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Abercrombie, RE, Agnew DC, Wyatt FK.  1995.  Testing a model of earthquake nucleation. Bulletin of the Seismological Society of America. 85:1873-1878. AbstractWebsite

Some laboratory models of slip find that a critical amount (or velocity) of slow slip is required over a nucleation patch before dynamic failure begins. Typically, such patch sizes, when extrapolated to earthquakes, have been thought to be very small and the precursory slip undetectable. Ohnaka (1992, 1993) has proposed a model in which foreshocks delineate a growing zone of quasi-static slip that nucleates the dynamic rupture and suggests that it could be large enough (similar to 10 km across) to be detectable and thus useful for short-term earthquake prediction. The 1992 Landers earthquake (M 7.3) had a distinctive foreshock sequence and initiated only 70 km from the strain meters at the Pinon Flat Observatory (PFO). We use this earthquake to investigate the validity and usefulness of Ohnaka's model. The accurate relocations of Dodge et al. (1995) show that the foreshock zone can be interpreted as expanding from an area of 800 m (along strike) by 900 m (in depth), to 2000 by 3200 m in the 6.5 hr before the mainshock. We have calculated the deformation signals expected both at PFO and 20 km from the foreshock zone, assuming either constant slip or constant stress drop on a circular patch expanding at 5 cm/sec over 6.5 hr. We find the slips or stress drops would have to have been implausibly high (meters or kilobars) to have been detectable on the strain meters at PFO. Slightly better Limits are possible only 20 lan from the source. Even though the distance from Landers to PFO is small compared with the average spacing of strain meters in California, we are unable to prove or disprove Ohnaka's model of earthquake nucleation. This suggests that even if the model is valid, it will not be useful for shortterm prediction.

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.  1997.  NLOADF: A program for computing ocean-tide loading. Journal of Geophysical Research-Solid Earth. 102:5109-5110.   10.1029/96jb03458   AbstractWebsite

The loading of the Earth by the ocean tides produces several kinds of signals which can be measured by geodetic technique. In order to compute these most accurately; a combination of global and local models of the ocean tides may be needed. The program NLOADF convolves the Green functions for loading with ocean tide models using a station-centered grid with fixed dimensions, making it easy to combine different ocean models without overlap in the convolution. The program computes all the quantities of interest (gravity, displacement, tilt, and strain) and includes the case where measurements are made beneath the surface of the ocean.

Agnew, DC.  1986.  Strainmeters and Tiltmeters. Reviews of Geophysics. 24:579-624. AbstractWebsite
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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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Agnew, DC.  2004.  Robert Fitzroy and the myth of the 'Marsden Square': Transatlantic rivalries in early marine meteorology. Notes and Records of the Royal Society of London. 58:21-46.   10.1098/rsnr.2003.0223   AbstractWebsite

Marine data (especially in meteorology) are often grouped geographically using a set of numbered 10degrees latitude-longitude squares known as Marsden squares, which are usually attributed to William Marsden, Secretary of the Admiralty (and Vice-President of The Royal Society), who supposedly invented them early in the nineteenth century. Available records suggest that this system was in fact probably invented by Robert FitzRoy soon after his appointment as head of the British Meteorological Office in 1854. FitzRoy felt that early English work in marine meteorology was being ignored, notably by the American Matthew Fontaine Maury, who had pioneered the collecting of marine meteorological data from ship's logs. A desire to undo this wrong led FitzRoy to emphasize earlier (though abortive) British projects by A.B. Becher (in 1831) and by Marsden (probably in the 1780s), both of which involved grouping marine data geographically, though only over limited areas. FitzRoy's treatment of this earlier work seems to have created, much later, the belief that Marsden had invented the system of 10degrees squares. Given both Maury's and FitzRoy's desire to demonstrate priority in this field, it is ironic that the first clear proposal to collect and group data from ship's logs was made by the American (and British) natural philosopher Isaac Greenwood in 1728.

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.  2014.  Variable star symbols for seismicity plots. Seismological Research Letters. 85:775-780.   10.1785/0220130214   AbstractWebsite
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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.  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).

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.  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.

Agnew, D.  1998.  Tides, Earth. Sciences of the Earth: An Encyclopedia of places, People and Phenomenon. ( Good G, Ed.).:810-812.: Garland Publishing Abstract
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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.

Agnew, DC.  2007.  Before PBO: an overview of continuous strain and tilt measurements in the United States. Journal of the geodetic Society of Japan. 53:157-182. Abstract
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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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Agnew, DC.  2002.  History of Seismology. IASPEI international handbook of earthwuake engineering seismology. ( Lee WHK, Ed.).:3-13.: Academic Press Abstract
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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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Agnew, DC.  1986.  Detailed Analysis of Tide-Gauge Data - A Case-History. Marine Geodesy. 10:231-255. AbstractWebsite
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Agnew, DC.  2010.  Comment on "Changes of Reporting Rates in the Southern California Earthquake Catalog, Introduced by a New Definition of M(L)" by Thessa Tormann, Stefan Wiemer, and Egill Hauksson. Bulletin of the Seismological Society of America. 100:3320-3324.   10.1785/0120100027   AbstractWebsite

Earthquake catalogs can be inhomogeneous because of changes in the definition of earthquake magnitude. Provided that a sufficient number of events have magnitudes defined in more than one system, it is possible to apply a Monte Carlo method to the observed joint distribution to convert sets of magnitudes from one system to another, improving any statistical analysis of the catalog. I demonstrate the method for the southern California catalog, in which the definition of local magnitude has recently been changed. Monte Carlo magnitude mapping appears to eliminate temporal changes that are otherwise present.

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.

Agnew, DC, Ellsworth WL.  1991.  Earthquake Prediction and Long-Term Hazard Assessment. Reviews of Geophysics. 29:877-889. AbstractWebsite
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Agnew, DC.  2001.  Map Projections to show the possible effects of surface loading. Journal of the geodetic Society of Japan. 47:255-260. 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, 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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