Open-File Report 96-517
A major cause of land subsidence is the withdrawal of groundwater from compressible sediments. As water is withdrawn and the water table lowered, the effective pressure in the drained sediments is increased. Compressible layers then compact under the over-pressure burden that is no longer compensated by hydrostatic pressure. The resulting land subsidence is most pronounced in uncompacted sediments Throughout Southern California oil and gas extraction is also a cause of subsidence. In particular, the Torrance oil field extends from Redondo Beach in the northwest to Long Beach in the southeast. Subsidence at a rate of ~3 cm/yr at Redondo Beach has been attributed to petroleum extraction.
Several criteria were applied in this study to define subsidence features. Subsidence was identified where the elevation decreased between two or more pre-earthquake surveys more than the allowable random survey error. The subsidence feature had to occur over several BMs and, a consistent temporal and spatial pattern of deformation had to exist between the surveys. Subsidence rates were calculated relative to a BM which was adjacent to the subsidence feature and in incompressible rock. This stable BM thus formed a local base level. Dividing the amount of subsidence below the base level by the time interval yields the subsidence rate. Since subsidence is a dynamic process and its magnitude and extent is affected by water use, rainfall and urban development, the derived rates reflect the average trend of subsidence in the area. Where possible more than two surveys of the route were examined to ensure the trend was not a short term effect.
Most of the run-off from the San Gabriel and Santa Susana Mountains flows through the San Fernando Valley. The entire Valley is drained by the Los Angeles River system. Since the 1940s, water flow has been controlled by the Hansen and Sepulveda Flood Control basins. Unconsolidated deposits, between 1.5 million and 10,000 years old, underlie the valley floor and are the main water bearing units of the San Fernando Valley. The Saugus Formation, around the northern margin of the Valley and south side of the Santa Susana Range, is composed of continental and marine sediments which also contain water-bearing units. Consolidated rock units in the surrounding mountains define the boundary between water bearing and non-water bearing units. Groundwater is drawn from the unconsolidated sediment aquifer systems that underlie the San Fernando Valley, Los Angeles and Long Beach areas. The main demands for water arise from industrial and urban residential usage.
Water levels in the San Fernando Valley and Los Angeles area have been recorded since the early 1900's. Fluctuations in water levels are influenced by periods of drought and increases in water usage. In the Castaic junction area the depth to water decreased 15 m during 1969 (Well 7048 [ DPW, 1992] ), (Figure 2). From 1969 to 1984 water levels rose another 4.5 m. The period 1984 to 1991, however, saw the water-level drop to the 1969 level. A monitoring well in Canoga Park, west of Balboa Boulevard, has shown little fluctuation in water-level between 1960 and 1991. The general trend at the Canoga Park well, though, has been one of water-level decline (Well 4709, [DPW, 1992] ). Over a 31-year period the depth to water has increased by 3 m (Figure 4). A second well in the San Fernando Valley (Well 3872B, North Hollywood) has shown much greater fluctuations. The depth to water below the ground surface increased by 24 to 57 m between 1944 and 1967 [Watermaster, 1981]. Between 1967 and 1984, however, water levels rose to almost that of the 1940's level (Well 3872B, Figure 2) [DPW, 1992]. Water-levels in the well then dropped again by 6 m between 1987 and 1991. In the coastal plains, ground-water fluctuations vary greatly between localities. Two wells, 706C and 1346D, separated by 5 km ( Figure 2), indicate the depth to the water-table has decreased. Water-levels have steadily risen by 12 m at well 706C since 1964.At well 1346D the 1964 and 1991 water-levels are about the same [DPW, 1992]. Fluctuations over the 27-year period at well 1346D, however, have been large, rising 12 m above the 1964 level by 1976 and then dropping almost 15 m by 1978. In the Long Beach area, water levels remained constant between 1965 and 1974 (Well 906). Between 1975 and 1977 water levels dropped by an average of 9 m and remained at that level until 1982, when water levels began to increase again. Since 1985 the water levels have been close to the 1965-74 level.
Subsidence was examined along each of the following leveling routes. The route numbers and names below are keyed to Figure 2 and the National Geodetic Survey line L25468 part numbers. The deduced subsidence rates are summarized as a contoured map in Figure 5. Where the data is insufficient to identify subsidence the contours are dashed e.g., in the San Gabrielle Mountains or the Santa Susana Range.
Subsidence at a rate of about 4 mm/yr is occurring from Castaic Junction to San Fernando. The trend is evident in surveys made in 1985, 1984 and 1978. Subsidence is concentrated in the Newhall area and the Santa Clarita Valley. North of Castaic Junction, where the route lies in the bedrock of the San Gabriel Mountains, subsidence is absent. In the City of San Fernando, subsidence rates are 3 to 4 mm/yr. This rate decreases gradually southward along the Verdugo Mountains and towards the Santa Monica Mountains. Four leveling surveys made between 1971 and 1994 show that the vertical deformation across the Los Angeles Basin is complex.
Subsidence at a rate of 15 mm/yr occurred between 1971 and 1989, relative to BMs outside the basin and south of the San Gabriel Mountains. In contrast, no such deformation occurred between 1989 and 1994. These observations agree well with water-levels measured at Well 906 (Figure 2), where low water levels were measured between 1975 and 1982, after which water levels rose and maintained a steady level.
Leveling surveys made along this route between 1978 and 1975 indicate that up to 60 mm of subsidence occurred. The subsidence zone was 20 km wide and centered in the town of Piru. This deformation appears to have ceased after 1978. The difference between a 1989 and 1975 survey of the route shows at most 1.5 cm of deformation, where the greatest subsidence was observed between 1975 and 1978. The possibilities are that subsidence followed by recharge occurred and water levels have again reached those of 1975, or, that the 1978 survey contains systematic errors. A subsidence correction of only 1 mm/yr was applied to a 10 km section of the line based on the 1989 to 1975 elevation changes to remove the subsidence feature from the elevation changes. This results in a set of elevation changes which can then be attributed solely to the Northridge earthquake.
Observed subsidence along this route varied from 2 to 4 mm/yr and was greatest on the west side of the San Fernando Valley. A maximum subsidence correction of 4 mm/yr was applied along this leveling route. The observed subsidence corresponds to that measured along Interstate 5 where the two survey lines intersect (Figure 6).
The subsidence along this leveling route ranged from 0.6 mm/yr at Fillmore to 2.5 mm/yr at Thousand Oaks. This was consistent with that observed along leveling routes Saugus to Fillmore and Moorpark to San Fernando.
Subsidence was observed along this line from Chatsworth to about 3 km west of San Fernando. The subsidence rate increased from east to west across the San Fernando Valley and was greatest at Chatsworth where it was ~ 4 mm/yr.
Surveys made in 1974 and 1989 indicate subsidence along this route in the San Fernando Valley. The deformation is greatest from Chatsworth to Canoga Park and decreases as the leveling route rises into the Santa Monica Mountains. A hydrograph from Well 906 in Canoga Park (Figure 2) records a steady decline in water-level at about 3 m in 30 years. Subsidence or deformation caused by landslides occurred along a 4-km-long section of the leveling route from Glenview to 3 km north of Topanga on Highway 27 between 1974 and 1989. The deformation is not, however, evident in the coseismic elevation changes which were formed by differencing surveys made in 1994 and 1989.
Localized subsidence occurred at a rate of about 1 mm/yr in the Sepulveda Dam Recreation Area between 1968 and 1974. One BM close to the Recreation area indicated subsidence as high as 3 mm/yr for that period. Subsidence is concentrated near the Dam and is negligible at distances of 5 km from it.
There is evidence for subsidence in the vicinity of Redondo Beach. The deformation is evident over a 23-year period. The 1994 to 1989 elevation changes show that the feature is bowl shaped, centered at Redondo Beach and disappears towards Palos Verdes (Figure 7). The correction applied to this line is based on the 1994 to 1989 elevation changes, since the postseismic surveys yield high rates of subsidence which are not evident in the two most recent surveys of the area. The subsidence rate was thus estimated to be about 2 mm/yr at Redondo Beach. Redondo Beach lies at the northeast side of the Torrance oil field and it is probable that subsidence in Redondo Beach is due to oil and gas extraction [U.S. Army Corp of Engineers, 1990]. Groundwater levels have not declined significantly at Redondo Beach since 1975. Although groundwater pumping was extensive in the area, especially in the 1960's the effects have been diminished by sea water intrusions which helped recharge the aquifers [U.S. Army Corp of Engineers, 1990]. Non-tectonic deformation is also evident on the Palos Verdes peninsula. Surveys made in 1971, 1978 and 1989 revealed subsidence rates as high as 8 cm/yr in the vicinity of the Portuguese Bend landslide. Between 1994 and 1989 however, the deformation rate had decreased to 2.7 cm/yr.
Subsidence is occurring west of the San Gabriel River at a maximum rate of 4 mm/yr. East of the river there is no subsidence. The correction applied to this line was determined from the 1994 to 1989 elevation changes since it was difficult to establish a zero subsidence level using the only other preseismic survey which was made in 1946.
U.S. Department of the Interior |
U.S. Geological Survey
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