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Vertical crustal movements preceding and accompanying the San Fernando earthquake of February 9, 1971: A summary
Tectonophysics, 29 (1975) 127-140 0 Elsevier Scientific Publishing Company,
127 Amsterdam
- Printed
in The Netherlands
VERTICAL CRUSTAL MOVEMENTS PRECEDING AND ACCOMPANYING THE SAN FERNANDO EARTHQUAKE FEBRUARY 9,197l: A SUMMARY
ROBERT 0. CASTLE*, JACK P. CHURCH*, NANCY L. MORRISON**
MICHAEL
R. ELLIOTT*
OF
and
* U.S. Geological Survey, Menlo Park, Calif (U.S.A.) ** National Geodetic Survey, Rockville, Md. (U.S.A.) (Accepted
for publication
May 28, 1975)
ABSTRACT Castle, R.O., Church, J.P., Elliott, M.R. and Morrison, N.L., 1975. Vertical crustal movements preceding and accompanying the San Fernando earthquake of February 9, 1971: A summary. In: N. Pavoni and R. Green (Editors), Recent Crustal Movements. Tectonophysics, 29 (l-4): 127-140. Comparative elevations referred chiefly to a tidal bench mark with a history of relatively positive movement show that much of the Transverse Ranges of southern California sustained major changes in elevation both before and in association with the ML 6.4 San Fernando earthquake of February 9, 1971. Preseismic changes in elevation in the southern Transverse Ranges were almost uniformly positive and generally episodic. Maximum uplift measured between 1960/61 and 1968/69 was 0.205 m (observed) or 0.200 m (adjusted); it occurred about 30 km northeast of the 1971 epicenter. As much as 0.129 m (observed) of preseismic uplift was also measured about 30 km northwest of the epicenter between 1964 and 1968. A comparison between 1968 and 1969 elevation data revealed 0.078 m (observed) of differential uplift centered about 10 km west of the epicenter; earlier leveling indicates that this uplift began between February 1967 and May 1968. Changes in elevation measured during the interval 1968/69-71 (postearthquake) are interpreted as chiefly coseismic and were concentrated along a 15-km ruptured segment of the east-trending, north-dipping San Fernando fault. These changes were characterized by subsidence of as much as 0.111 m (observed) or 0.092 m (adjusted) south of the fault and a ridge of uplift of up to at least 2.195 m (observed) or 2.196 m (adjusted) immediately north of the fault. The more episodic preseismic movements are interpretable as deep-seated creep events on the San Fernando fault. Alternatively, these episodic movements may be due partly to the operation of dilatancy; the onset of the 196849 epicentral uplift accords closely with the initiation of a Vp/Vs anomaly recognized in this same area. The 1968/69-71 elevation changes can be attributed almost entirely to slip on the San Fernando fault.
INTRODUCTION
The February 9,197l San Fernando, California panied by both vertical and horizontal movements
earthquake was accomof an almost spectacular
128
nature that included vertical offsets of up to about 1 m and left-lateral displacements of roughly equal amount along the principal tectonic rupture zones produced during this shock (U.S. Geological Survey, 1971, pp. 55-61). Geodetic surveys begun almost immediately after the earthquake subsequently revealed elevation changes of up to more than +2 m and length changes of as much as -2 m (Burford et al., 1971, pp. 82--84; Morrison, 1974; Savage et al., 1974). Owing to the unusually detailed preexisting geodetic data base, the occurrence of this shock presented a unique opportunity for assessing earth movements associated with a moderate- to large-sized earthquake. This opportunity, moreover, proved especially applicable to the assessment of coseismic vertical movements, since the preexisting vertical control in this area was exceptionally detailed. The large number of repeated levelings that preceded this shock has permitted the detection and description of preseismic vertical movements as well, although hardly in the detail of those that accompanied the earthquake. This paper briefly summarizes the results of our investigation of the vertical movements that occurred within a broad region centering on the 1971 earthquake (Fig. 1) during the interval 1960-71.
GENERAL
FEATURES
OF THE EARTHQUAKE
The 1971 San Fernando earthquake was produced through movement (elastic strain release) on what has since been designated the San Fernando fault zone, one of a family of east-trending and generally north-dipping thrust or reverse faults associated with north-south shortening across the Transverse Ranges (Wentworth and Yerkes, 1971, p. 6,12-13). Seismolo&c and tectonic features of both the main shock and the abundant aftershocks have been most recently summarized by Whitcomb et al. (1973a) and Hanks (1974). The main shock has been assigned a local magnitude (ML) of 6.4 and an epicentral location of 34”24.7’N, llS”24,O’W (Whitcomb et al., 1973a, p. 694). Although both investigators agree fairly closely on the epicenter, the hypocenter is given as 8.4 km by Whitcomb et al. (1973a, p. 694) and 13 km by Hanks (1974). The best fit of P-wave first motions for the main shock produces the following thrust solution: strike N 67” (+6” )W; dip 52” (+2”)NE; rake 72”(67”-95”) left lateral (Whitcomb et al., 1973a, p. 702-706). The dip of the seismogenic fault surface, however, apparently shallows to about 35” toward the surface, and there is some evidence that it steepens with depth (Whitcomb et al., 1973a, p. 726; Hanks, 1974, p. 1217). A surface down-stepped to the west is the postulated spatial configuration of the active tectonic element most consistent with the surface rupture zone and the roughly semicircular aftershock pattern spread out to the north (Whitcomb et al., 1973a, p. 695).
129
[1968$69]
I
I
Fig. 1. Locations of primary vertical control lines within the area investigated in this report. Junction points shown by dot,; selected bench marks and profile end points (Figs. 3 and 4) shown by name or letter symbol. Pertinent dates of leveling since 1960 shown in brackets adjacent to each circuit. Leveling surveys continued into a second calendar year (even where interrupted by junction points) indicated by slash symbol. Unless otherwise specified, it is assumed that elevation differences measured during a specific leveling remained unchanged during the period of leveling. 1971 epicenter from Whitcomb et al. (1973a, p. 694).
DATAANALYZED
The vertical crustal movements we describe here arebased on comparisons between both observed and free-adjusted elevations-@%Sduced from leveling carried out within the area of Fig. 1 duringtheQ.period 1960-71 (postearthquake). All of the leveling on which we base ‘our regional comparisons meets the first-order standards of the National Geodetic’Survey and consists chiefly of information developed as a result of repeat&surveys af the primary vertical control net shown in Fig. 1; use of lower&denleveling has been confined to the epicentral area.
130
The basic elevation data consist of measured differences in elevation between adjacent bench marks. Those differences derived from first-order surveys have been appropriately corrected for temperature and any instrumental or rod bias. Orthometric corrections have been applied to all of the derivative first-order elevations; they have not generally been included, however, with elevations based on lower-order surveys unless elevations derived from comparative levelings have also included orthometric corrections. The corrected observed elevations used in this study have been reconstructed chiefly with respect to bench mark Tidal 8, San Pedro (Fig. 1). Free adjustments of these same orthometrically corrected elevations, in which Tidal 8 has been held fixed, have been produced for those data developed from leveling carried out in 1960/61,1968/69, and 1971; lower-order data developed within the epicentral area during the leveling epochs 1968/69 and 1971 have been prorated with respect to bench-mark elevations derived from the first-order free adjustments. Bench mark Tidal 8 is located adjacent to an automatic tide gauge that has been in operation since 1924. Because continuous sea-level records at this gauge show that this station (Los Angeles) has been characterized by slight but relatively positive movement with respect to other California tide stations south of Crescent City (Hicks, 1972, p. 23), the changes in elevation developed from the comparisons we describe are biased against the recognition of positive movement. RESULTS
AND INTERPRETATION
The results of our investigation show that the area centering on the 1971 San Fernando earthquake was associated not only with unexpectedly large vertical movements accompanying the earthquake, but also with easily detected and generally episodic movements generated in advance of this shock (Castle et al., 1974). These movements are most conveniently described through the use of a series of maps and profiles presented in essentially ‘chronologic sequence; the most complete areal coverage is for the leveling epochs 1960/61,1968/69, and 1971 (postearthquake). Although these illustrations are largely self-explanatory, several features deserve qualification and interpretation. Preseismic movements Regional changes in elevation that we interpret as having occurred largely during the interval 1960/61-68/69 (Fig. 2) were developed partly from leveling carried out during epochs other than 1960/61 and 1968/69. Thus we have incorporated 1964 leveling data between Saugus and Sandberg with the 1960/61 data and 1971 leveling data north of Azusa with the 1968/69 data (Fig. 1). An assumption of stability between Saugus and Sandberg between 1960/61 and 1964 is supported by the 1961-64 stability of Saugus with respect to Tidal 8 and the general stability of the reach between Saugus and
_ __
._-.
._
Fig. 2. Regional changes in elevation measured within the area of Fig. 1 during the interval 1960/61-68/69. Based on comparisons between free-adjusted elevations resulting from first-order leveling with respect to bench mark Tidal 8 as invariant in elevation. Major faults generalized from Jennings (1962), Jennings and Strand (1961), and Rogers (1967).
+ E
132
fer from those shown in Fig. 3, moreover,
they dif-
in that they are referred to bench mark X 53 (Fig. 1) rather than Tidal 8, since we have been unable to extend 1969 or later preseismic leveling south of Saugus. Nevertheless, the relative
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stability between X 53 and Tidal 8 during the interval 1968-71 (Fig. 3) suggests that the changes in elevation represented in Fig. 4 would differ only slightly if they could be referred directly to Tidal 8. The most dramatic feature shown by this profile is the sharply differential uplift of at least 78 mm centering in the Saugus area; it diminished to zero 25 km northwest of Saugus and to no more than lo-15 mm eastward toward the San Andreas. That this differential uplift persisted during the 1969 leveling is also shown by overlapping leveling carried out during 1969 (see left-hand inset, Fig. 4). Because Saugus and Palmdale apparently remained unchanged in elevation between 1965 and 1968, a conclusion based partly on the supposed invariance between Tidal 8 and bench mark L 1141 during the interval 1964-65, it had been previously assumed that this massive uplift could not have begun before 1968 (Castle et al., 1974, p. 62,64-65). Nevertheless, the short section of this line leveled in both February 1967 and May 1968 (see right-hand inset, Fig. 4) now indicates that this uplift could have begun as early as 1967 and certainly no later than May 1968. The broadly developed uplift concentrated locally along several recognized faults, and expressed especially well in the series of profiles shown in Fig. 3, may be interpreted as the product of simple contractional strain across the east-trending Transverse Ranges (Castle et al., 1974, p. 66). The regional down-to-the-south tilting hinging on the San Andreas (Fig. 2) is less easily explained, although it too could have developed, at least indirectly, as a result of continuing north-south shortening. The episodic preseismic movements (Figs. 3 and 4) are equally consistent with continuing contractional strain, although their episodic nature is subject to alternative interpretation. Thus the differential movement between Burbank and Saugus between 1964 and 65 (Fig. 3) or the 1968-69 uplift centering on Saugus (Fig. 4) could be interpreted as surface expressions of deep-seated creep events on the San Fernando fault (Jungels and Frazier, 1973, p. 5080; Savage et al., 1974). Alternatively, these episodic movements could be attributed to the operation of dilatancy (that is, volumetric increase due to the formation of cracks accompanying elastic deformation) that has been postulated to occur in advance of large earthquakes (Scholz et al., 1973). The 1968-69 uplift centering in the Saugus area probably qualifies as the episodic or anomalous event most consistent with dilatancy. This likelihood is supported by the occurrence of a VP/V, anomaly in this same area that reportedly developed in 1967 (Whitcomb et al., 197313). Hence the San Fernando earthquake may be the first major shock identified with two completely independent premonitory phenomena indicative of dilatancy outside of, perhaps, the Carm region. Coseismic movements Changes in elevation that we interpret as having accompanied earthquake are based chiefly on leveling carried out in 1968/69
the 1971 and 1971
34”lS’
Fig. 6. Changes in elevation during the interval 1968/69-71 (postearthquake) within the epicentral area (outlined in Fig. 5) of the 1971 San Fernando earthquake. Based on same comparisons as those developed for Fig. 5. Location of major faults generalized from Wentworth and Yerkes (1971, p. 3) and Jennings and Strand (1961); location of tectonic surface ruptures from Wentworth and Yerkes (1971, p. 8) and Barrows et al. (1974). 1971 epicenter from Whitcomb et al. (l973a, p. 694).
(p&earthquake). This is rigorously true as applied to those movements we identify as regional (Fig. 5), but is subject to qu~ification as applied to changes in elevation recognized in the epicentral region (Fig. 6). Thus in calculating elevation changes along level lines emanating from bench marks incorporated in the 1968/69 free adjustment, we have included certain epicentral data based on leveling begun, in one case, as early as 1929. For example, 1963 leveling data athwart the main rupture zone {Fig. 6) have been incorporated with the 1968/69 surveys. We base the acceptance of this earlier work as the equivalent of 1968/69 leveling on the fact that preseismic changes in elevation in the epicentral region were trivial as contrasted with the coseismic movements. The greater the distance from the rupture zone, however, the less valid is the preceding generalization. Furthermore, although it is tacitly assumed that the changes in elevation between 1968/69 and 1971 were entirely coseismic, the occurrence of at least some vertical movements that both predate the earthquake yet postdate the elevation measurements on
138
which we base the 1968/69 adjustment has already been demonstrated (Fig. 4). Circumstantial evidence suggests, in addition, that the area north of Castaic (Fig. 4) sustained differential uplift between 1968 and 1969 that was followed shortly thereafter by preearthquake collapse (Castle et al., 1974, p. 64-65). Hence the actual coseismic changes in elevation in this area may have been very close to zero (or even negative, Fig. 3), rather than the +2030 mm implied by the comparison between the adjusted elevations. The coseismic vertical movements were concentrated along the 1971 rupture zone (Figs. 5 and 6), and were characterized by subsidence of as much as 0.111 m (observed) or 0.092 m (adjusted) south of the rupture zone and a ridge of uplift of up to at least 2.195 m (observed) or 2.196 m (adjusted) immediately north of the fault; these movements can be attributed almost entirely to slip on the San Fernando fault. As shown in Fig. 5, the coseismic movements were concentrated almost entirely within what we define as the epicentral region (Fig. 6). They diminished to almost zero to the south and west and to relatively small values to the north and east. The existence of the very subdued uplift projecting eastward from the epicentral region is especially equivocal; its form is inferred chiefly from movement developed within the epicentral region and the fact that there was almost no change in elevation between the Palmdale and Azusa areas during the 1968169-71 interval. The nature of the vertical movements recorded in two areas within the epicentral region can be characterized as anomalous or at least unexpected. Thus the smoothly developed changes in elevation along the Santa Susana fault zone are seemingly inconsistent with the recognition of tectonic rupturing here. The absence of any clearly defined geodetic expression of this rupturing may be due to the relatively small vertical offsets recognized here (Barrows et al., 1974) and indicates, in any case, that the rupturing on the Santa Susana was no more than a distinctly secondary strain adjustment associated with movement on the main fault. The other area of seemingly anomalous vertical movement is expressed as an irregular west-trending ridge of positive movement localized within the northwest section of the epicentral region (Fig. 6). A linear zone of what has been interpreted as right-lateral tectonic rupturing roughly parallels the northeast-trending isobases within this uplifted area and is at the same time almost precisely coaxial with an exceptionally well defined left-lateral aftershock zone described by Whitcomb et al. (1973a, p. 708-709). Although the significance of this zone of positive surface movement is certainly elusive, it may be related to deepseated tear faulting compatible with Whitcomb et ah’s (1973a, p. 708-709) left-lateral aftershock solutions. Alternatively, it has been suggested (R.L. Wesson, oral communication, 1973) that this zone may be associated with movements at depth on the San Gabriel fault, and that this structure was in fact involved with the 1971 earthquake. Finally, there is a very good possibility that this seemingly coseismic uplift is nothing more than a residual preseismic effect of the 1968-69 uplift centering near Saugus (Fig. 4) (see also Savage et al., 1974).
139
CONCLUSION
The results of our investigation of the preseismic and coseismic vertical crustal movements identified with the 1971 San Fernando earthquake are in many ways consistent with those that could have been predicted from geologic experience and a general knowledge of the deformational processes that characterize contractional strain regions such as the Transverse Ranges. Those phenomena that we did not anticipate include the generally episodic nature of the aseismic movements that preceded the shock and the magnitude and complexity of the changes in elevation that accompanied the earthquake. Recognition of the episodic nature of much of this movement, whether aseismic or not, suggests that vertical-control data in areas such as this should be used judiciously in attempting to interpret tectonic processes. ACKNOWLEDGMENTS
The fieldwork on which this report is based was carried out by the Department of Public Works of the City of Los Angeles, the Los Angeles Department of Water and Power, the Los Angeles Flood Control District, the Los Angeles County Department of County Engineer, the Public Works Department of the County of Ventura, the National Geodetic Survey, and the U.S. Geological Survey. We thank James C. Savage and William H. Prescott for thoughtful review of an earlier version of the manuscript. REFERENCES Barrows, A.G., Kahle, J.E., Saul, R.B. and Weber, F.H., 1975. Surface effects map of the San Fernando earthquake area. In: G.B. Oakeshott (Editor), San Fernando Earthquake of 9 February 1971. Calif. Div. Mines Geol. Bull., 196 pl. 3. Burford, R.O., Castle, R.O., Church, J.P., Kinoshita, W.T., Kirby, S.H., Ruthven, R.T. and Savage, J.C., 1971. Preliminary measurements of tectonic movement. In: The San Fernando, California, Earthquake of February 9, 1971. U.S. Geol. Surv. Prof. Pap., 733: 80-85. Castle, R.O., Alt, J.N., Savage, J.C. and Balazs, E.I., 1974. Elevation changes preceding the San Fernando earthquake of February 9, 1971. Geology, 2: 61-66. Hanks, T.C., 1974. The faulting mechanism of the San Fernando earthquake. J. Geophys. Res., 79: 1215-1229. Hicks, S.D., 1972. On the classification and trends of long period sea level series. Shore Beach, 40 (1): 20-23. Jennings, C.W. (compiler), 1962. Geologic map of California, scale 1 : 250,000. Olaf P. Jenkins edition, Long Beach sheet. Calif. Div. Mines Geol. Jennings, C.W. and Strand, R.G. (compilers), 1961. Geologic map of California, scale 1 : 250,000. Olaf P. Jenkins edition, Los Angeles sheet. Calif. Div. Mines Geol. Jungels, P.H. and Frazier, G.A., 1975. Finite element analysis of the residual displacements for an earthquake rupture: Source parameters for the San Fernando earthquake. J. Geophys. Res., 78: 5062-5083. Morrison, N.L.,‘1973. Vertical crustal movements determined from surveys before and after the San Fernando earthquake. In: Geological and Geophysical Studies, 3, of: San
140 Fernando, California Earthquake of February 9, 1971. U.S. Dep. Comm., N.O.A.A., p. 295-324. Rogers, T.H. (compiler), 1967. Geologic map of California, scale 1 : 250,000. Olaf P. Jenkins edition, San Bernardino sheet. Calif. Div. Mines Geol. Savage, J.C., Burford, R.O. and Kinoshita, W.T., 1975. Earth movements from geodetic measurements. In: G.B. Oakeshott (Editor), San Fernando Earthquake of 9 February 1971. Calif. Div. Mines Geol. Bull., 196: 175-186. Scholz, C.H., Sykes, L.R. and Aggarwal, Y.P., 1973. Earthquake prediction: A physical basis. Science, 181: 803-810. U.S. Geological Survey, 1971. Surface faulting. In: The San Fernando, California, Earthquake of February 9, 1971. U.S. Geol. Surv. Prof. Pap., 733: 55-76. Wentworth, C.M. and Yerkes, R.F., 1971. Geologic setting and activity of faults in the San Fernando area, California. In: The San Fernando, California, Earthquake of February 9, 1971. U.S. Geol. Surv. Prof. Pap., 733: 6-16. Whitcomb, J.H., Allen, C.R., Garmany, J.D. and Hileman, J.A., 1973a. San Fernando earthquake series, 1971: Focal mechanisms and tectonics. Rev. Geophys. Space Phys., 11: 693-730. Whitcomb, J.H., Garmany, J.D. and Anderson, D.L., 1973b. Variation of seismic velocities before the San Fernando earthquake. Science, 180: 632-635.
127 Amsterdam
- Printed
in The Netherlands
VERTICAL CRUSTAL MOVEMENTS PRECEDING AND ACCOMPANYING THE SAN FERNANDO EARTHQUAKE FEBRUARY 9,197l: A SUMMARY
ROBERT 0. CASTLE*, JACK P. CHURCH*, NANCY L. MORRISON**
MICHAEL
R. ELLIOTT*
OF
and
* U.S. Geological Survey, Menlo Park, Calif (U.S.A.) ** National Geodetic Survey, Rockville, Md. (U.S.A.) (Accepted
for publication
May 28, 1975)
ABSTRACT Castle, R.O., Church, J.P., Elliott, M.R. and Morrison, N.L., 1975. Vertical crustal movements preceding and accompanying the San Fernando earthquake of February 9, 1971: A summary. In: N. Pavoni and R. Green (Editors), Recent Crustal Movements. Tectonophysics, 29 (l-4): 127-140. Comparative elevations referred chiefly to a tidal bench mark with a history of relatively positive movement show that much of the Transverse Ranges of southern California sustained major changes in elevation both before and in association with the ML 6.4 San Fernando earthquake of February 9, 1971. Preseismic changes in elevation in the southern Transverse Ranges were almost uniformly positive and generally episodic. Maximum uplift measured between 1960/61 and 1968/69 was 0.205 m (observed) or 0.200 m (adjusted); it occurred about 30 km northeast of the 1971 epicenter. As much as 0.129 m (observed) of preseismic uplift was also measured about 30 km northwest of the epicenter between 1964 and 1968. A comparison between 1968 and 1969 elevation data revealed 0.078 m (observed) of differential uplift centered about 10 km west of the epicenter; earlier leveling indicates that this uplift began between February 1967 and May 1968. Changes in elevation measured during the interval 1968/69-71 (postearthquake) are interpreted as chiefly coseismic and were concentrated along a 15-km ruptured segment of the east-trending, north-dipping San Fernando fault. These changes were characterized by subsidence of as much as 0.111 m (observed) or 0.092 m (adjusted) south of the fault and a ridge of uplift of up to at least 2.195 m (observed) or 2.196 m (adjusted) immediately north of the fault. The more episodic preseismic movements are interpretable as deep-seated creep events on the San Fernando fault. Alternatively, these episodic movements may be due partly to the operation of dilatancy; the onset of the 196849 epicentral uplift accords closely with the initiation of a Vp/Vs anomaly recognized in this same area. The 1968/69-71 elevation changes can be attributed almost entirely to slip on the San Fernando fault.
INTRODUCTION
The February 9,197l San Fernando, California panied by both vertical and horizontal movements
earthquake was accomof an almost spectacular
128
nature that included vertical offsets of up to about 1 m and left-lateral displacements of roughly equal amount along the principal tectonic rupture zones produced during this shock (U.S. Geological Survey, 1971, pp. 55-61). Geodetic surveys begun almost immediately after the earthquake subsequently revealed elevation changes of up to more than +2 m and length changes of as much as -2 m (Burford et al., 1971, pp. 82--84; Morrison, 1974; Savage et al., 1974). Owing to the unusually detailed preexisting geodetic data base, the occurrence of this shock presented a unique opportunity for assessing earth movements associated with a moderate- to large-sized earthquake. This opportunity, moreover, proved especially applicable to the assessment of coseismic vertical movements, since the preexisting vertical control in this area was exceptionally detailed. The large number of repeated levelings that preceded this shock has permitted the detection and description of preseismic vertical movements as well, although hardly in the detail of those that accompanied the earthquake. This paper briefly summarizes the results of our investigation of the vertical movements that occurred within a broad region centering on the 1971 earthquake (Fig. 1) during the interval 1960-71.
GENERAL
FEATURES
OF THE EARTHQUAKE
The 1971 San Fernando earthquake was produced through movement (elastic strain release) on what has since been designated the San Fernando fault zone, one of a family of east-trending and generally north-dipping thrust or reverse faults associated with north-south shortening across the Transverse Ranges (Wentworth and Yerkes, 1971, p. 6,12-13). Seismolo&c and tectonic features of both the main shock and the abundant aftershocks have been most recently summarized by Whitcomb et al. (1973a) and Hanks (1974). The main shock has been assigned a local magnitude (ML) of 6.4 and an epicentral location of 34”24.7’N, llS”24,O’W (Whitcomb et al., 1973a, p. 694). Although both investigators agree fairly closely on the epicenter, the hypocenter is given as 8.4 km by Whitcomb et al. (1973a, p. 694) and 13 km by Hanks (1974). The best fit of P-wave first motions for the main shock produces the following thrust solution: strike N 67” (+6” )W; dip 52” (+2”)NE; rake 72”(67”-95”) left lateral (Whitcomb et al., 1973a, p. 702-706). The dip of the seismogenic fault surface, however, apparently shallows to about 35” toward the surface, and there is some evidence that it steepens with depth (Whitcomb et al., 1973a, p. 726; Hanks, 1974, p. 1217). A surface down-stepped to the west is the postulated spatial configuration of the active tectonic element most consistent with the surface rupture zone and the roughly semicircular aftershock pattern spread out to the north (Whitcomb et al., 1973a, p. 695).
129
[1968$69]
I
I
Fig. 1. Locations of primary vertical control lines within the area investigated in this report. Junction points shown by dot,; selected bench marks and profile end points (Figs. 3 and 4) shown by name or letter symbol. Pertinent dates of leveling since 1960 shown in brackets adjacent to each circuit. Leveling surveys continued into a second calendar year (even where interrupted by junction points) indicated by slash symbol. Unless otherwise specified, it is assumed that elevation differences measured during a specific leveling remained unchanged during the period of leveling. 1971 epicenter from Whitcomb et al. (1973a, p. 694).
DATAANALYZED
The vertical crustal movements we describe here arebased on comparisons between both observed and free-adjusted elevations-@%Sduced from leveling carried out within the area of Fig. 1 duringtheQ.period 1960-71 (postearthquake). All of the leveling on which we base ‘our regional comparisons meets the first-order standards of the National Geodetic’Survey and consists chiefly of information developed as a result of repeat&surveys af the primary vertical control net shown in Fig. 1; use of lower&denleveling has been confined to the epicentral area.
130
The basic elevation data consist of measured differences in elevation between adjacent bench marks. Those differences derived from first-order surveys have been appropriately corrected for temperature and any instrumental or rod bias. Orthometric corrections have been applied to all of the derivative first-order elevations; they have not generally been included, however, with elevations based on lower-order surveys unless elevations derived from comparative levelings have also included orthometric corrections. The corrected observed elevations used in this study have been reconstructed chiefly with respect to bench mark Tidal 8, San Pedro (Fig. 1). Free adjustments of these same orthometrically corrected elevations, in which Tidal 8 has been held fixed, have been produced for those data developed from leveling carried out in 1960/61,1968/69, and 1971; lower-order data developed within the epicentral area during the leveling epochs 1968/69 and 1971 have been prorated with respect to bench-mark elevations derived from the first-order free adjustments. Bench mark Tidal 8 is located adjacent to an automatic tide gauge that has been in operation since 1924. Because continuous sea-level records at this gauge show that this station (Los Angeles) has been characterized by slight but relatively positive movement with respect to other California tide stations south of Crescent City (Hicks, 1972, p. 23), the changes in elevation developed from the comparisons we describe are biased against the recognition of positive movement. RESULTS
AND INTERPRETATION
The results of our investigation show that the area centering on the 1971 San Fernando earthquake was associated not only with unexpectedly large vertical movements accompanying the earthquake, but also with easily detected and generally episodic movements generated in advance of this shock (Castle et al., 1974). These movements are most conveniently described through the use of a series of maps and profiles presented in essentially ‘chronologic sequence; the most complete areal coverage is for the leveling epochs 1960/61,1968/69, and 1971 (postearthquake). Although these illustrations are largely self-explanatory, several features deserve qualification and interpretation. Preseismic movements Regional changes in elevation that we interpret as having occurred largely during the interval 1960/61-68/69 (Fig. 2) were developed partly from leveling carried out during epochs other than 1960/61 and 1968/69. Thus we have incorporated 1964 leveling data between Saugus and Sandberg with the 1960/61 data and 1971 leveling data north of Azusa with the 1968/69 data (Fig. 1). An assumption of stability between Saugus and Sandberg between 1960/61 and 1964 is supported by the 1961-64 stability of Saugus with respect to Tidal 8 and the general stability of the reach between Saugus and
_ __
._-.
._
Fig. 2. Regional changes in elevation measured within the area of Fig. 1 during the interval 1960/61-68/69. Based on comparisons between free-adjusted elevations resulting from first-order leveling with respect to bench mark Tidal 8 as invariant in elevation. Major faults generalized from Jennings (1962), Jennings and Strand (1961), and Rogers (1967).
+ E
132
fer from those shown in Fig. 3, moreover,
they dif-
in that they are referred to bench mark X 53 (Fig. 1) rather than Tidal 8, since we have been unable to extend 1969 or later preseismic leveling south of Saugus. Nevertheless, the relative
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135
136
stability between X 53 and Tidal 8 during the interval 1968-71 (Fig. 3) suggests that the changes in elevation represented in Fig. 4 would differ only slightly if they could be referred directly to Tidal 8. The most dramatic feature shown by this profile is the sharply differential uplift of at least 78 mm centering in the Saugus area; it diminished to zero 25 km northwest of Saugus and to no more than lo-15 mm eastward toward the San Andreas. That this differential uplift persisted during the 1969 leveling is also shown by overlapping leveling carried out during 1969 (see left-hand inset, Fig. 4). Because Saugus and Palmdale apparently remained unchanged in elevation between 1965 and 1968, a conclusion based partly on the supposed invariance between Tidal 8 and bench mark L 1141 during the interval 1964-65, it had been previously assumed that this massive uplift could not have begun before 1968 (Castle et al., 1974, p. 62,64-65). Nevertheless, the short section of this line leveled in both February 1967 and May 1968 (see right-hand inset, Fig. 4) now indicates that this uplift could have begun as early as 1967 and certainly no later than May 1968. The broadly developed uplift concentrated locally along several recognized faults, and expressed especially well in the series of profiles shown in Fig. 3, may be interpreted as the product of simple contractional strain across the east-trending Transverse Ranges (Castle et al., 1974, p. 66). The regional down-to-the-south tilting hinging on the San Andreas (Fig. 2) is less easily explained, although it too could have developed, at least indirectly, as a result of continuing north-south shortening. The episodic preseismic movements (Figs. 3 and 4) are equally consistent with continuing contractional strain, although their episodic nature is subject to alternative interpretation. Thus the differential movement between Burbank and Saugus between 1964 and 65 (Fig. 3) or the 1968-69 uplift centering on Saugus (Fig. 4) could be interpreted as surface expressions of deep-seated creep events on the San Fernando fault (Jungels and Frazier, 1973, p. 5080; Savage et al., 1974). Alternatively, these episodic movements could be attributed to the operation of dilatancy (that is, volumetric increase due to the formation of cracks accompanying elastic deformation) that has been postulated to occur in advance of large earthquakes (Scholz et al., 1973). The 1968-69 uplift centering in the Saugus area probably qualifies as the episodic or anomalous event most consistent with dilatancy. This likelihood is supported by the occurrence of a VP/V, anomaly in this same area that reportedly developed in 1967 (Whitcomb et al., 197313). Hence the San Fernando earthquake may be the first major shock identified with two completely independent premonitory phenomena indicative of dilatancy outside of, perhaps, the Carm region. Coseismic movements Changes in elevation that we interpret as having accompanied earthquake are based chiefly on leveling carried out in 1968/69
the 1971 and 1971
34”lS’
Fig. 6. Changes in elevation during the interval 1968/69-71 (postearthquake) within the epicentral area (outlined in Fig. 5) of the 1971 San Fernando earthquake. Based on same comparisons as those developed for Fig. 5. Location of major faults generalized from Wentworth and Yerkes (1971, p. 3) and Jennings and Strand (1961); location of tectonic surface ruptures from Wentworth and Yerkes (1971, p. 8) and Barrows et al. (1974). 1971 epicenter from Whitcomb et al. (l973a, p. 694).
(p&earthquake). This is rigorously true as applied to those movements we identify as regional (Fig. 5), but is subject to qu~ification as applied to changes in elevation recognized in the epicentral region (Fig. 6). Thus in calculating elevation changes along level lines emanating from bench marks incorporated in the 1968/69 free adjustment, we have included certain epicentral data based on leveling begun, in one case, as early as 1929. For example, 1963 leveling data athwart the main rupture zone {Fig. 6) have been incorporated with the 1968/69 surveys. We base the acceptance of this earlier work as the equivalent of 1968/69 leveling on the fact that preseismic changes in elevation in the epicentral region were trivial as contrasted with the coseismic movements. The greater the distance from the rupture zone, however, the less valid is the preceding generalization. Furthermore, although it is tacitly assumed that the changes in elevation between 1968/69 and 1971 were entirely coseismic, the occurrence of at least some vertical movements that both predate the earthquake yet postdate the elevation measurements on
138
which we base the 1968/69 adjustment has already been demonstrated (Fig. 4). Circumstantial evidence suggests, in addition, that the area north of Castaic (Fig. 4) sustained differential uplift between 1968 and 1969 that was followed shortly thereafter by preearthquake collapse (Castle et al., 1974, p. 64-65). Hence the actual coseismic changes in elevation in this area may have been very close to zero (or even negative, Fig. 3), rather than the +2030 mm implied by the comparison between the adjusted elevations. The coseismic vertical movements were concentrated along the 1971 rupture zone (Figs. 5 and 6), and were characterized by subsidence of as much as 0.111 m (observed) or 0.092 m (adjusted) south of the rupture zone and a ridge of uplift of up to at least 2.195 m (observed) or 2.196 m (adjusted) immediately north of the fault; these movements can be attributed almost entirely to slip on the San Fernando fault. As shown in Fig. 5, the coseismic movements were concentrated almost entirely within what we define as the epicentral region (Fig. 6). They diminished to almost zero to the south and west and to relatively small values to the north and east. The existence of the very subdued uplift projecting eastward from the epicentral region is especially equivocal; its form is inferred chiefly from movement developed within the epicentral region and the fact that there was almost no change in elevation between the Palmdale and Azusa areas during the 1968169-71 interval. The nature of the vertical movements recorded in two areas within the epicentral region can be characterized as anomalous or at least unexpected. Thus the smoothly developed changes in elevation along the Santa Susana fault zone are seemingly inconsistent with the recognition of tectonic rupturing here. The absence of any clearly defined geodetic expression of this rupturing may be due to the relatively small vertical offsets recognized here (Barrows et al., 1974) and indicates, in any case, that the rupturing on the Santa Susana was no more than a distinctly secondary strain adjustment associated with movement on the main fault. The other area of seemingly anomalous vertical movement is expressed as an irregular west-trending ridge of positive movement localized within the northwest section of the epicentral region (Fig. 6). A linear zone of what has been interpreted as right-lateral tectonic rupturing roughly parallels the northeast-trending isobases within this uplifted area and is at the same time almost precisely coaxial with an exceptionally well defined left-lateral aftershock zone described by Whitcomb et al. (1973a, p. 708-709). Although the significance of this zone of positive surface movement is certainly elusive, it may be related to deepseated tear faulting compatible with Whitcomb et ah’s (1973a, p. 708-709) left-lateral aftershock solutions. Alternatively, it has been suggested (R.L. Wesson, oral communication, 1973) that this zone may be associated with movements at depth on the San Gabriel fault, and that this structure was in fact involved with the 1971 earthquake. Finally, there is a very good possibility that this seemingly coseismic uplift is nothing more than a residual preseismic effect of the 1968-69 uplift centering near Saugus (Fig. 4) (see also Savage et al., 1974).
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CONCLUSION
The results of our investigation of the preseismic and coseismic vertical crustal movements identified with the 1971 San Fernando earthquake are in many ways consistent with those that could have been predicted from geologic experience and a general knowledge of the deformational processes that characterize contractional strain regions such as the Transverse Ranges. Those phenomena that we did not anticipate include the generally episodic nature of the aseismic movements that preceded the shock and the magnitude and complexity of the changes in elevation that accompanied the earthquake. Recognition of the episodic nature of much of this movement, whether aseismic or not, suggests that vertical-control data in areas such as this should be used judiciously in attempting to interpret tectonic processes. ACKNOWLEDGMENTS
The fieldwork on which this report is based was carried out by the Department of Public Works of the City of Los Angeles, the Los Angeles Department of Water and Power, the Los Angeles Flood Control District, the Los Angeles County Department of County Engineer, the Public Works Department of the County of Ventura, the National Geodetic Survey, and the U.S. Geological Survey. We thank James C. Savage and William H. Prescott for thoughtful review of an earlier version of the manuscript. REFERENCES Barrows, A.G., Kahle, J.E., Saul, R.B. and Weber, F.H., 1975. Surface effects map of the San Fernando earthquake area. In: G.B. Oakeshott (Editor), San Fernando Earthquake of 9 February 1971. Calif. Div. Mines Geol. Bull., 196 pl. 3. Burford, R.O., Castle, R.O., Church, J.P., Kinoshita, W.T., Kirby, S.H., Ruthven, R.T. and Savage, J.C., 1971. Preliminary measurements of tectonic movement. In: The San Fernando, California, Earthquake of February 9, 1971. U.S. Geol. Surv. Prof. Pap., 733: 80-85. Castle, R.O., Alt, J.N., Savage, J.C. and Balazs, E.I., 1974. Elevation changes preceding the San Fernando earthquake of February 9, 1971. Geology, 2: 61-66. Hanks, T.C., 1974. The faulting mechanism of the San Fernando earthquake. J. Geophys. Res., 79: 1215-1229. Hicks, S.D., 1972. On the classification and trends of long period sea level series. Shore Beach, 40 (1): 20-23. Jennings, C.W. (compiler), 1962. Geologic map of California, scale 1 : 250,000. Olaf P. Jenkins edition, Long Beach sheet. Calif. Div. Mines Geol. Jennings, C.W. and Strand, R.G. (compilers), 1961. Geologic map of California, scale 1 : 250,000. Olaf P. Jenkins edition, Los Angeles sheet. Calif. Div. Mines Geol. Jungels, P.H. and Frazier, G.A., 1975. Finite element analysis of the residual displacements for an earthquake rupture: Source parameters for the San Fernando earthquake. J. Geophys. Res., 78: 5062-5083. Morrison, N.L.,‘1973. Vertical crustal movements determined from surveys before and after the San Fernando earthquake. In: Geological and Geophysical Studies, 3, of: San
140 Fernando, California Earthquake of February 9, 1971. U.S. Dep. Comm., N.O.A.A., p. 295-324. Rogers, T.H. (compiler), 1967. Geologic map of California, scale 1 : 250,000. Olaf P. Jenkins edition, San Bernardino sheet. Calif. Div. Mines Geol. Savage, J.C., Burford, R.O. and Kinoshita, W.T., 1975. Earth movements from geodetic measurements. In: G.B. Oakeshott (Editor), San Fernando Earthquake of 9 February 1971. Calif. Div. Mines Geol. Bull., 196: 175-186. Scholz, C.H., Sykes, L.R. and Aggarwal, Y.P., 1973. Earthquake prediction: A physical basis. Science, 181: 803-810. U.S. Geological Survey, 1971. Surface faulting. In: The San Fernando, California, Earthquake of February 9, 1971. U.S. Geol. Surv. Prof. Pap., 733: 55-76. Wentworth, C.M. and Yerkes, R.F., 1971. Geologic setting and activity of faults in the San Fernando area, California. In: The San Fernando, California, Earthquake of February 9, 1971. U.S. Geol. Surv. Prof. Pap., 733: 6-16. Whitcomb, J.H., Allen, C.R., Garmany, J.D. and Hileman, J.A., 1973a. San Fernando earthquake series, 1971: Focal mechanisms and tectonics. Rev. Geophys. Space Phys., 11: 693-730. Whitcomb, J.H., Garmany, J.D. and Anderson, D.L., 1973b. Variation of seismic velocities before the San Fernando earthquake. Science, 180: 632-635.