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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.

Fialko, Y, Simons M, Agnew D.  2001.  The complete (3-D) surface displacement field in the epicentral area of the 1999 M(w)7.1 Hector Mine earthquake, California, from space geodetic observations. Geophysical Research Letters. 28:3063-3066.   10.1029/2001gl013174   AbstractWebsite

We use Interferometric Synthetic Aperture Radar (InSAR) data to derive continuous maps for three orthogonal components of the co-seismic surface displacement field due to the 1999 M-w 7.1 Hector Mine earthquake in southern California. Vertical and horizontal displacements are both predominantly antisymmetric with respect to the fault plane, consistent with predictions of linear elastic models of deformation for a strike-slip fault. Some deviations from symmetry apparent in the surface displacement data may result from complexity in the fault geometry.

Agnew, DC.  1983.  Conservation of Mass in Tidal Loading Computations. Geophysical Journal of the Royal Astronomical Society. 72:321-325.   10.1111/j.1365-246X.1983.tb03786.x   AbstractWebsite

A re-examination of methods for including mass conservation in tidal loading shows that the spherical harmonic correction of Farrell is incorrect. The effect of unconserved mass for a nearly ocean-covered earth shows that the proper spherical harmonic expansion of the Newtonian Green function is the average of the internal and external expansions.

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.

Johnston, MJS, Linde AT, Agnew DC.  1994.  Continuous Borehole Strain in the San-Andreas Fault Zone Before, During, and After the 28 June 1992, M(W)7.3 Landers, California, Earthquake. Bulletin of the Seismological Society of America. 84:799-805. AbstractWebsite

High-precision strain was observed with a borehole dilational strainmeter in the Devil's Punchbowl during the 11:58 UT 28 June 1992 M(w) 7.3 Landers earthquake and the large Big Bear aftershock (M(w) 6.3). The strainmeter is installed at a depth of 176 m in the fault zone approximately midway between the surface traces of the San Andreas and Punchbowl faults and is about 100 km from the 85-km-long Landers rupture. We have questioned whether unusual amplified strains indicating precursive slip or high fault compliance occurred on the faults ruptured by the Landers earthquake, or in the San Andreas fault zone before and during the earthquake, whether static offsets for both the Landers and Big Bear earthquakes agree with expectations from geodetic and seismologic models of the ruptures and with observations from a nearby two-color geodimeter network, and whether postseismic behavior indicated continued slip on the Landers rupture or local triggered slip on the San Andreas. We show that the strain observed during the earthquake at this instrument shows no apparent amplification effects. There are no indications of precursive strain in these strain data due to either local slip on the San Andreas or precursive slip on the eventual Landers rupture. The observations are generally consistent with models of the earthquake in which fault geometry and slip have the same form as that determined by either inversion of the seismic data or inversion of geodetically determined ground displacements produced by the earthquake. Finally, there are some indications of minor postseismic behavior, particularly during the month following the earthquake.

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.

Agnew, DC, Owen S, Shen ZK, Anderson G, Svarc J, Johnson H, Austin KE, Reilinger R.  2002.  Coseismic Displacements from the Hector Mine, California, earthquake: Results from survey-mode global positioning system measurements. Bulletin of the Seismological Society of America. 92:1355-1364.   10.1785/0120000928   AbstractWebsite

We describe the collection and processing of Global Positioning System (GPS) data from 77 locations around the Hector Mine earthquake, which we use to estimate coseismic displacements related to this shock. The existence of pre-event GPS data, some collected to monitor postseismic displacements from the 1992 Landers earthquake and some to establish survey control in the meizoseismal area, provided a relatively dense coverage close to the rupture zone. The data available were collected mostly within the 2 years prior to the 1999 earthquake; we reobserved many points within a few days after the shock, and all within 6 months after. We include corrections for interseismic motion to provide the best value possible for coseismic motion caused by this earthquake. The displacements in general display the pattern expected for a strike-slip fault, though a few show significant vertical motion. The maximum horizontal displacement observed was 2 m; one station between fault ruptures showed little horizontal motion, but significant uplift.

Barbour, AJ, Agnew DC, Wyatt FK.  2015.  Coseismic strains on plate boundary observatory borehole strainmeters in Southern California. Bulletin of the Seismological Society of America. 105:431-444.   10.1785/0120140199   AbstractWebsite

Strainmeters can record offsets coincident with earthquakes, but how much these represent strain changes from elastic rebound, and how much they are contaminated by local effects, remains an open question. To study this, we use a probabilistic detection method to estimate coseismic offsets on nine borehole strainmeters (BSMs) operated by the Plate Boundary Observatory (PBO) in southern California, from 34 earthquakes with a wide range of magnitudes and distances. In general, the offsets estimated for the BSM data differ substantially from the static strain predicted by elastic dislocation theory, which is well supported by other techniques, though 10% of the observed offsets agree well with theory. For one earthquake, the BSM offsets significantly disagree with collocated long-base laser strainmeter data. Comparisons with collocated seismic data provide strong evidence that the absolute errors between the observed and predicted strains scale with the level of seismic energy density but also that relative errors (normalized by the model size) do not. We conclude that apparent strain offsets are induced by seismic waves, occurring presumably because of irreversible deformation, whether in the rock or cementing materials close to the BSMs, or in the instruments themselves. Coseismic offsets seen in PBO BSM data should therefore be viewed with caution before being used as a measure of large-scale coseismic deformation.

Fialko, Y, Sandwell D, Agnew D, Simons M, Shearer P, Minster B.  2002.  Deformation on nearby faults induced by the 1999 Hector Mine earthquake. Science. 297:1858-1862.   10.1126/science.1074671   AbstractWebsite

Interferometric Synthetic Aperture Radar observations of surface deformation due to the 1999 Hector Mine earthquake reveal motion on several nearby faults of the eastern California shear zone. We document both vertical and horizontal displacements of several millimeters to several centimeters across kilometer-wide zones centered on pre-existing faults. Portions of some faults experienced retrograde (that is, opposite to their long-term geologic slip) motion during or shortly after the earthquake. The observed deformation likely represents elastic response of compliant fault zones to the permanent co-seismic stress changes. The induced fault displacements imply decreases in the effective shear modulus within the kilometer-wide fault zones, indicating that the latter are mechanically distinct from the ambient crustal rocks.

Agnew, DC, Hodgkinson K.  2007.  Designing compact causal digital filters for low-frequency strainmeter data. Bulletin of the Seismological Society of America. 97:91-99.   10.1785/0120060088   AbstractWebsite

For the strainmeter component of the Plate Boundary Observatory, filters are needed to produce low-frequency series (5-minute samples) from the higher-frequency (1 Hz) data generated by the instruments. We present design methods for finding filters that are efficient, causal, and compact. We use standard methods for generating symmetric finite impulse response filters, followed by root finding, selection of roots, and reconstruction of the weights, using procedures that make these processes numerically stable. The final filters show appropriate performance even in the presence of large teleseismic signals, but introduce unavoidable artifacts for strain data from large local earthquakes.

Agnew, DC.  1986.  Detailed Analysis of Tide-Gauge Data - A Case-History. Marine Geodesy. 10:231-255. AbstractWebsite
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.

Barbour, AJ, Agnew DC.  2012.  Detection of Seismic Signals Using Seismometers and Strainmeters. Bulletin of the Seismological Society of America. 102:2484-2490.   10.1785/0120110298   AbstractWebsite

Using data from borehole and long-base strainmeters and from borehole and surface seismometers, we compare the seismic-wave detection capability of strainmeters and seismometers. We use noise spectra to determine the relative signal-to-noise ratios (SNRs) on different sensors, as a function of the phase velocity and frequency of a signal. For the instruments we analyze, signals with frequencies from 10(-3) to 10 Hz and phase velocities typical of (or higher than) surface and body waves will have lower SNRs on the strainmeters than on broadband seismometers. At frequencies from 0.1 to 10 Hz the borehole (short-period) seismometers have better SNRs than strainmeters for typical phase velocities; at lower frequencies strainmeter data signals would have higher SNRs.

Agnew, DC, Sieh KE.  1978.  Documentary Study of Felt Effects of Great California Earthquake of 1857. Bulletin of the Seismological Society of America. 68:1717-1729. AbstractWebsite
Agnew, DC, Wyatt FK.  2014.  Dynamic strains at regional and teleseismic distances. Bulletin of the Seismological Society of America. 104:1846-1859.   10.1785/0120140007   AbstractWebsite

We develop formulas for the size of dynamic strains caused by seismic waves from an earthquake of given magnitude and distance. These formulas include peak strain, peak dissipated power, and total dissipated energy, and they are applicable at regional and teleseismic distances. The formulas are fits to data from 89 large (6.5 <= M-w <= 9.0) shallow earthquakes, with source distances between 500 and 16,000 km, recorded between 1977 and 2013 by three long-base laser strainmeters at Pinon Flat Observatory in southern California; these strainmeters provide uniquely well-calibrated measurements of tensor strain. The residuals to the fits suggest that strain values can usually be predicted to within a factor of 2. These data also show that the strain tensor can be substantially different from that expected for plane waves: in particular, the extension perpendicular to the back azimuth, which should be zero, is always 20% or more of the extension along that azimuth. How much the strains resemble those for plane waves depends on their path, perhaps because inhomogeneities along different paths produce different amounts of multipathing. The observed strains are systematically 10%-30% larger at nearby laser strainmeter sites in the Salton trough, suggesting local amplification from inhomogeneous crustal structure.

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.

Agnew, DC, Ellsworth WL.  1991.  Earthquake Prediction and Long-Term Hazard Assessment. Reviews of Geophysics. 29:877-889. AbstractWebsite
Johnson, HO, Agnew DC, Hudnut K.  1994.  Extremal Bounds on Earthquake Movement from Geodetic Data - Application to the Landers Earthquake. Bulletin of the Seismological Society of America. 84:660-667. AbstractWebsite

We present a technique to place quantifiable bounds on the moment of an earthquake from geodetic data, assuming known fault geometry. Application of this technique to the 1992 Landers earthquake shows that the moment must have been between 0.84 and 1.15 x 10(20) Nm with 90% confidence (M 7.25 to 7.34). We also find that to satisfy the data to this same level of confidence, the slip on the fault must have exceeded 7 m in at least one location, in good agreement with field mapping of the surface rupture.

Jones, G, Hilde T, Sharman G, Agnew D.  1979.  Fault patterns in outer trench wals and their tectonic significance. Journal of Physical Earth. 26:S85-S101. Abstract
Agnew, DC, Larson KM.  2007.  Finding the repeat times of the GPS constellation. Gps Solutions. 11:71-76.   10.1007/s10291-006-0038-4   AbstractWebsite

Single-epoch estimates of position using GPS are improved by removing multipath signals, which repeat when the GPS constellation does. We present two programs for finding this repeat time, one using the orbital period and the other the topocentric positions of the satellites. Both methods show that the repeat time is variable across the constellation, at the few-second level for most satellites, but with a few showing much different values. The repeat time for topocentric positions, which we term the aspect repeat time, averages 247 s less than a day, with fluctuations through the day that may be as much as 2.5 s at high latitudes.

Agnew, DC.  2005.  GHAM: A compact global geocode suitable for sorting. Computers & Geosciences. 31:1042-1047.   10.1016/j.cageo.2005.02.007   AbstractWebsite

The GHAM code is a technique for labeling geographic locations based on their positions. It defines addresses for equal-area cells bounded by constant latitude and longitude, with arbitrarily fine precision. The cell codes are defined by applying Morton ordering to a recursive division into a 16 by 16 grid, with the resulting numbers encoded into letter-number pairs. A lexical sort of lists of points so labeled will bring near neighbors (usually) close together; tests on a variety of global datasets show that in most cases the actual closest point is adjacent in the list 50% of the time, and within 5 entries 80% of the time. (C) 2005 Elsevier Ltd. All rights reserved.

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
Gonzalez-Garcia, JJ, Prawirodirdjo L, Bock Y, Agnew D.  2003.  Guadalupe Island, Mexico as a new constraint for Pacific plate motion. Geophysical Research Letters. 30   10.1029/2003gl017732   AbstractWebsite

[1] We use GPS data collected on Isla de Guadalupe and in northern Baja California, Mexico, to estimate site velocities relative to Pacific plate motion. The velocities of all three geodetic monuments on Guadalupe fit a rigid Pacific plate model with residuals of 1 mm/yr. Using the Guadalupe data and data from five IGS stations on the Pacific plate ( CHAT, KOKB, KWJ1, MKEA, and THTI) we estimate an angular velocity for this plate that is consistent with other recently-published estimates. Our results indicate that Isla de Guadalupe lies on the Pacific plate, and that GPS data collection on the island usefully constrains Pacific plate motion and rigidity.

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.

Agnew, DC.  2018.  An improbable observation of the diurnal core resonance. Pure and Applied Geophysics. 175:1599-1609.   10.1007/s00024-017-1522-1   AbstractWebsite

The resonance associated with the ellipticity of the core-mantle boundary is usually measured with observations of either the Earth's nutations, or of tidal gravity, strain, or tilt. But, improbably, it can also be seen in a dataset collected and processed with older and simpler technologies: the harmonic constants for the ocean tides. One effect of the resonance is to decrease the ratio of the amplitude of the constituent to the amplitude of the constituent to 0.96 of the ratio in the equilibrium tidal potential. The compilation of ocean-tide harmonic constants prepared by the International Hydrographic Bureau between 1930 and 1980 shows considerable scatter in this ratio; however, if problematic stations and regions are removed, this dataset clearly shows a decreased ratio. While these data apply only a weak constraint to the frequency of the resonance, they also show that the effect could have been observed long before it actually was.