Tsunamis and Seiches
Impact 4.7.4
Land uses and new development or redevelopment under the proposed
General Plan Update is not expected to expose substantial numbers of
people and structures to hazards associated with seismically induced
tsunamis and seiches. This is considered a less than significant impact.
As noted above, tsunamis are ocean waves generated by certain undersea earthquakes,
volcanic eruptions, or landslides, and seiches are waves created in closed bodies of water, such
as lakes, by geologic instability. Potential for damage caused by tsunamis is considered low
given the City of San Mateo is not directly exposed to the open ocean. Tsunamis are relatively
rare in California due to the lack of submarine earthquake faults. An Alaska-generated tsunami
would have to reach a height of at least 20 feet at the Golden Gate to overtop San Mateo’s
levees with a minimum runup of 5 feet at higher high tide. The highest tsunami affecting the area
during the last 120 years had a height of 7.4 feet at the Golden Gate, causing a 2-foot runup
along the San Mateo shoreline.
Mitigation Measures
None required.
4.7.4 CUMULATIVE SETTING, IMPACTS, AND MITIGATION MEASURES
CUMULATIVE SETTING
Geotechnical impacts relative to expansive soils and seismic hazards tend to be site-specific
rather than cumulative in nature, and each development site would be subject to, at a
minimum, uniform site development and construction standards relative to seismic and other
geologic conditions that are prevalent within the region. However, exposure of additional
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residents and structures to seismic and geologic hazards in the region would be a cumulative
impact.
Impacts regarding surficial deposits, however, namely erosion and sediment deposition, can be
cumulative in nature within a watershed. The reader is referred to Section 4.8, Hydrology and
Water Quality, regarding cumulative water quality impacts from soil erosion.
CUMULATIVE IMPACTS AND MITIGATION MEASURES
Geologic and Seismic Hazards
Impact 4.7.8
Implementation of the proposed General Plan Update along with potential
development in the region could result in cumulative impacts to geologic
and seismic hazards. This impact is considered less than cumulatively
considerable.
As identified under Impacts 4.7.1 and 4.7.2, implementation of the proposed General Plan
Update would contribute to cumulative geologic/seismic hazards as a result of continued
growth, new development, and redevelopment in a region that is seismically active. Impacts
4.7.1 and 4.7.2 identify geologic hazards associated with seismic events (e.g., ground shaking
and ground failure).
General Plan Policies and Implementation Programs
The following policy provisions proposed in the General Plan Update address cumulative seismic
hazards.
S 1.1:
Geologic Hazards. Require site specific geotechnical and engineering studies,
subject to the review and approval of the City Engineer and Building Official, for
development proposed on sites identified in Figure S-1 as having moderate or high
potential for ground failure. Permit development in areas of potential geologic
hazards only where it can be demonstrated that the project will not be endangered
by, nor contribute to, the hazardous condition on the site or on adjacent properties.
S 1.4:
Unreinforced Masonry Buildings. Maintain the program which requires mandatory
modifications of existing unreinforced masonry buildings identified as being
potentially hazardous, and similar unsafe building conditions, to reduce the
associated life safety hazards. The mandatory structural modifications should be
designed to be in character with the existing architectural style.
C/OS 3.1: Hillside Development Principles. Minimize the impact of hillside development through
conformance with the City’s Zoning and Site Development Codes, and by utilizing
the following principles:
a. Limit the development of steep slopes through conformance with City regulations
which consider slope in the determination of appropriate minimum lot area for
subdivisions and parcel maps, permitted floor area ratio (FAR), and density of
multi-family development.
b. Cluster development to preserve steep slopes as private or common open
spaces.
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c. Preserve the form of the existing topography by limiting cuts and fills, and the
height and visibility of new development.
d. Comply with Safety Element Policies S 1.1, S 1.2, and S 1.3 regarding site stability.
C/OS 3.2: Low-Impact Development. Regulate the location, density, and design of
development throughout the City in order to preserve topographic forms and to
minimize adverse impacts on vegetation, water, and wildlife resources.
S 1.2:
Hillside Development Standards. Regulate hillside development consistent with the
S 1.3:
Erosion Control. Require erosion control measures for all development sites where
City’s Site Development Code and Open Space/Conservation Policy 3.1.
grading activities are occurring, including those having landslide deposits, past
erosion problems, the potential for storm water quality impacts, or slopes of 15% or
greater which are to be altered. Control measures shall retain natural topographic
and physical features of the site if feasible.
City of San Mateo Municipal Code
The Site Development Code (Municipal Code Chapter 23.40) seeks to minimize risk of harm to
persons or property by requiring appropriate engineering and/or hydrologic studies and
recommendations for development located on slopes 15 percent or greater, or within a slope
setback area as defined in the Site Development Code. These slope setback areas also apply to
slopes adjacent to water features.
Implementation of above policy provisions, in addition to the provisions of the CBC, would
reduce the potential hazards associated with seismic ground shaking and ground failure. During
small and moderate seismic events, the impacts of seismic ground shaking would be reduced to
less than cumulatively considerable for new development and redevelopment consistent with
the General Plan Update. These codes and policies require site-specific geologic, hydrologic, or
seismic evaluations for development proposals that establish appropriate recommendations
and requirements to reduce potential hazards.
Mitigation Measure
None required.
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REFERENCES
California Division of Mines and Geology. 1987. Stinson, Melvin C. and Manson, M. Part
III: Mineral Land Classification: Aggregate Materials in the San FranciscoMonterey Bay Area: Classification of Aggregate Resource Areas: North San
Francisco Bay Production-Consumption Region. Special Report 146.
California Division of Mines and Geology. 1996. Probabilistic Seismic Hazard Assessment
for the State of California. Open-file Report 96-08.
City of San Mateo. 2009. Draft Safety Element of the General Plan.
City of San Mateo. 2009. Site Development Code (Municipal Code Chapter 23.40).
United States Geologic Survey (USGS). 1998. Geology of the Onshore Part of San Mateo
County, California: Derived from the Digital Database Open File 98-137.
United States Geologic Survey (USGS), Working Group on California Earthquakes. 1999.
Earthquake Probabilities in the San Francisco Bay Region: 2000 to 2030 – A
Summary of Finding.
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