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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.  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, DC, Ellsworth WL.  1991.  Earthquake Prediction and Long-Term Hazard Assessment. Reviews of Geophysics. 29:877-889. 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.  2007.  Earth Tides. Treatise on Geophysics and Geodesy. ( Herring TA, Ed.).:163-195., New York: Elsevier 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, D.  1998.  Instruments, Gravity. Sciences of the Earth: An Encyclopedia of places, People and Phenomenon. ( Good G, Ed.).:453-455.: Garland Publishing Abstract
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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, 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.  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, DC.  1995.  Ocean-Load Tides at the South-Pole - A Validation fo Recent Ocean-Tide Models. Geophysical Research Letters. 22:3063-3066.   10.1029/95gl03074   AbstractWebsite

Small diurnal and semidiurnal gravity tides are seen at the South Pole because of the loading by and attraction of the ocean tides. These data provide a check on the quality of ocean-tide models, especially in the southernmost ocean, which has historically been the most lacking in tidal data. Ocean-tide models developed in the 1980's did not predict the gravity tides at this location very well. Recently-developed models based on the Topex/Poseidon altimetric data and improved hydrodynamical modeling agree much better with the observations, provided that the tides beneath the ice shelves are included. The level of agreement at this remote location suggests that, loads from very local tides aside, the new generation of ocean-tide models can predict the loading tides to very high accuracy.

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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Agnew, DC.  2013.  Realistic Simulations of Geodetic Network Data: The Fakenet Package. Seismological Research Letters. 84:426-432.   10.1785/0220120185   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.  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, 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, 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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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.  1978.  1852 Fort Yuma Earthquake - 2 Additional Accounts. Bulletin of the Seismological Society of America. 68:1761-1762. AbstractWebsite
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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.  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.

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.  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.  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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