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1994
Johnson, HO, Agnew DC, Wyatt FK.  1994.  Present-Day Crustal Deformation in Southern California. Journal of Geophysical Research-Solid Earth. 99:23951-23974.   10.1029/94jb01902   AbstractWebsite

The effects of laterally homogeneous mantle electrical conductivity have been included in steady. Using an extensive set of precise geodetic measurements, we have developed a detailed picture of present-day deformation rates in southern California. This large set of measurements, amounting to nearly 2000 repeated distance measurements over the period 1973 to 1991, comes from the U.S. Geological Survey's Geodolite trilateration program, involving their combined Anza, Joshua Tree, and Salton networks. Building on previous results from these data, we are able to present the deformation field as estimates of the rate of horizontal strain accumulation in small four-station subnetworks of the overall 89-station network.; Using this technique, the spatial details of the 18-year average strain rate field can be determined. By correlating these spatial details with the tectonics of the region we are able to understand better how deformation is partitioned across this highly complex margin between the Pacific and North American tectonic plates. Some of the more interesting findings of this study are that (1) the vast majority of strain rate estimates show a pattern of nearly pure shear as would be expected in a transcurrent environment, (2) the fastest accumulation of surface strain in southern California is along the San Jacinto Fault west of the Salton Sea, not along the San Andreas Fault, (3) strain accumulation rate along the length of the San Jacinto Fault increases toward the southeast as the fault enters the Imperial Valley, (4) a large area near the southern end of the Salton Sea, where the San Andreas Fault meets the Brawley Seismic Zone, is undergoing areal dilatation, which is in part consistent with the formation of crust at a spreading center, and (5) deformation at the transition zone between the San Andreas Fault and the Eastern California Shear Zone also appears to be the result of crustal spreading.

1993
Bock, Y, Agnew DC, Fang P, Genrich JF, Hager BH, Herring TA, Hudnut KW, King RW, Larsen S, Minster JB, Stark K, Wdowinski S, Wyatt FK.  1993.  Detection of Crustal Deformation from the Landers Earthquake Sequence Using Continuous Geodetic Measurements. Nature. 361:337-340.   10.1038/361337a0   AbstractWebsite

THE measurement of crustal motions in tectonically active regions is being performed increasingly by the satellite-based Global Positioning System (GPS)1,2, which offers considerable advantages over conventional geodetic techniques3,4. Continuously operating GPS arrays with ground-based receivers spaced tens of kilometres apart have been established in central Japan5,6 and southern California to monitor the spatial and temporal details of crustal deformation. Here we report the first measurements for a major earthquake by a continuously operating GPS network, the Permanent GPS Geodetic Array (PGGA)7-9 in southern California. The Landers (magnitude M(w) of 7.3) and Big Bear (M(w) 6.2) earthquakes of 28 June 1992 were monitored by daily observations. Ten weeks of measurements, centred on the earthquake events, indicate significant coseismic motion at all PGGA sites, significant post-seismic motion at one site for two weeks after the earthquakes, and no significant preseismic motion. These measurements demonstrate the potential of GPS monitoring for precise detection of precursory and aftershock seismic deformation in the near and far field.

Larsen, SC, Agnew DC, Hager BH.  1993.  Strain Accumulation in the Santa-Barbara Channel - 1970-1988. Journal of Geophysical Research-Solid Earth. 98:2119-2133.   10.1029/92jb02043   AbstractWebsite

Geodetic observations between 1970 and 1988 indicate appreciable strain accumulation in the Santa Barbara Channel, California. Eleven line-length changes from a six-station geodetic network spanning the eastern two-thirds of the channel were determined from electronic distance measurements in 1970/1971 and Global Positioning System observations in 1987/1988. Within this network the strains observed are spatially nonuniform. In the easternmost channel the strain is nearly uniaxial, with convergence of 6.4 +/- 0.9 mm/yr oriented N25-degrees-E +/-5-degrees; this direction is consistent with the seismicity, which is dominated by thrust mechanisms with P axes directed to the northeast. In the central channel the strain is less well determined, but appears to include a significant component of shear that is left-lateral when resolved on an east-west plane.

Feigl, KL, Agnew DC, Bock Y, Dong D, Donnellan A, Hager BH, Herring TA, Jackson DD, Jordan TH, King RW, Larsen S, Larson KM, Murray MH, Shen ZK, Webb FH.  1993.  Space Geodetic Measurement of Crustal Deformation in Central and Southern California, 1984-1992. Journal of Geophysical Research-Solid Earth. 98:21677-21712.   10.1029/93jb02405   AbstractWebsite

We estimate the velocity field in central and southern California using Global Positioning System (GPS) observations from 1986 to 1992 and very long baseline interferometry (VLBI) observations from 1984 to 1991. OUT core network includes 12 GPS sites spaced approximately 50 km apart, mostly in the western Transverse Ranges and the coastal Borderlands. The precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95% confidence ellipse with a semiminor axis of approximately 2 mm/yr oriented roughly north-south, and a semimajor axis of approximately 3 mm/yr oriented east-west. For other stations, occupied fewer than 5 times, or occupied during experiments with poor tracking geometries, the uncertainty is larger. These uncertainties are calibrated by analyzing the scatter in three types of comparisons: (1) multiple measurements of relative position (''repeatability''), (2) independent velocity estimates from separate analyses of the GPS and VLBI data, and (3) rates of change in baseline length estimated from the joint GPS+VLBI solution and from a comparison of GPS with trilateration. The dominant tectonic signature in the velocity field is shear deformation associated with the San Andreas and Garlock faults, which we model as resulting from slip below a given locking depth. Removing the effects of this simple model from the observed velocity field reveals residual deformation that is not attributable to the San Andreas fault. Baselines spanning the eastern Santa Barbara Channel, the Ventura basin, the Los Angeles basin, and the Santa Maria Fold and Thrust Belt are shortening at rates of up to 5 +/- 1, 5 +/- 1, 5 +/- 1, and 2 +/- 1 mm/yr, respectively. North of the Big Bend, some compression normal to the trace of the San Andreas fault can be resolved on both sides of the fault. The rates of rotation about vertical axes in the residual geodetic velocity field differ by up to a factor of 2 from those inferred from paleomagnetic declinations. Our estimates indicate that the ''San Andreas discrepancy'' can be resolved to within the 3 mm/yr uncertainties by accounting for deformation in California between Vandenberg (near Point Conception) and the westernmost Basin and Range. Strain accumulation of 1-2 mm/yr on structures offshore of Vandenberg is also allowed by the uncertainties. South of the Transverse Ranges, the deformation budget must include 5 mm/yr between the offshore islands and the mainland.

1992
Agnew, DC.  1992.  The Time-Domain Behavior of Power-Law Noises. Geophysical Research Letters. 19:333-336.   10.1029/91gl02832   AbstractWebsite

The power spectra of many geophysical phenomena are well approximated by a power-law dependence on frequency or wavenumber. I derive a simple expression for the root-mean-square variability of a process with such a spectrum over an interval of time or space. The resulting expression yields the power-law time dependence characteristic of fractal processes, but can be generalized to give the temporal variability for more general spectral behaviors. The method is applied to spectra of crustal strain (to show what size of strain events can be detected over periods of months to seconds) and of sea level (to show the difficulty of extracting long-term rates from short records).

1991
Larson, KM, Agnew DC.  1991.  Application of the Global Positioning System to Crustal Deformation Measurement 1. Precision and Accuracy. Journal of Geophysical Research-Solid Earth. 96:16547-16565.   10.1029/91jb01275   AbstractWebsite

In this paper we assess the precision and accuracy of interstation vectors determined using the Global Positioning System (GPS) satellites. These vectors were between stations in California separated by 50-450 km. Using data from tracking the seven block I satellites in campaigns from 1986 through 1989, we examine the precision of GPS measurements over time scales of a several days and a few years. We characterize GPS precision by constant and length dependent terms. The north-south component of the interstation vectors has a short-term precision of 1.9 mm + 0.6 parts in 10(8); the east-west component shows a similar precision at the shortest distances, 2.1 mm, with a larger length dependence, 1.3 parts in 10(8). The vertical precision has a mean value of 17 mm, with no clear length dependence. For long-term precision, we examine interstation vectors measured over a period of 2.2 to 2.7 years. When we include the recent results of Davis et al. (1989) for distances less than 50 km, we can describe long-term GPS precision for baselines less than 450 km in length as 3.4 mm + 1.2 parts in 10(8), 5.2 mm + 2.8 parts in 10(8), 11.7 mm + 13 parts in 10(8) in the north- south, east-west, and vertical components. Accuracy has been determined by comparing GPS baseline estimates with those derived from very long baseline interferometry (VLBI). A comparison of eight interstation vectors shows differences ranging from 5 to 30 mm between the mean GPS and mean VLBI estimates in the horizontal components and less than 80 mm in the vertical. A large portion of the horizontal differences can be explained by local survey errors at two sites in California. A comparison which suffers less from such errors is between the rates of change of the baselines. The horizontal rates estimated from over 4 years of VLBI data agree with those determined with 1-2 years of GPS data to within one standard deviation. In the vertical, both GPS and VLBI find insignificant vertical motion.

Larson, KM, Webb FH, Agnew DC.  1991.  Application of the Global Positioning System to Crustal Deformation Measurement 2. The Influence of Errors in Orbit Determination Networks. Journal of Geophysical Research-Solid Earth. 96:16567-16584.   10.1029/91jb01276   AbstractWebsite

Global Positioning System (GPS) measurements of a geodetic network in southern and central California have been used to investigate the errors introduced by adopting different sets of stations as fixed. Such fixed points, called fiducial stations, are necessary to eliminate the errors of imprecise satellites orbits, which otherwise would dominate the error budget for distances greater than tens of kilometers. These fiducial stations also define the reference frame of the crustal deformation network. Establishing the magnitude of the effect of changing the fiducial network is essential for crustal deformation studies, so that these artifacts of the differences between fiducial networks used for the data analyses are not interpreted as geophysical signals. Solutions for a crustal deformation network spanning distances up to 350 km were computed with a variety of fiducial networks. We use fiducial coordinates determined from very long baseline interferometry (VLBI). We compare these solutions by computing the equivalent uniform strain and rotation that best maps one solution into another. If we use a continental-scale fiducial network with good geometry, the distortions between the solutions are about 10(-8), largely independent of the exact choice of stations. The one case of a large-scale fiducial network where the distortions are larger is when the three fiducial stations chosen all lie close to a great circle. Use of a fiducial network no larger than the crustal deformation network can produce apparent strains of up to 10(-7). Our work suggests that fiducial coordinates determined from GPS data analysis may be used, although they should be determined using a consistent reference frame, such as provided by VLBI and satellite laser ranging.

King, NE, Agnew DC.  1991.  How Large is the Retrograde Annual Wobble. Geophysical Research Letters. 18:1735-1738.   10.1029/91gl01882   AbstractWebsite

We compare recent measurements of polar motion (wobble) made by space-geodetic methods with older optical measurements. Multitaper spectra of these data show that the retrograde (clockwise) annual wobble is much larger in the older data than the newer data, implying systematic errors in the older techniques. Two additional analyses show evidence for a small retrograde motion in the newer data, which appears to be consistent between data types. This implies that the excitation of the retrograde wobble must be about half of the prograde excitation. None of the available estimates of the excitation shows full agreement with these observations.

Cummins, P, Wahr JM, Agnew DC, Tamura Y.  1991.  Constraining Core Undertones Using Stacked IDA Gravity Records. Geophysical Journal International. 106:189-198. AbstractWebsite

A search is made for oscillatory gravity signals, possibly associated with core undertones, in the frequency range between the diurnal and semidiurnal tidal bands. IDA tidal data recorded during 6 month intervals following four large earthquakes were used, and the data from different stations were stacked according to spherical harmonic surface amplitude patterns with angular order l less-than-or-equal-to 4. The detection level of a signal in the diurnal-semidiurnal frequency band was roughly 10-20 ngal, but the only signal detected was a 9 ngal signal of 9.54 hr period whose origin remains unexplained. Two of the earthquakes were chosen because of claims that core undertones were excited following these events, and it is established that signals of the required amplitude, if they exist, can only be associated with a few spherical harmonic patterns for which the Brussels instrument is near an antinode. We also used NCAR barometric data to correct the gravity recordings for the effects of atmospheric pressure variations, which reduces the noise level in the diurnal-semidiurnal frequency band by 2-3 dB.

Agnew, DC, Jones LM.  1991.  Prediction Probabilities from Foreshocks. Journal of Geophysical Research-Solid Earth and Planets. 96:11959-11971.   10.1029/91jb00191   AbstractWebsite

When any earthquake occurs, the possibility that it might be a foreshock increases the probability that a larger earthquake will occur nearby within the next few days. Clearly, the probability of a very large earthquake ought to be higher if the candidate foreshock were on or near a fault capable of producing that very large mainshock, especially if the fault is towards the end of its seismic cycle. We derive an expression for the probability of a major earthquake characteristic to a particular fault segment, given the occurrence of a potential foreshock near the fault. To evaluate this expression, we need: (1) the rate of background seismic activity in the area, (2) the long-term probability of a large earthquake on the fault, and (3) the rate at which foreshocks precede large earthquakes, as a function of time, magnitude, and spatial location. For this last function we assume the average properties of foreshocks to moderate earthquakes in California: (1) the rate of mainshock occurrence after foreshocks decays roughly as t-1, so that most foreshocks are within three days of their mainshock, (2) foreshocks and mainshocks occur within 10 km of each other, and (3) the fraction of mainshocks with foreshocks increases linearly as the magnitude threshold for foreshocks decreases, with 50% of the mainshocks having foreshocks with magnitudes within three units of the mainshock magnitude (within three days). We apply our results to the San Andreas, Hayward, San Jacinto, and Imperial faults, using the probabilities of large earthquakes from the report of the Working Group on California Earthquake Probabilities (1988). The magnitude of candidate event required to produce a 1% probability of a large earthquake on the San Andreas fault within three days ranges from a high of 5.3 for the segment in San Gorgonio Pass to a low of 3.6 for the Carrizo Plain.

Agnew, D.  1991.  How complete is the pre-instrumental record of earthquakes in southern California? Environmental perils, San Diego Region. ( Abbott PL, Elliott WJ, Eds.).:75-88., [San Diego, Calif.]: Published for the Geological Society of America Annual Meeting by the San Diego Association of Geologists Abstract
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Agnew, DC, Ellsworth WL.  1991.  Earthquake Prediction and Long-Term Hazard Assessment. Reviews of Geophysics. 29:877-889. AbstractWebsite
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1990
Dixon, T, Blewitt G, Larson K, Agnew D, Hager B, Kroger P, Krumega L, Strange W.  1990.  GPS measurements of regional deformation in southern California. EOS Trans. AGU. 71:1051-1053,1056. Abstract
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1989
Rojstaczer, S, Agnew DC.  1989.  The Influence of Formation Material Properties on the Response of Water Levels in Wells to Earth Tides and Atmospheric Loading. Journal of Geophysical Research-Solid Earth and Planets. 94:12403-12411.   10.1029/JB094iB09p12403   AbstractWebsite

The water level in an open well can change in response to deformation of the surrounding material, either because of applied strains (tidal or tectonic) or surface loading by atmospheric pressure changes. Under conditions of no vertical fluid flow and negligible well bore storage (static-confined conditions), the sensitivities to these effects depend on the elastic properties and porosity which characterize the surrounding medium. For a poroelastic medium, high sensitivity to applied areal strains occurs for low porosity, while high sensitivity to atmospheric loading occurs for high porosity; both increase with decreasing compressibility of the solid matrix. These material properties also influence vertical fluid flow induced by areally extensive deformation and can be used to define two types of hydraulic diffusivity which govern pressure diffusion, one for applied strain and one for surface loading. The hydraulic diffusivity which governs pressure diffusion in response to surface loading is slightly smaller than that which governs fluid flow in response to applied strain. Given the static-confined response of a water well to atmospheric loading and Earth tides, the in situ drained matrix compressibility and porosity (and hence the one-dimensional specific storage) can be estimated. Analysis of the static-confined response of five wells to atmospheric loading and Earth tides gives generally reasonable estimates for material properties.

Agnew, DC.  1989.  Robust Pilot Spectrum Estimation for the Quality-Control of Digital Seismic Data. Bulletin of the Seismological Society of America. 79:180-188. AbstractWebsite
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Agnew, DC, Wyatt FK.  1989.  The 1987 Superstition Hills Earthquake Sequence - Strains and Tilts at Pinon Flat Observatory. Bulletin of the Seismological Society of America. 79:480-492. AbstractWebsite
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Knopoff, L, Rydelek PA, Zurn W, Agnew DC.  1989.  Observations of Load Tides at the South-Pole. Physics of the Earth and Planetary Interiors. 54:33-37.   10.1016/0031-9201(89)90184-2   AbstractWebsite

The use of tidal observations to study the ‘nearly diurnal free wobble’ mode of the Earth is possible if oceanic effects in the tidal record are accurately removed. We have analyzed vertical gravity data from the South Pole to determine the amplitude and phase of the small daily and semidaily tides observed at the Pole. Since these signals at the Pole are most probably caused by oceanic tides, our observations provide an excellent target for the oceanic models. A comparison with the best models of the oceans now available indicates the need for improvement in measuring and modeling the southern oceans.

Anderson, JG, Rockwell TK, Agnew DC.  1989.  Past and possible future earthquakes of significance to the San Diego region. Earthquake Spectra. 5:299-335. Abstract
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Agnew, D.  1989.  Seismic instrumentation. The Encyclopedia of solid earth geophysics. ( James DE, Ed.).:1033-1037., New York: Van Nostrand Reinhold Abstract
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Agnew, D.  1989.  Seismology: History. The Encyclopedia of solid earth geophysics. ( James DE, Ed.).:1198-1202., New York: Van Nostrand Reinhold Abstract
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1988
King, NE, Agnew DC, Wyatt F.  1988.  Comparing Strain Events - A Case-Study for the Homestead Valley Earthquakes. Bulletin of the Seismological Society of America. 78:1693-1706. AbstractWebsite
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Wyatt, FK, Morrissey ST, Agnew DC.  1988.  Shallow Borehole Tilt - A Reprise. Journal of Geophysical Research-Solid Earth and Planets. 93:9197-9201.   10.1029/JB093iB08p09197   AbstractWebsite

We describe results from nearly a decade of tilt measurements produced by two arrays of shallow borehole tiltmeters: one in a semiarid environment at Piñon Flat Observatory (PFO), California (depth of burial 4.5 m) and the other in a maritime-Arctic environment at Adak, Alaska (depth of burial 2 m). Although renovation and reinstallation of the instruments at the two sites reduced thermal noise, it did not change the secular records significantly. This implies that the large tilts observed reflect instability of the ground rather than the sensor, so that deeper installations should give better results. The PFO data show large rainfall-related tilts (caused by near-surface weathering) and also periodic temperature-related tilts (from several thermoelastic effects). The Adak data are dominated by tilts from the annual temperature cycle (though by a smaller amount than at PFO), but at other frequencies they show essentially the same power levels as at PFO. Both data sets confirm earlier results that burial at such shallow depths, even in apparently stable material, is inadequate for the measurement of tectonic tilts.

1987
Davis, PM, Rydelek PA, Agnew DC, Okamura AT.  1987.  Observation of Tidal Tilt on Kilauea Volcano, Hawaii. Geophysical Journal of the Royal Astronomical Society. 90:233-244.   10.1111/j.1365-246X.1987.tb00682.x   AbstractWebsite

We have analysed the east-west tilt components, O1, K1, N2, M2 and S2 from a continuously recording tiltmeter located in Uwekahuna Vault on Kilauea Volcano, Hawaii, for the period 1971—79. Detailed analysis of the M2 component gives values of 30.9 ± 2.0 (95 per cent) nrad and 116.0 ± 2.0° for the amplitude and phase, respectively, compared to values of 48.5 nrad and 139.4° for the equilibrium tide. the total theoretical tide, found by summing the equilibrium and load tides, amounts to 37.2 nrad at a phase of 121.7°. the 20 per cent discrepancy with that observed may be due to an inaccurate cotical chart, cavity effects in the vault, strain—tilt coupling or an inappropriate solid earth model. In the vicinity of Hawaii (≤ 3°) two independent cotidal charts give almost identical results for the near field ocean load. At greater distances, we use the Schwiderski (1978) cotidal chart. We estimate that local cavity and strain—tilt coupling effects are less than 12 per cent owing to the agreement between geodetically determined and instrumental tilt but we can not rule out regional effects. Assuming these are small and the cotical charts correct, we find that the M2 results are brought into satisfactory agreement if, instead of using an average oceanic earth model in the (< 75 km) vicinity of Hawaii, we use an earth model with nearly one-half the oceanic rigidity. Such a low upper mantle and crustal rigidity is consistent with Kilauea's position above the thermal upwelling associated with the Hawaiian hotspot.

Agnew, D.  1987.  The continuous measurement of crustal deformation. Methods of experimental physics 24, Part B, Geophysics. Field measurements. ( Sammis CG, Henyey TL, Celotta R, Eds.).:409-439., London; New-York: Academic press ; Abstract
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