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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.

Wyatt, F, Agnew D, Linde A, Sacks IS.  1983.  Borehole Stranimeter studies in Pinon flat observatory. Carnegie Institute of Washington, Yearbook 82, Washington DC. :533-538. Abstract
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Wyatt, FK, Agnew DC, Gladwin M.  1994.  Continuous Measurements of Crustal Deformation for the 1992 Landers Earthquake Sequence. Bulletin of the Seismological Society of America. 84:768-779. AbstractWebsite

We describe, and attempt to interpret, continuous measurements of strains and tilts made at Pinon Flat Observatory (PFO) before, during, and after the Landers and Joshua Tree earthquake sequences. These data show substantial transient deformation following the Landers mainshock, with a total amplitude of several percent of the co-seismic deformation, and a decay time of at least several days. Comparing data from the many types of instruments at PFO allows us to infer possible sources for this deformation. The immediate postseismic transient was nearly the same size on three long-base strainmeters, suggesting either broad-scale deformation or local motion near one part of the observatory. The latter can largely be ruled out by the similarity of many other measurements in the area covered by these strainmeters and the observations by others of significant postseismic displacements nearer the source. Possible mechanisms for broad-scale deformation include postseismic fault slip, time-dependent creep in near-surface rocks, and elastic or thermal responses to water-table changes. The first two agree best with the observations from PFO, but if postseismic fault slip is the source, it must have been distributed differently than the co-seismic slip, and may have included faults other than those that ruptured seismically. If one of the other mechanisms is the main source, the PFO data imply that the postseismic slip must have been very much smaller than the seismic slip, perhaps 2% or less. No significant preseismic deformation was observed, at a level of 2 X 10(-3) of the co-seismic deformation, for the days to minutes before the earthquake.

Wyatt, F, Bilham R, Beavan J, Sylvester AG, Owen T, Harvey A, Macdonald C, Jackson DD, Agnew DC.  1984.  Comparing Tiltmeters for Crustal Deformation Measurement - A Preliminary Report. Geophysical Research Letters. 11:963-966.   10.1029/GL011i010p00963   AbstractWebsite

A collection of high-precision tiltmeters is being operated at Piñon Flat Observatory, southern California, both to compare instruments and to measure tectonic deformation. We report on 1.2 years of data from four of these: two Michelson-Gale long fluid tiltmeters, one long center-pressure tiltmeter, and a shallow borehole tiltmeter. The three long-base instruments are all located on the same baseline, with a precise leveling line running between their end-monuments. At nontidal frequencies, only the two Michelson-Gale instruments show some coherence (γ² = .3 for periods of 2 to 4 days), while the center-pressure instrument is correlated with air temperature at periods from a few days to a few weeks. The most stable tilt record shows a secular rate of 0.28 µrad/a, which may be real. Over much longer times, leveling to specially stabilized benchmarks should confirm this. Comparing instruments has identified more and less successful measurement techniques; it appears that low-noise data will most probably be produced only by relatively complex and expensive instruments, though even for these, the operating costs over any reasonable lifetime will exceed the capital cost. Even the best existing sensors must be improved to measure continuous tectonic motions.

Wyatt, F, Cabaniss G, Agnew DC.  1982.  A Comparison of Tiltmeters at Tidal Frequencies. Geophysical Research Letters. 9:743-746.   10.1029/GL009i007p00743   AbstractWebsite

Different techniques for measuring crustal deformation are now being tested at Piñon Flat Geophysical Observatory in southern California. As a part of this comparison, we have analyzed five months of data from four different tiltmeters: three borehole instruments, two 4.5 m and one 26 m deep; and a long-base (535 m) instrument at the surface. For periods from 1 hour to 10 days, the signals from the deep borehole and long-base instruments show comparable noise levels; the shallow instruments are much noisier. Tidal analyses give results for the different sensors which agree to within the errors, which are several percent.

Wang, TH, Cochran ES, Agnew D, Oglesby DD.  2013.  Infrequent triggering of tremor along the San Jacinto Fault near Anza, California. Bulletin of the Seismological Society of America. 103:2482-2497.   10.1785/0120120284   AbstractWebsite

We examine the conditions necessary to trigger tremor along the San Jacinto fault (SJF) near Anza, California, where previous studies suggest triggered tremor occurs, but observations are sparse. We investigate the stress required to trigger tremor using continuous broadband seismograms from 11 stations located near Anza, California. We examine 44 M-w >= 7.4 teleseismic events between 2001 and 2011; these events occur at a wide range of back azimuths and hypocentral distances. In addition, we included one smaller-magnitude, regional event, the 2009 M-w 6.5 Gulf of California earthquake, because it induced extremely high strains at Anza. We find the only episode of triggered tremor occurred during the 3 November 2002 M-w 7.8 Denali earthquake. The tremor episode lasted 300 s, was composed of 12 tremor bursts, and was located along SJF at the northwestern edge of the Anza gap at approximately 13 km depth. The tremor episode started at the Love-wave arrival, when surface-wave particle motions are primarily in the transverse direction. We find that the Denali earthquake induced the second highest stress (similar to 35 kPa) among the 44 teleseismic events and 1 regional event. The dominant period of the Denali surface wave was 22.8 s, at the lower end of the range observed for all events (20-40 s), similar to periods shown to trigger tremor in other locations. The surface waves from the 2009 M-w 6.5 Gulf of California earthquake had the highest observed strain, yet a much shorter dominant period of 10 s and did not trigger tremor. This result suggests that not only the amplitude of the induced strain, but also the period of the incoming surface wave, may control triggering of tremors near Anza. In addition, we find that the transient-shear stress (17-35 kPa) required to trigger tremor along the SJF at Anza is distinctly higher than what has been reported for the well-studied San Andreas fault.

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Vidale, JE, Agnew DC, Johnston MJS, Oppenheimer DH.  1998.  Absence of earthquake correlation with Earth tides: An indication of high preseismic fault stress rate. Journal of Geophysical Research-Solid Earth. 103:24567-24572.   10.1029/98jb00594   AbstractWebsite

Because the rate of stress change from the Earth tides exceeds that from tectonic stress accumulation, tidal triggering of earthquakes would be expected if the final hours of loading of the fault were at the tectonic rate and if rupture began soon after the achievement of a critical stress level. We analyze the tidal stresses and stress rates on the fault planes and at the times of 13,042 earthquakes which are so close to the San Andreas and Calaveras faults in California that we may take the fault plane to be known. We find that the stresses and stress rates from Earth tides at the times of earthquakes are distributed in the same way as tidal stresses and stress rates at random times. While the rate of earthquakes when the tidal stress promotes failure is 2% higher than when the stress does not, this difference in rate is not statistically significant. This lack of tidal triggering implies that preseismic stress rates in the nucleation zones of earthquakes are at least 0.15 bar/h just preceding seismic failure, much above the long-term tectonic stress rate of 10(-4) bar/h.

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Shen, ZK, King RW, Agnew DC, Wang M, Herring TA, Dong D, Fang P.  2011.  A unified analysis of crustal motion in Southern California, 1970-2004: The SCEC crustal motion map. Journal of Geophysical Research-Solid Earth. 116   10.1029/2011jb008549   AbstractWebsite

To determine crustal motions in and around southern California, we have processed and combined trilateration data collected from 1970 to 1992, VLBI data from 1979 to 1992, and GPS data from 1986 to 2004: a long temporal coverage required in part by the occurrence of several large earthquakes in this region. From a series of solutions for station positions, we have estimated interseismic velocities, coseismic displacements, and postseismic motions. Within the region from 31 N to 38 N. and east to 114 W, the final product includes estimated horizontal velocities for 1009 GPS, 190 trilateration, and 16 VLBI points, with ties between some of these used to stabilize the solution. All motions are relative to the Stable North American Reference Frame (SNARF) as realized through the velocities of 20 GPS stations. This provides a relatively dense set of horizontal velocity estimates, with well-tested errors, for the past quarter century over the plate boundary from 31 N to 36.5 N. These velocities agree well with those from the Plate Boundary Observatory, which apply to a later time period. We also estimated vertical velocities, 533 of which have errors below 2 mm/yr. Most of these velocities are less than 1 mm/yr, but they show 2-4 mm/yr subsidence in the Ventura and Los Angeles basins and in the Salton Trough. Our analysis also included estimates of coseismic and postseismic motions related to the 1992 Landers, 1994 Northridge, 1999 Hector Mine, and 2003 San Simeon earthquakes. Postseismic motions increase logarithmically over time with a time constant of about 10 days, and generally mimic the direction and relative amplitude of the coseismic offsets.

Sandwell, DT, Sichoix L, Agnew D, Bock Y, Minster JB.  2000.  Near real-time radar interferometry of the Mw 7.1 Hector Mine Earthquake. Geophysical Research Letters. 27:3101-3104.   10.1029/1999gl011209   AbstractWebsite

The Hector Mine Earthquake (Mw 7.1, 16 October 1999) ruptured 45 km of previously mapped and unmapped faults in the Mojave Desert. The ERS-2 satellite imaged the Mojave Desert on 15 September and again on 20 October, just 4 days after the earthquake. Using a newly-developed ground station we acquired both passes and were able to form an interferogram within 20 hours of the second overflight. Estimates of slip along the main rupture are 1-2 meters greater than slip derived from geological mapping. The gradient of the interferometric phase reveals an interesting pattern of triggered slip on adjacent faults as well as a 30 mm deep sink hole along Interstate 40.

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Rolandone, F, Burgmann R, Agnew DC, Johanson IA, Templeton DC, d'Alessio MA, Titus SJ, DeMets C, Tikoff B.  2008.  Aseismic slip and fault-normal strain along the central creeping section of the San Andreas fault. Geophysical Research Letters. 35   10.1029/2008gl034437   AbstractWebsite

We use GPS data to measure the aseismic slip along the central San Andreas fault (CSAF) and the deformation across adjacent faults. Comparison of EDM and GPS data sets implies that, except for small-scale transients, the fault motion has been steady over the last 40 years. We add 42 new GPS velocities along the CSAF to constrain the regional strain distribution. Shear strain rates are less than 0.083 +/- 0.010 mu strain/yr adjacent to the creeping SAF, with 1 - 4.5 mm/yr of contraction across the Coast Ranges. Dislocation modeling of the data gives a deep, long-term slip rate of 31 - 35 mm/yr and a shallow (0 - 12 km) creep rate of 28 mm/yr along the central portion of the CSAF, consistent with surface creep measurements. The lower shallow slip rate may be due to the effect of partial locking along the CSAF or reflect reduced creep rates late in the earthquake cycle of the adjoining SAF rupture zones.

Rolandone, F, Burgmann R, Agnew DC, Johanson IA, Templeton DC, d'Alessio MA, Titus SJ, DeMets C, Tikoff B.  2009.  Reply to comment by J. C. Savage on "Aseismic slip and fault-normal strain along the creeping section of the San Andreas Fault''. Geophysical Research Letters. 36   10.1029/2009gl039167   AbstractWebsite
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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.

Riedesel, MA, Agnew D, Berger J, Gilbert F.  1980.  Stacking for the frequencies and Qs of 0S0 and 1S0. Geophysical Journal International. 62:457-471.   10.1111/j.1365-246X.1980.tb04867.x   AbstractWebsite

Using nine IDA records for the Indonesian earthquake of 1977 August 19, we have formed an optimal linear combination of the records and have measured the frequency and Q of 0S0 and 1S0. The frequency was measured using the moment ratio method. The attenuation was measured by the minimum width method and by the time-lapse method. The frequency and attenuation were measured simultaneously by varying them to obtain a best fit to the data. A 2000-hr stack, the sum of nine individual records, for 0S0 gave a frequency of 0.814664 mHz±4 ppm. The values for the Q of 0S0 for the three different methods of measurement were 5600,5833 and 5700, respectively. The error in the estimates of Q-1 is about 5 per cent for the minimum power method. For 1S0 a 300-hr stack yielded a frequency of 1.63151 mHz±30 ppm. The values of Q for this mode were 1960, 1800 and 1850, respectively, with an error in Q-1 of about 12 per cent for the minimum power method.

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Owen, S, Anderson G, Agnew DC, Johnson H, Hurst K, Reilinger R, Shen ZK, Svarc J, Baker T.  2002.  Early postseismic deformation from the 16 October 1999 M-w 7.1 Hector Mine, California, earthquake as measured by survey-mode GPS. Bulletin of the Seismological Society of America. 92:1423-1432.   10.1785/0120000930   AbstractWebsite

The 16 October 1999 (M-w 7.1) Hector Mine earthquake was the largest earthquake in California since the 1992 (M-w 7.3) Landers event. The Hector Mine earthquake occurred in the eastern Mojave Desert, where the density of permanent Global Positioning System (GPS) stations is relatively low. Since the earthquake, groups from the United States Geological Survey, University of Southern California, University of California, Los Angeles, University of California, San Diego, and Massachusetts Institute of Technology have made postseismic survey-mode observations to increase the spatial coverage of deformation measurements. A total of 55 sites were surveyed, with markers from a few meters to 100 km from the surface rupture. We present velocity estimates for the 32 sites that had enough repeated observations between 17 October 1999 and 26 March 2000 to provide reliable results; these survey-mode data complement the temporal and spatial coverage provided by newly installed Southern California Integrated Geodetic Network permanent GPS stations and future Interferometric Synthetic Aperture Radar postseismic results. We then use the postseismic velocity estimates to compute a simple afterslip model. Results of inversions show that the observed velocities are consistent with deep afterslip occuring underneath the coseismic rupture area.

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Nikolaidis, RM, Bock Y, de Jonge PJ, Shearer P, Agnew DC, vanDomselaar M.  2001.  Seismic wave observations with the Global Positioning System. Journal of Geophysical Research-Solid Earth. 106:21897-21916.   10.1029/2001jb000329   AbstractWebsite

We describe the direct measurement of ground displacement caused by the Hector Mine earthquake in southern California (M-w 7.1, October 16, 1999). We use a new method of instantaneous positioning, which estimates site coordinates from only a single epoch of Global Positioning System (GPS) data, to measure dynamic as well as static displacements at 24 stations of the Southern California Integrated GPS Network (SCIGN), with epicentral distances from 50 to 200 km. For sites outside the Los Angeles basin the observed displacements are well predicted by an elastic half-space model with a point shear dislocation; within the sedimentary basin we observe large displacements with amplitudes up to several centimeters that last as long as 3-4 min. Since we resolve the GPS phase ambiguities and determine site coordinates independently at each epoch, the GPS solution rate is the same as the receiver sampling rate. For the SCIGN data this is 0.033 Hz (once per 30 s), though sample rates up to 2 Hz are possible with the SCIGN receivers. Since the GPS phase data are largely uncorrelated at I s, a higher sampling rate would offer improved temporal resolution of ground displacement, so that in combination with inertial seismic data, instantaneous GPS positioning would in many cases significantly increase the observable frequency band for strong ground motions.

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Meltzner, AJ, Sieh K, Abrams M, Agnew DC, Hudnut KW, Avouac JP, Natawidjaja DH.  2006.  Uplift and subsidence associated with the great Aceh-Andaman earthquake of 2004. Journal of Geophysical Research-Solid Earth. 111   10.1029/2005jb003891   AbstractWebsite

Rupture of the Sunda megathrust on 26 December 2004 produced broad regions of uplift and subsidence. We define the pivot line separating these regions as a first step in defining the lateral extent and the downdip limit of rupture during that great M(w) approximate to 9.2 earthquake. In the region of the Andaman and Nicobar islands we rely exclusively on the interpretation of satellite imagery and a tidal model. At the southern limit of the great rupture we rely principally on field measurements of emerged coral microatolls. Uplift extends from the middle of Simeulue Island, Sumatra, at similar to 2.5 degrees N, to Preparis Island, Myanmar (Burma), at similar to 14.9 degrees N. Thus the rupture is similar to 1600 km long. The distance from the pivot line to the trench varies appreciably. The northern and western Andaman Islands rose, whereas the southern and eastern portion of the islands subsided. The Nicobar Islands and the west coast of Aceh province, Sumatra, subsided. Tilt at the southern end of the rupture is steep; the distance from 1.5 m of uplift to the pivot line is just 60 km. Our method of using satellite imagery to recognize changes in elevation relative to sea surface height and of using a tidal model to place quantitative bounds on coseismic uplift or subsidence is a novel approach that can be adapted to other forms of remote sensing and can be applied to other subduction zones in tropical regions.

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Liu, HP, Sembera ED, Westerlund RE, Fletcher JB, Reasenberg P, Agnew DC.  1985.  Tidal Variation of Seismic Travel-Times in a massachusetts Granite Quarry. Geophysical Research Letters. 12:243-246.   10.1029/GL012i005p00243   AbstractWebsite

Conflicting results on tidal variation of seismic travel times exist in the literature. With improved methods, we have conducted a seismic survey at a Massachusetts granite quarry. The survey was conducted in the intervals (230d 23h, 231d11h) and (231d22h, 233d10h), 1983 (U.T.) along a 148 m baseline situated in nearly flat topography. The source for the present experiment was an air gun placed in a mud-filled pit. Travel times for the first five body wave extrema were analyzed. The results are: I. The 1st, 2nd, 4th, and 5th extrema show travel time variations; the 3rd extremum remains constant throughout the experiment. The magnitude of the fractional travel time variation, Δt/t, ranges from 0.5 to 0.9%. II. Changes in travel times of the 1st and 2nd extrema correspond to opposite changes in travel times of the 4th and 5th extrema. III. Two sets of nearly orthogonal joint systems are observed in the granite; the theoretical tidal strain in the direction perpendicular to the nearly vertical joints matches the travel time variations of the 1st and 2nd extrema whereas the tidal strain in the direction perpendicular to the nearly horizontal sheets matches the travel time variations of the 4th and 5th extrema, when a 4-hr delay is introduced for all the tidal strains. These results are interpreted in terms of the velocity changes of seismic rays as the two joint systems open and close due to the tidal stress.

Legg, M, Agnew D.  1979.  The 1862 earthquake in San Diego. Earthquake and Other perils: San Diego region. ( Abbott PL, Elliott WJ, Eds.).:139-141., San Diego: San Diego Association of Geologists Abstract
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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.

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.

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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.

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.

King, NE, Argus D, Langbein J, Agnew DC, Bawden G, Dollar RS, Liu Z, Galloway D, Reichard E, Yong A, Webb FH, Bock Y, Stark K, Barseghian D.  2007.  Space geodetic observation of expansion of the San Gabriel Valley, California, aquifer system, during heavy rainfall in winter 2004-2005. Journal of Geophysical Research-Solid Earth. 112   10.1029/2006jb004448   AbstractWebsite

[1] Starting early in 2005, the positions of GPS stations in the San Gabriel valley region of southern California showed statistically significant departures from their previous behavior. Station LONG moved up by about 47 mm, and nearby stations moved away from LONG by about 10 mm. These changes began during an extremely rainy season in southern California and coincided with a 16-m increase in water level at a nearby well in Baldwin Park and a regional uplift detected by interferometric synthetic aperture radar. No equivalent signals were seen in GPS station position time series elsewhere in southern California. Our preferred explanation, supported by the timing and by a hydrologic simulation, is deformation due to recharging of aquifers after near-record rainfall in 2004 - 2005. We cannot rule out an aseismic slip event, but we consider such an event unlikely because it requires slip on multiple faults and predicts other signals that are not observed.