Publications

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1987
1986
Agnew, DC.  1986.  Strainmeters and Tiltmeters. Reviews of Geophysics. 24:579-624. AbstractWebsite
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Agnew, DC.  1986.  Detailed Analysis of Tide-Gauge Data - A Case-History. Marine Geodesy. 10:231-255. AbstractWebsite
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Agnew, DC, Berger J, Farrell WE, Gilbert JF, Masters G, Miller D.  1986.  Project IDA: a decade in review. EOS Trans. AGU. 67:203-212. Abstract
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1985
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.

1984
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.

1983
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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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.

1982
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.

1981
Agnew, DC.  1981.  Nonlinearity in Rock - Evidence from Earth Tide. Journal of Geophysical Research. 86:3969-3978.   10.1029/JB086iB05p03969   AbstractWebsite

The earth is sinusoidally stressed by tidal forces; if the stress-strain relation for rock is nonlinear, energy should appear in an earth tide record at frequencies which are multiples of those of the larger tidal lines. An examination of the signals to be expected for different nonlinear deformation laws shows that for a nonlinear response without dissipation, the largest anomalous signal should occur at twice the forcing frequency, whereas for nonlinear laws involving dissipation (cusped hysteresis loops) the anomalous signal will be greatest at 3 times this frequency. The size of the signal in the dissipative case depends on the amount by which dissipation affects the particular response being measured. For measurements of strain tides this depends on whether dissipation is assumed to be present throughout the earth or localized around the point of measurement. An analysis of 5.7 years of strain tide records from Piñon Flat, California, shows a small signal at twice the frequency of the largest (M2) tide. Most of the observed signal can be explained by loading from nonlinear water tides in the Gulf of California and the Pacific Ocean; the residual nonlinear tide is 65 dB less than the M2 tide. The signal at 3 times the M2 frequency is compatible with a linear model or with nonlinear hysteresis loops provided that nonlinear dissipation occurs throughout the earth. Nonlinear dissipation in the rocks near the strainmeter would produce a larger signal than is seen.

1980
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.

1979
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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Agnew, D. C; Legg, SM; C.  1979.  Earthquake history of San Diego. Earthquake and Other perils: San Diego region. ( Abbott PL, Elliott WJ, Eds.).:123-138., San Diego: San Diego Association of Geologists Abstract
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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
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Berger, J, Agnew DC, Parker RL, Farrell WE.  1979.  Seismic System Calibration 2. Cross-Spectral Calibration Using Random Binary Signals. Bulletin of the Seismological Society of America. 69:271-288. AbstractWebsite
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Agnew, D.  1979.  Tsunami history of San Diego. Earthquake and Other perils: San Diego region. ( Abbott PL, Elliott WJ, Eds.).:117-122., San Diego: San Diego Association of Geologists Abstract
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1978
Agnew, DC.  1978.  1852 Fort Yuma Earthquake - 2 Additional Accounts. Bulletin of the Seismological Society of America. 68:1761-1762. AbstractWebsite
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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
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Agnew, DC, Farrell WE.  1978.  Self-Consistent Equilibrium Ocean Tides. Geophysical Journal of the Royal Astronomical Society. 55:171-181.   10.1111/j.1365-246X.1978.tb04755.x   AbstractWebsite

We compute the static response of the world ocean to an external zonal gravitational potential. The computation includes the effects of the self-attraction of the ocean, and the yielding of the Earth caused both by the external potential and the change in ocean load. We compare the computed tide with measurements of the fortnightly and monthly ocean tides. The short-wavelength departures from equilibrium found by Wunsch are still present. An average of observations at Pacific islands shows that the fortnightly tide departs significantly from equilibrium but the monthly may not. We have also calculated the effects of our computed tide on measurements of tidal gravity and tidal fluctuations in the length of day. Existing tidal gravity data are too imprecise to enable us to determine whether or not the spatial average of the ocean tides departs from equilibrium. The length of day data suggest that the monthly tide is farther from equilibrium than the fortnightly. We have not been able to resolve the apparent discrepancy between the length of day and ocean tide data.

Agnew, DC, Berger J.  1978.  Vertial Seismic Noise at Very Low-Frequencies. Journal of Geophysical Research. 83:5420-5424.   10.1029/JB083iB11p05420   AbstractWebsite

We use records from the Project IDA modified LaCoste gravimeters to investigate ground noise at. frequencies from 1 to 10 mHz. At most sites the level between 2 and 10 mHz is nearly flat and close to 2 × 10−18 m2 s−3 Much higher values which are observed at island and coastal stations are due to loading by waves trapped along the shore. Data from the superconducting gravimeter at Piñon Flat Geophysical Observatory show that the noise power increases as ƒ−2.7 for frequencies between 1 and 0.001 mHz.

1976
Agnew, D, Berger J, Buland R, Farrell W.  1976.  International deployment of accelerometers: a network for very long period seismology. EOS Trans. AGU. 57:171-181. Abstract
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