Download 3.8 Geology/Soils - City of Pasadena

3.8 Geology/Soils
This section examines whether implementation of the 2004 Land Use Mobility Elements, Zoning
Code Revisions, and Central District Specific Plan the will expose people or structures to hazardous
geologic or seismic conditions.1
This chapter does not address the 2004 Mobility Element. At the local level, policies in the 2004
Mobility Element do not directly relate to geology and soils issues.
Environmental Setting
Topography
Pasadena is located in the northwestern portion of the San Gabriel Valley, an area characterized by
sharp contrasts in terrain. The northwestern valley is defined by the steeply rising range front of the
San Gabriel Mountains on the north, by highlands known as the San Rafael Hills on the west, and
by a string of east-trending knolls along the south. The most heavily populated area of Pasadena lies
on the gently sloping valley floor, while the unincorporated community of Altadena generally
separates most of Pasadena from the mountain front. The western edge of the City includes the
San Rafael Hills which, due to the terrain, have been developed with large-lot single-family homes.
The northeastern corner of the City reaches the base of the San Gabriel Mountains, encompassing
the lower part of Eaton Canyon and extending up to the mouth of Hastings Canyon.
Elevations in the valley range from about 560 feet above mean sea level at the southernmost point,
to about 1,280 feet near the point where the Arroyo Seco emerges from the mountain front. The
San Gabriel Mountains are part of the Transverse Ranges Province of Southern California. The
characteristic features that define this province are a series of predominantly east-west trending
mountain ranges and their intervening valleys. The highest point within the City limits is
approximately 1,600 feet above seal level.
Geology and Soils
Pasadena is located at the boundary between two of Southern California’s geomorphic provinces,
in an area that is being compressed by geologic forces associated with movement of the Pacific and
North American tectonic plates. Bounded by the San Andreas fault system on the north and the
Sierra Madre fault zone on the south, the mountains are essentially a large block of the Earth’s crust
that has been squeezed up and thrust over the valley floor by north-south compression along the
San Andreas tectonic plate boundary.
Displacements on faults at the northern edge of the Los Angeles Basin and the San Gabriel Valley
are mainly of the thrust or thrust-oblique type, causing older geologic units to be pushed up along a
series of faults that dip northward beneath the mountains they form. The most dramatic example of
1
The information presented in this section is drawn from the Technical Background Report to the Safety Element of the
General Plan prepared by Earth Consultants International, June, 2002.
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this in the Pasadena area is the Sierra Madre fault zone, which has thrust ancient crystalline rocks
onto and over younger sediments filling the valley.
The San Gabriel Valley is composed of alluvial fan sediments that have a range of ages coincident
with the rise of the San Gabriel Mountains. Because the fans were built up naturally with sediments
shed from the mountains, their composition reflects the rocks eroded by various streams. Most of
Pasadena is underlain by old alluvium consisting of unconsolidated gravel, sand, silt and clay
containing decomposed boulders of granitic rock from the mountains to the north and west that
form the upland portions of the City. The upper alluvium is usually a loose to medium dense silty
sand underlain by discontinuous beds of moderately dense sand and gravelly sand.
Seismicity
Pasadena is located in one of the most seismically active areas of Southern California. Several
seismic sources in the vicinity have the potential to cause strong seismic groundshaking. Figure 22
illustrates the geologic and seismic hazards in the Pasadena area. The main faults in the Pasadena
area include the Sierra Madre fault, a reverse fault that extends across the City’s northern boundary,
and the left-lateral strike-slip Raymond fault that locally extends into Pasadena’s southern and
eastern boundaries. The eastern and central sections of the Raymond fault are designated AlquistPriolo Earthquake Fault Zones by the California Geological Survey. This designation, under State
law, limits the types of construction and other activities that can occur within the fault zone. The
San Gabriel Fault, Eagle Rock Fault, and two other unnamed faults also traverse the City.
In addition to these faults, the Verdugo, Hollywood, Whittier, and Elysian Park fault zones are
located in and around Pasadena (Figure 22). The San Andreas Fault is located about 28 miles to the
northeast, and the Newport-Inglewood Fault runs approximately 18 miles southwest of the City.
The San Andreas Fault has the highest probability of causing a major earthquake, and although it is
located farther away from Pasadena, it has the potential to cause some damage within Pasadena.
Geologic Hazards from Seismic Groundshaking
Liquefaction
Liquefaction typically occurs in loose, saturated sediments consisting primarily of sandy composition
in the presence of ground accelerations caused by earthquakes. When liquefaction occurs, the
sediments involved have a total or substantial loss of shear strength and behave like a liquid or semiviscous substance. Three general conditions must be met for liquefaction to occur: (1) strong
seismic groundshaking of relatively long duration; (2) loose, or unconsolidated, recently deposited
sediments consisting primarily of silty sand and sand; and (3) water-saturated sediments within
about 50 feet of the surface.
In accordance with the Seismic Safety Mapping Act, the California Geological Survey has evaluated
the liquefaction susceptibility for the Pasadena area. Areas near existing stream channels, such as
Eaton Wash and the Arroyo Seco, may be especially vulnerable to liquefaction because these areas
contain loose, recently deposited sediments. Shallow groundwater also occurs in the canyons
emanating from the San Rafael Hills and along the Raymond Fault in the southernmost portion of
the City. Areas subject to liquefaction are shown in Figure 22.
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Seismically Induced Settlement
Strong groundshaking can cause the densification of soils, resulting in local or regional settlement of
the ground surface. During strong groundshaking, soil grains may become more tightly packed due
to the collapse of voids or pore spaces, resulting in a reduction in the thickness of the soil column.
This type of ground failure typically occurs in loose granular, cohesionless soils, and can occur in
either wet or dry conditions. Artificial fills may also experience seismically induced settlement.
Those portions of Pasadena that may be susceptible to seismically induced settlement are the
alluvial surfaces and larger drainages that are underlain by late Quarternary alluvial sediments,
similar to the liquefaction-susceptible areas. Sites near the base of the San Rafael Hills and along
the margins of the larger drainage channels may be particularly vulnerable.
Landslides
Strong ground motions can worsen existing unstable slope conditions, particularly if coupled with
saturated ground conditions. Although numerous types of earthquake-induced landslides have
been identified, the most widespread type generally consists of shallow failures involving surficial
soils and the uppermost weathered bedrock in moderate to steep hillside terrain. Rock falls and
rock slides on very steep slopes are also common.
Several areas in Pasadena have been identified as vulnerable to seismically induced slope failure
(see Figure 22). The mountainous region along the northern reaches of the City is susceptible to
slope failure due to the steep terrain. The crystalline bedrock that crops out in the northern and
central portions of the San Rafael Hills is highly fractured and weathered. In steep areas, strong
groundshaking can cause slides or rock falls of this material. Slope failures can also occur in the
southwestern portion of the City, where steep terrain is combined with weak sedimentary rock
units. Numerous small landslides can be expected to occur in these areas in response to an
earthquake on the Sierra Madre or other nearby faults. Oversteepened slopes along the large
drainage channels, such as the Arroyo Seco and Eaton Canyon, are also locally susceptible.
Thresholds Used to Determine Level of Impact
Implementation of the 2004 Land Use Element, Zoning Code Revisions, and the Central District
Specific Plan will result in a significant impact if land use and other policies:
ƒ
Risk exposure of people or structures to potential substantial adverse effects involving
strong seismic groundshaking or seismic-related ground failure, including landslides and
liquefaction
ƒ
Lead to developments that create substantial soil erosion or loss of topsoil
ƒ
Place people or structures on a geologic unit or soil that is unstable and potentially results in
on- or off-site landslide, lateral spreading, subsidence, liquefaction, or collapse
ƒ
Allow development on expansive soil
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Environmental Impact
Seismic Groundshaking
2004 Land Use Element, Zoning Code Revisions, and Central District Specific Plan
Pasadena is located in a seismically active area, as is all of Southern California. Therefore, the
adopted Safety Element includes the following goals and policies to minimize the impact of seismic
groundshaking:
Goal S-1
Minimize injury and loss of life, property damage, and other impacts caused by
seismic shaking, fault rupture, ground failure, earthquake induced landslides, and
other earthquake-induced ground deformation.
Policy S-1
The City will monitor development or re-development within the Fault Hazard
Management Zones identified for both the Sierra Madre and Raymond faults.
Policy S-2
The City will ensure that current geologic knowledge and State-certified professional
review are incorporated into the design, planning, and construction stages of a
project and that site-specific data are applied to each project.
Policy S-3
The City will strive to ensure that the design of new and the performance of existing
structures address the appropriate earthquake standards.
Policy S-4
The City will ensure the fullest extent possible that, in the event of a major disaster,
essential structures and facilities remain safe and functional, as required by current
law. Essential facilities include hospitals, police stations, fire stations, emergency
operation centers, communication centers, generators and substations, and
reservoirs.
Policy S-5
The City will continue earthquake strengthening and provisions for alternate or backup essential services, such as water, sewer, electricity, and natural gas pipelines and
connections throughout the City. First priority for this program should be for the
essential services within the identified fault hazard management zones.
Projects developed pursuant to 2004 land use policies, Zoning Code Revisions and the Central
District Specific Plan will expose additional people and structures to groundshaking hazards
associated with earthquakes. Any groundshaking that will occur will be similar throughout the City
and is not considered an unusual or unique risk because all of Southern California is at risk of
seismic groundshaking. Per City and State building codes, all new development will be required to
incorporate appropriate design and construction measures to guard against groundshaking hazards.
All projects and structures will be constructed in compliance with existing seismic safety regulations
of the California Uniform Building Code, which requires the use of site-specific engineering and
construction standards identified for each class of seismic hazard.
In addition, the City requires geological and geotechnical investigations in areas of potential seismic
or geologic hazards as part of the environmental and development review process. Proposals and
projects for development or redevelopment which do not provide for mitigation of seismic or
geologic hazards to the satisfaction of responsible agencies will not be approved. The City will
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continue to require preliminary geological investigations by State-registered geotechnical engineers
and certified engineering geologists (in accordance with the Uniform Building Code).
Conclusion:
Compliance with General Plan Safety Element policies and building code regulations will ensure a
less than significant impact.
Erosion
2004 Land Use Element and Zoning Code Revisions
The 2004 Land Use Element, and the Zoning Code Revisions as the implementing tool, will allow
some new construction on vacant land and undeveloped parcels, including property in the hillside
areas surrounding the Arroyo Seco and Eaton Canyon. During the construction phase of individual
projects, grading and earthwork will be necessary to prepare sites for development. Exposed rock
or soil may increase the chance of erosion, landslides, and mudslides if these surfaces are left
unprotected during periods of high winds and rain.
The City currently has in place guidelines and standards for development in hillside areas to
minimize the adverse impacts of grading. Also, Section 14.05.084 of the Pasadena Municipal Code
requires the control of runoff from all construction sites to guard against erosion.
Conclusion:
Continued implementation of standard erosion control and engineering techniques during
construction of individual projects will reduce erosion impact to a less than significant level.
Central District Specific Plan
The Central District Specific Plan area does not include any hillside areas or areas that are
susceptible to or have historically experienced landslides, mudslides, collapsible soils, or slope
failure. Thus, the probability of slope failure and collapse in the Central District Specific Plan area is
negligible.
Conclusion:
Impact will be less than significant.
Geology and Soils
2004 Land Use Element, Zoning Code Revisions, and Central District Specific Plan
Portions of Pasadena are underlain with weak, semi-consolidated bedrock and loose,
unconsolidated and often saturated alluvial sediments. These soil types have the potential for
liquefaction and collapse in the event of a major groundshaking event. The fine-grained
components of the bedrock units are potentially expansive. The weak soil, combined with steep
slopes and saturated drainage channels, make areas of Pasadena susceptible to landslides and
mudflows (Figure 20). Seismic groundshaking has historically caused landslides, rock falls, and
liquefaction. Unstable soils, landslides, soil erosion, debris flows, and mudflows are common in the
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hillside areas. To address these issues, the adopted General Plan Safety Element includes the
following goal and policies:
Goal G-1
Minimize the risk to life or limb, and property damage resulting from soil and
slope instability.
Policy G-1
Whenever possible, mitigation of geologic hazard will be conducted without
violating the property owners’ rights to modify or improve their investment, along
with preserving the aesthetic or natural conditions of the area through minimal
grading. When these goals are in conflict, protection of life and property will take
precedence.
Policy G-2
The City will continue to participate in regional programs designed to protect the
groundwater resources of the Raymond Basin while protecting the area from the
hazard of regional ground subsidence.
The City requires geological and geotechnical investigations on properties where new development
is proposed and seismic and geologic hazards are of concern. Liquefaction assessment studies are
also required in areas identified as susceptible to liquefaction. Compliance with General Plan Safety
Element goals and policies and implementation of existing regulations will ensure that impacts can
be avoided. Impact will be less than significant.
Conclusion:
Impact will be less than significant, and no further mitigation beyond compliance with applicable
policies, regulations, and standards is required.
Mitigation Measures
The adopted General Plan Safety Element policy and current City development review practices
adequately address geologic and seismic hazards within in the entire planning area, including the
Central District Specific Plan area. No mitigation is required.
Level of Impact after Mitigation
Impact is less than significant.
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