Ester Azucena
Geo 309
Daly City Dilemma
Stage 1
The Mussel Rock landslide is made up of 112 acres in the southwest corner of
Daly City. The landslide is made up of slumps shed that form the lower Merced
Formation. Here, the San Andreas Fault trends northwestward out to the sea. Natural
coastal processes such as cliff erosion, slope failure, wave erosion, and landslides have all
been associated with this area. Wave erosion is the main erosive process along the coast.
The waves erode the bottom part of the sea cliffs in which it oversteeps and destabilizes
the slopes above. Landslides are secondary in the erosive process, they move loosened
rock material down the slope to the beaches below, where the material is eroded away by
wave activity. Most of the coast ground water that seeps from the cliff face (cliff ranges
from grater than 35m high) softens and loosens even resistant bedrock material, which
helps contribute to the erosion process.
In 1874, Richard M. Tobin made a tunnel through a coastal headland in the area.
The tunnel is located in the southwest corner of the area and is 90 feet long, 11ft high,
and 10.5 ft. wide. Because of the seasonal fluctuations of beach sand, it caused the tunnel
floor to be 3 ft. above the beach in the summer and the route was abandoned.
From 1905 -1920, the Ocean Shoreline Railroad went along the coastline and part
of the route passed the toe platform of Mussel Rock landslide. The
tracks had to be cleared often because of the debri of landslides. In the 1906 earthquake
parts of the railroad were destroyed.
During the 1906 earthquake, according to the website abag, the area where
Mussel Rock lies had a very violent shaking intensity(Fig 1). The majority of the coastal
area experienced this intensity (Fig 2), especially in the area where the San Andreas
Fault runs through. In the 1957 earthquake the epicenter was in this area. Not much
information could be found but I am sure it affected the area probably by making new
cracks on the site and or extending old ones. In the 1989 Loma Prieta Earthquake the area
felt a moderate shaking intensity (Fig 3). This resulted in extensive re-cracking and
residential damage (a large extended crack, diagonally northwest, caused extensive
foundation damage to a house).
In 1936 the old State Highway 1 was built where the railroad route use to be. The
highway also had problems with landslides and was abandoned after the 1957
earthquake.
In the early 1960’s, urban development covered the crown surrounding the
Mussel Rock landslide. In 1963, a sanitary landfill began to operate there. Today the area
is used as a construction dump site.
Mussel Rock consists of Franciscan rocks and lies at the north side of the San
Andreas Fault Zone. The rocks in the fault zone are mainly crushed soft sand stone and
shale of the Merced Formation that have slumped to form the large landslide. The
Franciscan formation is the oldest formation exposed, and the out crops of the Franciscan
complex show it to be 100 ft. thick. The Mussel Rock area is a sea stack in the Franciscan
Complex. At the out crop in the southwest corner, an arch has been cut into a sea stack of
Franciscan rock. The Merced Formation forms steep, partially-vegetated cliffs that
surround the landslides deposits. The lower Merced Formation dips gently to the
northwest and is approximately 600 ft. thick. Mussel Rock is underlain by greenstone of
the Franciscan Complex and easily crumbles to weak sedimentary rocks of the Merced
Formation. These rocks are covered by weakly cemented marine terrace deposits,
landslides debris, colluviums (loose earth material that has accumulated at the base of a
slope), and sand dune deposits. Both the Merced and Franciscan formation on the Mussel
Rock area have been disturbed by the San Andreas Fault.
The area is in a major fault zone, the rocks in the fault zone are loose and
unstable and the bluff is steep and high. Seismic slope stability is a major concern. The
San Andreas Fault traverses the eastern side of the site. If an earthquake on the San
Andreas happened it is predicted that most of the affected area would be along the
northern side of the site. According to Raymond Wilson, a research geologist with the
United States Geological Survey’s Menlo Park,” If there is movement on the San
Andreas, that neighborhood would be toast”(San Francisco Chronicle, Feb. 24,2000).
There are four known faults in the area. In 1999 there was a discovery of a fault
line underneath the neighborhood, called the Mussel Rock Fault. The Mussel Rock Fault
contact between two subunits of the Merced Formation is also evidence that the Mussel
Rock landslide lies within the San Andreas Fault Zone. A number of small, low angle
normal and reverse faults are exposed locally at the fossil crop. None of the faults show
displacement of over 10 ft. Some displacements are due to the settling of the ground after
movements along the San Andreas and the Mussel Rock Fault (there is a vertical fault
zone exposed in the sea cliffs). The Mussel Rock Fault is 100 ft. wide predominately
vertically oriented shears and faults that cut through the Franciscan Complex greenstone
and overlying terrace deposits. The Mussel Rock Fault strikes sub parallel to the San
Andreas Fault. The San Gregorio is a young active slip fault that lies west of the San
Andreas Fault, mostly offshore, it’s structural zone is an area of major thrust fault
deformation. The Pilarcitos Fault is an inactive young strike slip fault that lies between
the San Andreas and the San Gregorio Fault and trends offshore.
Deep ancient landslide complex appears to have underlain the site prior to
construction. This landslide likely developed in pre-historic times along the over steeped
southern flank of Mussel Rock Landslide Complex. The lower Merced Formation is the
source of the landslide deposits and is divided into two by the Mussel Rock Fault. Due to
erosion, slumping, and faulting in the area the poorly cemented Merced sediments have
been reduced into a large active landslide. The steep, unstable rocks have caused a major
landslide in the bluff. Because of the landslides, the edge of the bluff is rapidly receding,
at a rate of up to 3 ft. per year (the cliff retreated 13m landward while the toe of the cliff
retreated 10m). Landslides accelerate when the ground is wet from heavy winter waves
(groundwater has a substantial impact upon the stability of the landslides). Active
landslides movements are occurring at great depths, at about 80 -120 ft. below existing
ground surface. The sliding zones are generally interpreted to occur along thin seams of
very weak clay interbeds within the underlying siltstone bedrock formation. The first
landslide, as described by Jacobs Associates, “Based on the slide, morphology may be
characterized by a clock slide-earth flow complex involving oblique slip along bedding
surfaces in the Merced Formation possibly a slump-earth flow complex. The predominant
direction of sliding appears oriented at about N29W, and the estimated maximum depth
and volume of slide material is on the order of 80 ft. and 400,000 cubic yards,
respectively.”( pg 7)
The second landslide appears to project a 90 degree bend. It is truncated by
ground cracks, the direction of sliding appears oriented at N60W with the maximum
depth and total volume of slide material in the order of 75 ft. and 300,000 yards. “Based
on bedding structure in the Merced formation, the main landslide mechanism may
involve wedge sliding or possibly slumping. The secondary mass may represent a
geologically youthful slide that has developed in response to the more extensive
bordering slope movements.(J&A pg 7)” Strong earthquake shaking may potentially
cause sudden and extensive ground cracking within landslides and along boundaries.
Substantial rains can infiltrate the ground crack, it causes more adverse ground water
conditions and increases the severity of ground movement.
The landslides “Consists of brown, poorly cemented, fine grained litharenites
interbedded with layers and lenses of shale, coarse sandstones, and well-rounded gravel
(Geology of Mussel Rock Landslide, Smelser).” The layers and lenses average one foot
in thickness “At Mussel Rock Fault, this litharenite subunit is in fault contact with
another subunit consisting of gray, moderately cemented fine grained litharenties
interbedded with pebbly and fossiliferous layers that dip about 19 degrees northwest
(Geology of Mussel Rock Landslide, Smelser).” The plane that separates these two
subunits is vertical.
Landslides deposits in Mussel Rock area are surrounded by steep cliffs, of the
Merced Formation. The primary mechanism for downhill movement of the landslide
deposits is by continuous small-scale slumping. These deposits, which are roughly nine
million cubic yards, are poorly consolidated and uncemented matrix-supported slumps
and debris flows. The landslide deposits are dissected by numerous V-shaped drainage
channels, which were formed by intermittent drainage, indicating vigorous erosion. This
drainage channels promote further slumping as evidence by pull-apart cracks visible on
top of surface of slide.
There are two basic stream systems that extend east to west down slope. The first
flowing down the center of the landslide deposits and the other flowing down the
northern side.( Sediments form a small out wash plain between the landslide deposits and
the landfill deposits. In the winter this plain is an area of deep mud and standing water. In
the summer the plane is a mud-cracked platform with thin crust of salt.) These drainage
systems flow only in winter and spring. Most downhill movement of the slide probably
occurs during the wet season.
During a period of seven years the eastern crown was measured, at the end of the
period it showed that locally up to 20 ft. of the crown had slumped, at an average of 3 ft.
per year. The eastern scarp ( steep cliff face that is formed by a slump) is the steepest and
most barren it is also compromised of moderately to very steeply slopes. “ The scarp
ranges from about 50W at the most stable portion to 90W at the most active portion and
is, therefore, the most environmentally hazardous scarp in Mussel Rock landslide area
(Geology of Mussel Rock Landslide).” Northern crown is composed of a series of narrow
V-shaped gullies. The crevasses on the northern scarp indicate active erosion caused by
slumping. On the western boundary of the area has a maximum site elevation of
approximately 450 ft. On the southern crown many houses were built on landfill and the
deepest area of fill occurs here, mainly south of Westline Drive. Ground movements that
have occurred since construction, appear to be related primarily to the destabilizing
effects of this surcharge and possibly less favorable post construction groundwater
conditions. Ground movement occurred during the wet winter of 1962-63, the depth of
the groundwater recorded ranged 50 -100 ft. From Oct. ’59 - Nov. ’63 roadway
displacements were found on the order of 0.5 ft. Many houses have been removed due to
active slumping. There are houses threaten due to undercutting by slumping. Cracking on
sidewalks and streets indicate that the southern crown is slowly and continuously
slumping. This scarp has shallower slopes than the other scarps and it also has larger
slump blocks.
Fault activity, erosion and slumping have reduced the strength of the rocks of the
Merced Formation into a larger composite landslide. If a major earthquake occurs in or
near this area during the wet winter, this area would be dramatically activated. The
Mussel Rock area strongly indicates that the land is unsuitable for permanent structures.
Works Cited
1. Jacobs and Associated Engineers/Consultants. November 5, 1998. Westline Drive Site
Study, Daly City, California. Letter Report Summarizing Phase 1 Services.
2. Geology of Mussel Rock Landslides. Mark G. Smelser. March 1987.
3. San Francisco Chronicle. February 24, 2000.