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Section 2.0 Geology 1960050-03 111 2.0 GEOLOGY Regional Geology The project site is located on the Palos Verdes Peninsula, near the boundary of the North American and Pacific plates. This plate boundary is marked by the San Andreas Fault zone, which is located approximately 60 miles east of the peninsula. The peninsula is the result of geological processes that began approximately 16 million years ago in the Middle Miocene. Divergent motion along the fault developing at the plate boundary created a basin-and-range type topography. The sediments that would become the Monterey Formation were deposited in one of these basins during a period of about 12 million years (from 16 million years ago until the early Pliocene; about 4 million years before present). Widespread volcanism until about 14.5 million years before present contributed to the sediments of the Monterey Formation (Conrad and Ehlig, 1987) . In the early Pliocene, the depositional basin was folded and uplifted into an anticline along the south side of the Palos Verdes Fault, until it formed an island, separated from the mainland by a shallow sea. Sediments were deposited along the north and northeast flanks of the island, gradually filling the low-lying Los Angeles Basin and connecting the Palos Verdes Peninsula with the. mainland. During the Pleistocene, uplift of the peninsula continued, and changes in sea level due to glacial retreats carved a series of thirteen recognized terraces into the flanks of the peninsula. Currently, wave action is continuing to cut into the peninsula, creating steep, near-vertical cliffs up to 150 feet high along the shoreline. The bedrock of the Palos Verdes Peninsula consists of a core of Mesozoic_Age Catalina Schist, overlain by the Monterey Formation (Figure 2). It has a gentle to moderate dip, approximately 15 to 30 degrees toward the south on the seaward side. The rock has smaller folds within the limbs of the anticline that forms the Palos Verdes Hills. The Monterey Formation has been divided into three subordinate units or members. The oldest unit is the Altamira Shale which is overlain by the Valmonte Diatomite, which is, in turn, overlain by the youngest of the three members, the Malaga Mudstone. The Altamira Shale is the most prominent member of the Monterey Formation exposed in Rancho Palos Verdes. It consists of beds of tuffaceous shale, siltstone, tuff. and tuffaceous siltstone that are intruded by basaltic dikes and sills, as well as tuffs. The tuffs can form distinct marker beds. One of these, the Portuguese Tuff, has an average thickness of about 55 feet and is an important marker bed in the study area. Many of the tuff beds have been altered to bentonite clay. Another bedrock unit overlying the Monterey Formation is the Pliocene Reppetto Formation which outcrops in the northern part of the peninsula. The Pliocene Lomita Marl, Timms Point Silt, and the San Pedro Sand are found in the north and east edges of the peninsula. 1960050-03 Landslide Geologv and Geomorphology Coastal portions of the Palos Verdes Peninsula have been affected by episodic landsliding for approximately the last 600,000 years (Jahns and Vonder Linden. 1973). The largest of these ancient landslides was the Pleistocene-Age Ancient Abalone Cove Landslide. The ancient landslide occupies a two square mile, bowl-shaped area between the Pacific Ocean and the top of the Palos Verdes hills. The area contains a complex of landslides that vary in size from less than an acre to hundreds of acres. The recently active Abalone Cove and Portuguese Bend Landslides are portions of the large Pleistocene-Age Abalone Cove Landslide (Figure 3). Other nearby recently active landslides include the Klondike Canyon and Flying Triangle landslides. Portions of the ancient Abalone Cove Landslide became unstable and began to move approximately 120,000 years ago (Jahns and Vonder Linden, 1973; Ehlig, 1987 and 1992). Initially, the landslide began moving as a single sheet (Ehlig, 1992). This is supported by the stratigraphic continuity of the ancient landslide rupture surface between upslope and downslope areas, as well as laterally across the landslide complex. Borings drilled throughout the landslide complex have found the ancient landslide rupture surface at or near the top of the Portuguese Tuff(Leighton, 1990, 1996, and Law/Crandall, 1991). Subsequently, the landslide has broken into several large blocks which moved separately. The recently active (1974-1982) Abalone Cove Landslide is the western portion of the ancient • (Pleistocene Age) Abalone Cove mega-landslide. The ancient complex overrode the bedrock of Portuguese and Inspiration Point before wave action eroded the cove between the two points. Later sliding appears to have moved around the points, leaving ancient slide debris resting over terrace deposits and bedrock on the two points. The recently active Abalone Cove Landslide consists of slide debris from the ancient Pleistocene landslide that was remobilized. Sliding was first noticed in 1974 at the toe, and over the next two years, progressed inland to Palos Verdes Drive South. By 1980, the active sliding had propagated inland of Palos Verdes Drive South, with a maximum area of about 80 acres. Reduction of groundwater within the landslide by dewatering wells began in early 1980, and, by the end of 1981, most of the movement had stopped. The Klondike Canyon Landslide lies just to the east of the active Portuguese Bend Landslide, and is part of the larger, ancient landslide complex (Kerwin, 1982). The landslide was reactivated in 1979 following Vseveral heavy rainfall seasons. Frictional drag along the edge of the Portuguese Bend Landslide may have contributed to reactivation. Kerwin (1982) considers the landslide to be an eastward extension of the active Portuguese Bend Landslide. The 50-acre landslide does not appear to be moving at the present time. The active Portuguese Bend Landslide is the largest of the recent slides in the ancient Pleistocene-age Abalone Cove Landslide area (Figure 3). The active landslide has an area of 0F 1.4 QA-4 - �` LFJGNTONAND ASSOCIATES,INC. 1960050-03 approximately 260 acres. It is a reactivated portion of the ancient landslide complex. which has been continually moving since August 1956. The initial failure in 1956 was due to the loading on the head of the active landslide with fill materials. The continued movement of the landslide is attributed to the high groundwater table, the continual shoreline erosion. and the low shear strength of the bentonite. The Portuguese Bend Landslide is located between Klondike Canyon on the east, and Portuguese Point on the west. It is separated from the recently active Abalone Cove Landslide by a diffuse ill-defined contact near Portuguese Point. Neither Portuguese Point nor Inspiration Point have been affected by the sliding. The rupture surface of the active Portuguese Bend Landslide is underlain by bedrock east of Portuguese Canyon, and inactive landslide debris west of Portuguese Canyon.. Since its reactivation, several efforts have been made to stabilize the landslide, including grading, dewatering, and construction of gabions to protect the toe. These efforts are complicated by multiple factors contributing to the continued movement. The most significant factor causing continuing movement is the presence of groundwater within the landslide. The water causes a reduction in the strength of the bentonite clays, due to adsorption of the water by smectites within the clay. Ground water also reduces the effective stresses along the rupture surface, and increases the weight of the landslide material. In addition to high ground water, the slightly undulatory, seaward-dipping rupture surface reduces the stability of the landslide mass. The toe of the landslide is continually eroded by the ocean, removing resisting forces at the landslide toe and allowing continued downslope movement. The Role of the Portuguese Tuff The Miocene-Age, Portuguese Tuff is a distinctive marker unit throughout the project area. The Portuguese Tuff consists of a thick bentonitic tuff bed approximately 55 feet thick. The tuff grades upward from a coarse ash, often with pumice fragments, at the base to a fine, reworked ash at the top of the unit. After deposition, most of the tuff has devitrified and weathered to a nearly pure bentonite. The upper portion of the unit is a distinctively waxy bentonite. The bentonitic composition and thickness of the Portuguese Tuff make it a nearly perfect aquiclude. Groundwater migrates downward until reaching the tuff, which prevents continued downward migration. Borings drilled within the landslide often find local groundwater perched on top of the bentonite. Borings drilled at the toe of the adjacent Klondike Canyon Landslide encountered groundwater under artesian pressures beneath the Portuguese Tuff. Following dewatering, the Klondike Canyon Landslide stabilized. Extensive drilling within the active Portuguese Bend Landslide and the ancient Abalone Cove Landslide area demonstrate that the ancient landslide rupture surface is near the top of the Portuguese Tuff. Recent mapping by Dr. Ehlig indicates that a bentonite layer approximately 30 feet above the top of the Portuguese Tuff may be the active rupture surface. • 5111 1.4 hig I VICUTIMI AIM ACCncmrrs ar.. 1960050-03 Faulting. and Seismicity410. Southern California, which includes the Palos Verdes Peninsula and the project site, is in a seismically active area due to its proximity to the San Andreas Fault zone. This fault zone forms the boundary between the North American and Pacific plates. Movement alone this boundary occurs along a broad zone of faults throughout Southern California (Figure 5). Major active faults include the Newport-Inglewood, Whittier-Elsinore, San Fernando, and the San Andreas Faults. Other major active faults in the area include the Palos Verdes, Malibu Coast. Santa Monica, Elysian Park. and Verdugo Faults. Major potentially active faults include the Cabrillo, Redondo Canyon,Hollywood, and San Pedro Faults. To evaluate the seismicity of the project area, a deterministic seismic analysis was performed using EQSEARCH and EQFAULT. EQSEARCH tabulates the location and magnitude of all historical earthquakes near the site since 1880. The program also calculates the peak ground acceleration (PGA) at the site from each of the old earthquakes using a selected attenuation equation. EQSEARCH was also used to generate Figure 6, which shows the location of historical earthquakes and their magnitude. The computer calculated the distance from the site to active faults within a•radius of 62 miles. The program then used the published maximum credible (MCE) and maximum probable (MPE) earthquakes for each fault to determine the peak ground acceleration at the site, assuming that the MCE and MPE were to occur at the closest approach of the fault to the site. Both computer programs were written by Thomas Blake. As shown in Table 1, the maximum site acceleration felt at the study area is estimated to have occurred as a result of the December 8, 1812 earthquake that had an epicenter offshore of Orange County. This earthquake, with an estimated magnitude of 6.9, destroyed the mission at San Juan Capistrano. The site acceleration at Portuguese Bend is estimated to have been 0.138 g. As shown on Table 2, the maximum accelerations expected from the maximum credible or maximum probable earthquakes is 0.535 g and 0.419 g, respectively. In both cases the causative fault is the Palos Verdes Fault,with the epicenter approximately 3 miles from the site. Groundwater Conditions Groundwater plays a critical role in the continuing movement of the Portuguese Bend Landslide as well as other landslides within the region. Nearly all of the adjacent landslides have been active during periods of high ground water in the late 1970's and early 1980's. As ground water levels were lowered following a program of withdrawals, both the Klondike Canyon and Abalone Cove Landslides stabilized and ceased moving (Ehlig, 1992). Most of the Palos Verdes Peninsula consists of dense, impermeable bedrock that does not store groundwater; hence, there is little data on regional groundwater levels for the south side of the peninsula. Site specific groundwater data is available, however, for the study area(Appendix D). The Groundwater Contour Map (Figure 7) shows present day groundwater levels in the LEIGHTONANOASSOCIATES,INC. 1960050-03 IP Portuguese Bend Landslide. In general. the groundwater table parallels the ground surface and groundwater flows south and southwestward, toward the ocean. Figure 7 also shows the wells and well designations that form the basis of the groundwater contour map. The water table slopes seaward, and varies from an elevation of about 375 feet MSL at the north edge of the landslide to sea level at the shoreline. Comparison of the Groundwater Contour Map (Figure 7) and the Structure Contour Map (Figure 17) show that the groundwater levels are generally substantially higher than the rupture surface. In most cases, the elevation difference between the rupture surface and the groundwater elevation is greatest where the rupture surface is deepest. In the east-central portion of the active landslide, for example, a depression in the underlying bedrock surface is located just north of Palos Verdes Drive South. In this area, the rupture surface is located approximately 90 feet below the ground water surface. South of Palos Verdes Drive South, the water table drops to sea level. There do not appear to be artesian pressures beneath the landslide rupture surface, except near the shoreline along the eastern edge of the Portuguese Bend Landslide (Ehlig, 1992). High levels of groundwater within the landslide result from the relatively high permeability of the broken and fractured landslide mass. Cracks, fissures, and abundant coarse debris allow for easy filtration of surface water. Rainfall, runoff along canyon bottoms, and residential runoff are sources of inflow to the water table (Ehlig and Bean. 1982). Groundwater levels are higher today than they have been in the past. According to Ehlig (Ehlig and Yen, 1997), current ground water levels in the northern and western portions of the landslide are generally at the levels observed in the few observation stations available in 1956. Water levels in the eastern portion of the landslide, however, are much higher today than in 1956. Following activation of the landslide in 1956, the ground surface was quickly riven by fissures and cracks. According to Ehlig (Ehlig and Yen, 1997), by 1968 the groundwater table had risen to near its present level. Heavy rainfall during 1977 through 1983 resulted in a significant rise in the groundwater table. As a consequence of this rise in the groundwater table, the landslide accelerated to its highest recorded rate of movement(Figure 21). Groundwater withdrawal in the study area is very difficult due to low permeability of the clay and silt that make up the landslide mass, and dewatering wells have been only moderately successful. Production from all 14 dewatering wells in the Portuguese Bend Landslide averages 20,000 gallons per day. In comparison, the 18 dewatering wells in the adjacent Abalone Cove Landslide Abatement District average a total of 220,000 gallons per day. Increased production at Abalone Cove is proportional to increased rainfall in most cases, with the exception of two wells located within the west-central subslide. Production rates from these wells vary with no relation to precipitation (Ehlig, 1992). Figures 21 through 23 compare monthly and annual rainfall totals with groundwater levels for four monitoring wells in the Portuguese Bend Landslide. The difference in production volumes is the result of a higher quantity of bentonite in the • landslide debris below the water table at Portuguese Bend. The fine grained clays retain more of CIMMIII AYfl ACC/IPIA TLC WI 1960050-03 the groundwater than fine sands and silts. Permeability rates calculated by EhligEhlig and Yen. 1997) are 7 x 10 -5 cm/sec ba Ehlig (Ehlig as d on production rates from the dewatering wells. Ehlig notes that laboratory testing generally results in hydraulic conductivities one to two magnitudes lower than the yields based on well production. The lower production of the dewatering system indicates that dewatering wells have a limited effect that will decrease as the groundwater levels become lower. Therefore, as the groundwater levels decline below some, as yet undefined. level, the additional withdrawal effort will have a progressively decreasing result in lowering the water table. • LEIGHTON AND ASSOCIATES,INC.