Elva Valencia
GEOL/METR 309
Final Exam- Daly City Dilemma
May 20, 2003.
Geologic Setting
The geologic setting of Westline Drive in Daly City is underlain by greenstone (metamorphosed ultramafic basalts) of the Cretaceous Franciscan Complex, friable to weak sedimentary rocks of the Merced Formation (deposited in the Pliocene), and serpentine. These rocks are covered by weakly cemented marine terrace deposits, landslide debris, colluvuim (slope wash deposits), and sand dunes deposits. A prominent escarpment north of Westline Drive defines the southern boundary of Mussel Rock Landslide Complex, an approximately nine million cubic yard mass movement that occupies the open space at Mussel Rock (Jacob Associates 1998).
Merced Formation (lower Pleistocene and upper Pilocene) is a medium-gray to yellowish gray and yellowish orange, medium to very fine-grained, poorly indurated to friable sandstone, siltstone, and claystone, with some conglomerate lenses and a few friable beds of white volcanic ash. In many places, sandstone is silty, claylike, or conglomeratic. Some of the conglomerate, especially where fossiliferous, is well cemented. Volcanic ash is in beds as much as 2 m thick and consists largely of glass shards (USGS).
Serpentine is a greenish-gray to bluish-green sheared serpentine, enclosing variably abundant blocks of unsheared rock. Blocks are commonly less than 3 m in diameter, but range in size from several centimeters to several meters; they consist of greenish-black serpentine, schist, rodingite, ultramafic rock, and silica-carbonate rock, nearly all of which are too small to be shown on the a map (USGS).
Due to erosion, slumping, and faulting in the area, the poorly cemented Merced sediments have been reduced into a large active landslide. Landslide deposits in this area create many small knolls in the topography that are heavily vegetated. Steep cliffs, most of which are either landslide or fault scarps, surround these deposits. An old landfill pit underlies most of the toe platform. Riprap has been placed at the landslide toe to prevent landfill material from washing into the ocean (Palmer).
Continual fault activity, erosion, and slumping have made large composite landslides in the Merced Formation. The toes, bodies, scarps, and crowns of these landslides show evidence of recent and continued slumping. Almost every human enterprise above the slide area is threatened: a tunnel, the Ocean Shore Railroad, Coastal Highway 1, and an ever-increasing number of houses (Palmer).
Faults
The faults of San Mateo County are characterized by both strike-slip and dip-slip components of displacements. There are three major faults systems that display large right-lateral offsets: the San Andreas, the Pilarcitos, and the San Gregorio fault zones. These faults trend roughly N30W. Most of these fault systems include many strands in a broad zone. The San Andreas Fault was displaced up to several meters in San Mateo County during the 1906 earthquake. Currents estimates of total offset is about 35 km for the San Andreas fault zone in San Mateo County, 120 km for the Pilarcitos fault zone, and 155 km for the San Gregorio fault zone (USGS).
The 1906 trace of the fault trends about 48 degrees northwest through the Mussel Rock area. The San Andreas Fault is generally considered to be about a mile wide and the entire Mussel Rock landslide area probably lies within the fault zone (Burns, Cooper, Carlson, McCulloch pg 85).
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San Gregorio fault
San Gregorio fault zone is part of a system of strike-slip faults than include the Hosgri and Sur Faults, which extend from Point Arguello north of Santa Barbara to Monterrey Bay to Bolinas. The fault may have become active by the middle Miocene, and has accumulated at least 150 km of offset since then. Onshore trenching studies have shown a slip rate of about 6 to 8 mm/yr on the Sea Cove Fault and regional geologic corrections give a similar slip rate of about 6mm/yr (Burns, Cooper, Carlson, McCulloch pg 85).
Pilaracitos Fault
The onshore Pilarcitos Fault continues on a trend of about N. 50º W. into the offshore basement and may merge with the San Gregorio Fault. This fault is now an inactive fault (Burns, Cooper, Carlson, McCulloch pg 110).
Westline Drive
The Westline Drive Study area is located along the Pacific coastline in a seismically active region of California that is astride the San Andreas Fault Zone. The active trace of the San Andreas Fault Zone is trending to the northwest through the interaction of Westline and Longview Drives, and most of the residences are within the Alquist-Priolo fault-rupture hazard zone established along the San Andreas Fault. The San Andreas Fault represents a significant geologic hazard that could produce a violent ground shaking in the area and result in fault ground rupture (Jacobs Associates 1998).
Topographic Setting
Stability and Erosion
Heavy surf and rains severely erode beaches and fragile sea cliffs along the coast of San Mateo County, California. During a storm season, the main coastal highway gets damaged and closes for a considerable period of time due to landslides, and many buildings and homes are damaged or threatened by sea-cliff retreat and slope failure.
Coastal Stability
Segments of the San Mateo County coast are classified as stable and unstable based on the inherent resistance of the exposed rocks to wave erosion and slope failure. The stable coastal segments in the northern part of the county are backed by resistant granitic rocks, which form high bluff to low cliffs. The stable coastal segments in the southern part of the county are backed by resistant sedimentary rocks that form low cliffs.
Unstable coastal segments are backed by weakly indurated or highly fractured sedimentary rocks that form high bluffs, weakly indurated terrace deposits that form low cliff, and loose beach and dune sand that forms spits and low cliffs. Along much of the San Mateo County coast, fragile terrace and loose dune deposits overlie resistant to moderately resistant bedrock. In those areas the base of the cliff is stable and the upper part is unstable. If the cliff is sufficiently high that wave surge does not erode the terrace deposits exposed in the upper part of the cliff face, that coastal segment is classified as stable.
Natural coastal processes such as cliff erosion, slope failure, wave erosion, landslides, block falls, debris slides, and cave collapse are illustrated diagrammatically on the cross sections. Wave erosion is the primary or initiating erosive process along the San Mateo County coast. Waves erode the base of the sea cliffs or coastal bluffs, destabilizing the slopes above. Landslides, debris slides, and block falls are secondary erosive agents that move loosened rock material down the slopes to the beaches below, where the material is eventually eroded away by wave activity. Along most of the San Mateo County coast groundwater that seeps from the cliff face softens and loosens even otherwise resistant bedrock material, thus contributing to the erosion process (JacobsAssociates 1998).
Westline Drive
The Pacific Ocean defines the western boundary of the area. The maximum site elevation is approximately 450 feet along the eastern boundary that coincides with Longview Drive. In the western portion of the area, the topography slopes moderately to very steep from the backyards of residences along Westline Drive, down toward the ocean. Downhill of the escarpment, the topography slopes moderately to very-steeply downward the northwest to the level open space at Mussel Rock. The maximum relief between Westline Drive and open space is approximately 200 feet.
Earthquake history
The shaking intensity for Daly City during the 1906 earthquake was measured at IX-violent to X –very violent (Refer to map # 1). By visiting one of the geologic sites at Mussel Rock, Longview Playground via Longview Drive, one can observe a fault that was formed during the 1906 San Francisco Earthquake. The fault zone consists of hundreds of offsets along the San Andreas Fault. The most recent of these offsets was caused by the 1906 Earthquake. The trace of the 1906 fault crosses Longview Drive 100 feet south of the Longview Playground and then continues northwest down the landslide scarp just to the north of the steep gully below the playground. The trace then crosses the north end of the landfill and goes into the ocean about 500 feet north of the landfill. There is little evidence of the trace now, since it is largely covered by houses, roads, landfill and beach sand (Geologic Trips pg. 117).
A M5.3 earthquake occurred near Daly City on March 22, 1957, the largest event on the northern San Francisco peninsula since 1906. It was located within a kilometer of the "locked" section of the San Andreas Fault and roughly 20 km southeast of the presumed epicenter area of the 1906 earthquake, offshore from the Golden Gate. Available local and regional first motion data were originally interpreted to suggest primarily dip-slip movement on a near vertical and a near horizontal plane indicating that, despite its location, this event was not a typical right--lateral strike slip San Andreas earthquake.
During the 1989 Loma Prieta Earthquake the shaking intensity for Daly City was measured at VI- moderate (Refer to map #2). The residents of #35 Westline Drive reported that the earthquake resulted in extensive re-cracking of Westline Drive and residential damage.
Land movements and landslides
Land movements
On November 9, 1999, Jacobs Associates were hired to survey the Westline Drive Site in Daly City by the City of Daly City- Engineering Division. The following information was taken from their report given to the Division. The survey team found evidence that active landslide movement was occurring at great depths. Their studies also indicated that groundwater had substantial impact upon the stability of the landslides.
Substantial land movements occurred during the winter of 1962-1963. Landslide movements continued during the next few years, and in 1966, the residences at # 34 through #56 Westline Drive, together were removed from the northern side of the roadway. Periodic landslide movements have occurred from the 1970’s to the present time. The most significant ground movements were occurring at the time of the survey (1999), involving movement of part of the street and associated infrastructure.
Landslides
Ground movements affecting Westline Drive, the residence along Westline, and the utilities beneath Westline Drive appeared to be related to two distinct landslide complexes. The predominant landslide appeared to include # 28 and #30, along the northern side of Westline Drive, and #33 through # 77 on the southern side. The landslide toe coincided with the topographic break-in-slope bordering the level open space to the north. Based on the slide morphology and bedding structure in the Merced Formation, the mechanism of landsliding could be characterized by a block slide-earth flow complex involving oblique slip along bedding surfaces in the Merced Formation. The sliding appeared oriented at about N20ºW, and estimated maximum depth and volume of slide material is on the order of 80 feet and 400,000 cubic yards.
The second landslide scarp appears to project into the 90-degree bend in Westline Drive, where it is truncated by ground cracks associated with Landslide # 1. The predominate direction of sliding appeared oriented to about N60° W, and the estimated maximum depth and total volume of slide material is on order of 75 feet and 300,000 cubic yards. The main landslide mechanism may involve wedge sliding or slumping. The secondary slide mass may represent a geologically youthful slide that has developed in response to more extensive bordering slope movements (Jacobs Associates).
Development History
The area is part of a residential subdivision that was developed by Henry Doelger Builders, Inc. in the late 1950’s and early 60’s. Based on the subdivision-grading plan, fills up to about 50 feet deep were placed in the area to establish pads for the residential structures. Most of the fill was placed to raise the grade through a northwest-trending topographic saddle. As fill was locally placed to the edge of the escarpment forming the southern boundary of the Mussel Rock Landslide Complex, and the deepest area fill occurs south of Westline Drive, beneath house # 37 through # 45. In a report addressing the 1962-63 ground movements, WCS (1965) concluded that an earthquake on the San Andreas Fault could cause 10 feet of sudden ground movement and potential loss of life (Jacobs Associates).
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Works Cited
Burns, T, Cooper, A, Carlson, P, McCulloch D. Structures of the Submerged San Andreas and San Gregorio Fault zones in the Gulf of the Farallons off San Francisco, California, from High Resolution Seismic-Reflection Data.
Description of Map Units for Geology for San Mateo County.
Konigsmark, Ted. Geologic Trips. GeoPress. Gualala, California
Lajoie R. Kenneth, Mathieson A. Scott. 1982-83 El Nino Coastal Erosion: San Mateo County, California.
Palmer Jerome. Stop 2: Mussel Rock Fault Splay & Landslides.
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