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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.

Hart, RHG, Gladwin MT, Gwyther RL, Agnew DC, Wyatt FK.  1996.  Tidal calibration of borehole strain meters: Removing the effects of small-scale inhomogeneity. Journal of Geophysical Research-Solid Earth. 101:25553-25571.   10.1029/96jb02273   AbstractWebsite

We investigate the estimation of Earth strain from borehole strain meter data in a study of tidal calibration of the Gladwin borehole tensor strain meter (BTSM) at Pinon Flat. Small-scale geological inhomogeneity is one of several effects examined that cross couple remote areal/shear strain into measured areal/shear strain. A methodology is developed for incorporating cross coupling into the strain meter calibration. Using the measured strain tides from the colocated laser strain meter (LSM) as a reference, we show that calibration of the BTSM with cross coupling removes systematic errors of up to 30% in the borehole strain meter tides. This calibration accurately relates the BTSM measurements to strains at the scale length of the LSM, about 1 km. The calibration technique provides a solution to a major criticism of all short-baseline strain measurements, namely, that tectonic strains are not representatively sampled due to small-scale inhomogeneities. The technique removes errors potentially greater than 50% in observed strain offsets from fault slip, permitting improved constraint of slip mechanisms. We show that current theoretical estimates of strain tides in the instrument locality are not yet of sufficient accuracy for cross-coupled calibration. Comparison of theoretical tides with measurements from the LSM suggest that at least half of the error is in the ocean load tide estimates.

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.

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.  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, 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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