Robert O. Castle

Aseismic Uplift in Southern California

Preliminary examination of the historic geodetic record has disclosed crustal uplift of 0.15 to 0.25 meter that apparently began around 1960 and has since grown to include at least 12,000 square kilometers of southern California. This uplift extends at least 150 kilometers west-northwestward along the San Andreas Fault from Cajon to Maricopa, southward from the San Andreas into the northern Transverse Ranges, and eastward from Lebec into and including much of western Mojave block. It seems to have grown spasmodically eastward from a center near the junction of the San Andreas and Garlock faults and has occurred largely within an area that has remained virtually aseismic since at least 1932. Although much of this area has been characterized by crustal mobility since at least the turn the century, the described uplift seems to be an unusually large and probably unique event superimposed the existing pattern of continuing deformation.

Seismicity and faulting attributable to fluid extraction

The association between fluid injection and seismicity has been well documented and widely publicized. Less well known, but probably equally widespread are faulting and shallow seismicity attributable solely to fluid extraction, particularly in association with petroleum production. Two unequivocable examples of seismicity and faulting associated with fluid extraction in the United States are: The Goose Creek, Texas oil field event of 1925 (involving surface rupture); and the Wilmington, California oil field events (involving subsurface rupture) of 1947, 1949, 1951 (2), 1955, and 1961. Six additional cases of intensity I–VII earthquakes (M < 4.6) without reported faulting may be attributable to shallow production from other large oil and gas fields. In addition to these examples are thirteen cases of apparently aseismic surface rupture associated with production from California and Texas oil fields. Small earthquakes in the Eloy—Picacho area of Arizona may be attributable to withdrawal of groundwater, but their relation to widespread fissuring is enigmatic. The clearest example of extraction-induced seismicity outside of North America is the 1951 series of earthquakes associated with gas production from the Po River delta near Caviga, Italy. Faulting and seismicity associated with fluid extraction are attributed to differential compaction at depth caused by reduction of reservoir fluid pressure and attendant increase in effective stress. Surface and subsurface measurements and theoretical and model studies show that differential compaction leads not only to differential subsidence and centripetally-directed horizontal displacements, but to changes in both vertical- and horizontal-strain regimes. Study of well-documented examples indicates that the occurrence and nature of faulting and seismicity associated with compaction are functions chiefly of: (1) the pre-exploitation strain regime, and (2) the magnitude of contractional horizontal strain centered over the compacting materials relative to that of the surrounding annulus of extensional horizontal strain. The examples cited include natural systems strained only by extraction of fluids, as well as some subsequently subjected to injection. Faulting and seismicity have accompanied both decrease and subsequent increase of fluid pressures; reversal of fluid-pressure decline by injection may enhance the likelihood of subsurface faulting and seismicity due chiefly to earlier fluid pressure reduction. A consistent common denominator appears to be continuing compaction at depth; the relative effects of fluid extraction followed by injection are not easily separated.

Elevation Changes Preceding the San Fernando Earthquake of February 9, 1971

Comparative elevation studies based on levelings in 1961, 1964, 1965, 1968, and 1969 show that the central Transverse Ranges of southern California sustained major elevation changes prior to the 1971 San Fernando earthquake. These changes are referred chiefly to bench mark Tidal 8, San Pedro, as invariant in elevation; they have been almost uniformly positive and generally episodic. Maximum uplift was recorded during the interval 1961–1964; it occurred south of Palmdale, about 28 km east-northeast of the 1971 epicenter, and measured 207 mm. Maximum values of uplift recorded along the main north-south level line through the central Transverse Ranges were 96 mm measured between 1961 and 1968, 13 km southwest of the epicenter, and 129 mm measured between 1964 and 1968, 30 km northwest of the epicenter. Uplift of 78 mm in reference to a control point 40 km northwest of Saugus was recorded 12 km west-northwest of the epicenter during the interval 1968–1969. The crustal movements recognized here are most simply interpreted as the product of continuing north-south contractional strain. Alternatively, the more episodic movements in particular may derive from dilatancy (inelastic volume increases) postulated to occur before earthquakes. Because the uplift near Palmdale has continued more or less independently of that to the south and west, it is probably associated with mechanically separable strain accumulation on the San Andreas fault.

Four-dimensional modeling of recent vertical movements in the area of the southern california uplift

Abstract This paper describes an analytical technique that utilizes scattered geodetic relevelings and tide-gauge records to portray Recent vertical crustal movements that may have been characterized by spasmodic changes in velocity. The technique is based on the fitting of a time-varying algebraic surface of prescribed degree to the geodetic data treated as tilt elements and to tide-gauge readings treated as point movements. Desired variations in time can be selected as any combination of powers of vertical movement velocity and episodic events. The state of the modeled vertical displacement can be shown for any number of dates for visual display. Statistical confidence limits of the modeled displacements, derived from the density of measurements in both space and time, line length, and accuracy of input data, are also provided. The capabilities of the technique are demonstrated on selected data from the region of the southern California uplift.

Vertical crustal movements preceding and accompanying the San Fernando earthquake of February 9, 1971: A summary

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 (1—4): 127—140.

Elevation Changes in the Central Transverse Ranges Near Ventura, California*

Abstract Profiles of elevation changes developed from repeated levelings in the east-trending Transverse Ranges near Ventura, California, reveal three general types of vertical movements: 1. (1) broadly defined regional tilting; 2. (2) sharply defined differential movements across recently active faults; and 3. (3) differential subsidence centering on producing oil fields. Down-to-the-southeast tilting is evident in profiles along the coast this sense of movement, however, is the inverse of that that may have prevailed during late Pleistocene time. Profiles along lines extending north and northwest from Ventura show prominent inflections formed by up-to-the-north differential movements that coincide roughly with the Red Mountain fault; this fault is a north-dipping reverse fault that displaces a Holocene(?) soil zone and along which scarps and sag ponds are preserved. A similar inflection coincides with the Padre Juan fault; post-Pleistocene activity on the Padre Juan, however, is uniquely indicated by the geodetic data. Contemporary integrity of the structural block extending northward from the Red Mountain fault is suggested by the apparent absence of differential movements across the Munson Creek, Tule Creek, Santa Ynez, and Arroyo Parida faults since at least 1934. Subsidence is recognized over both the Ventura and Rincon oil fields; although maximum subsidence has not been recorded in either case, 277 mm of differential subsidence was measured within the Ventura field between 1934 and 1968.

Tectonic state: its significance and characterization in the assessment of seismic effects associated with reservoir impounding

Abstract Any analysis of seismicity associated with the filling of large reservoirs requires an evaluation of the natural tectonic state in order to determine whether impoundment is the basic source, a mechanically unrelated companion feature, or a triggering stimulus of the observed seismicity. Several arguments indicate that the associated seismicity is usually a triggered effect. Among the elements of tectonic state considered here (existing fractures, accumulated elastic strain, and deformational style), deformational style is especially critical in forecasting the occurrence of impoundment-induced seismicity. The observational evidence indicates that seismicity associated with impounding generally occurs in areas that combine steeply dipping faults, relatively high strain rates, and either extensional or horizontal-shear strain. Simple physical arguments suggest: (1) that increased fluid pressures resulting from increased reservoir head should enhance the likelihood of seismic activity, whatever the tectonic environment; (2) that stress changes resulting from surface loading may increase the likelihood of crustal failure in areas of normal and transcurrent faulting, whereas they generally inhibit failure in areas of thrust faulting. Comparisons with other earthquake-producing artificial and natural processes (underground explosions, fluid injection, underground mining, fluid extraction, volcanic emissions) indicate that reservoir loading may similarly modify the natural tectonic state. Subsurface loading resulting from fluid extraction may be a particularly close analogue of reservoir loading; “seismotectonic” events associated with fluid extraction have been recognized in both seismically active and otherwise aseismic regions. Because the historic record of seismicity and surface faulting commonly is short in comparison with recurrence intervals of earthquake and fault-slip events, tectonic state is most reliably appraised through combined studies of historic seismicity and faulting, instrumentally measured strain, and the geological record, especially that of the Quaternary. Experience in California and elsewhere demonstrates that the character and activity of recognized faults can be assessed by means of: instrumental earthquake investigations, repeated geodetic measurements, written history, archeological studies, fault topography, and local stratigraphic relations. Where faults are less easily distinguished, appraisals of tectonic state may be based on both the regional seismicity and the regional history of vertical movement as shown by: repeated levelling and sea-level measurements, written history, archeologic investigations, terrace and shoreline deformation, and denudation and sedimentation studies.

An exsolution silica-pump model for the origin of myrmekite

Myrmekite, as defined here, is the microscopic intergrowth between vermicular quartz and modestly anorthitic plagioclase (calcic albite-oligoclase), intimately associated with potassium feldspar in plutonic rocks of granitic composition. Hypotheses previously invoked in explanation of myrmekite include: (1) direct crystallization; (2) replacement; (3) exsolution. The occurrence of myrmekite in paragneisses and its absence in rocks devold of discrete grains of potassium feldspar challenge those hypotheses based on direct crystallization or replacement. However, several lines of evidence indicate that myrmekite may in fact originate in response to kinetic effects associated with the exsolution of calcic alkali feldspar into discrete potassium feldspar and plagioclase phases. Exsolution of potassium feldspar system projected from [AlSi2O8] involves the exchange CaAlK-1Si-1, in which the AlSi-1 tetrahedral couple is resistant to intracrystalline diffusion. By contrast, diffusion of octahedral K proceeds relatively easily where it remains uncoupled to the tetrahedral exchange. We suggest here that where the ternary feldspar system is open to excess silica, the exchange reaction that produces potassium feldspar in the ternary plane is aided by the net-transfer reaction K+Si=Orthoclase, leaving behind indigenous Si that reports as modal quartz in the evolving plagioclase as the CaAl component is concomitantly incorporated in this same phase. Thus silica is “pumped” into the reaction volume from a “silica reservoir”, a process that enhances redistribution of both Si and Al through the exsolving ternary feldspar.

Estimation of rod scale errors in geodetic leveling

Comparisons among repeated geodetic levelings have often been used for detecting and estimating residual rod scale errors in leveled heights. Individual rod-pair scale errors are estimated by a two-step procedure using a model based on either differences in heights, differences in section height differences, or differences in section tilts. It is shown that the estimated rod-pair scale errors derived from each model are identical only when the data are correctly weighted, and the mathematical correlations are accounted for in the model based on heights. Analyses based on simple regressions of changes in height versus height can easily lead to incorrect conclusions. We also show that the statistically estimated scale errors are not a simple function of height, height difference, or tilt. The models are valid only when terrain slope is constant over adjacent pairs of setups (i.e., smoothly varying terrain). In order to discriminate between rod scale errors and vertical displacements due to crustal motion, the individual rod-pairs should be used in more than one leveling, preferably in areas of contrasting tectonic activity. From an analysis of 37 separately calibrated rod-pairs used in 55 levelings in southern California, we found eight statistically significant coefficients that could be reasonably attributed to rod scale errors, only one of which was larger than the expected random error in the applied calibration-based scale correction. However, significant differences with other independent checks indicate that caution should be exercised before accepting these results as evidence of scale error. Further refinements of the technique are clearly needed if the results are to be routinely applied in practice.

A revised configuration of the southern California uplift

Recently recovered archival leveling data from southwestern Arizona and adjacent parts of California compel major revisions in the configuration and interpretation of the southern California uplift, both at its culmination and following its partial collapse. Re-examination of the older vertical-control record indicates that similar modifications may be equally appropriate in any reconsideration of the early-20th-century analogue of the southern California uplift. The impact of these revisions is limited to the southeastern part of the uplift, which is now believed to have projected well into northern Mexico. The chief changes in our earlier reconstructions appear as a sharply diminished isobase gradient south of Cottonwood Pass, dramatically reduced tectonic subsidence centering on the Salton Sea, and short-lived uplift of at least 0.3 m at the latitude of El Centro. A newly refined qualitative interpretation of the data implies contraction and decoupling at the base of the seismogenic zone, in conjunction with right-stepping movement and extension between an en echelon transform pair through the Salton Basin, combining to produce both the regional uplift and concurrently developed and relatively localized differential subsidence within the Salton Trough.