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Sandwell, DT, Sichoix L, Agnew D, Bock Y, Minster JB.  2000.  Near real-time radar interferometry of the Mw 7.1 Hector Mine Earthquake. Geophysical Research Letters. 27:3101-3104.   10.1029/1999gl011209   AbstractWebsite

The Hector Mine Earthquake (Mw 7.1, 16 October 1999) ruptured 45 km of previously mapped and unmapped faults in the Mojave Desert. The ERS-2 satellite imaged the Mojave Desert on 15 September and again on 20 October, just 4 days after the earthquake. Using a newly-developed ground station we acquired both passes and were able to form an interferogram within 20 hours of the second overflight. Estimates of slip along the main rupture are 1-2 meters greater than slip derived from geological mapping. The gradient of the interferometric phase reveals an interesting pattern of triggered slip on adjacent faults as well as a 30 mm deep sink hole along Interstate 40.

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.

Barbour, AJ, Agnew DC.  2012.  Noise Levels on Plate Boundary Observatory Borehole Strainmeters in Southern California. Bulletin of the Seismological Society of America. 101:2453-2466.   10.1785/0120110062   AbstractWebsite

To establish noise levels for the borehole strainmeters of the Plate Boundary Observatory (PBO), we have analyzed data recorded by eight of these instruments, all in the Anza region of southern California. We determine time-varying power spectra for frequencies from 10(-3) to 10 Hz, using a new method that combines multitaper spectrum estimation, smoothing by local regression, and computation of cumulative distribution functions. From about 2 Hz to the Nyquist frequency of 10 Hz, the noise floor is set by instrument resolution; for frequencies between 0.1 Hz and 1 Hz, it is set by microseisms. The lowest noise level is between 0.01 and 0.1 Hz, with a rapid increase at lower frequencies. However, in most instruments this low-noise range also contains narrowband noise that appears to be caused by power supply fluctuations. We compare these results with noise spectra from other types of strainmeters, which suggest two conclusions: (1) they are in agreement with results for surficial, long-baseline instruments; and (2) other subsurface strainmeters have lower noise in the seismic band than the PBO instruments do.

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 PiƱon 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.