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Gonzalez-Garcia, JJ, Prawirodirdjo L, Bock Y, Agnew D.  2003.  Guadalupe Island, Mexico as a new constraint for Pacific plate motion. Geophysical Research Letters. 30   10.1029/2003gl017732   AbstractWebsite

[1] We use GPS data collected on Isla de Guadalupe and in northern Baja California, Mexico, to estimate site velocities relative to Pacific plate motion. The velocities of all three geodetic monuments on Guadalupe fit a rigid Pacific plate model with residuals of 1 mm/yr. Using the Guadalupe data and data from five IGS stations on the Pacific plate ( CHAT, KOKB, KWJ1, MKEA, and THTI) we estimate an angular velocity for this plate that is consistent with other recently-published estimates. Our results indicate that Isla de Guadalupe lies on the Pacific plate, and that GPS data collection on the island usefully constrains Pacific plate motion and rigidity.

Gomberg, J, Wech A, Creager K, Obara K, Agnew D.  2016.  Reconsidering earthquake scaling. Geophysical Research Letters. 43:6243-6251.   10.1002/2016gl069967   AbstractWebsite

The relationship (scaling) between scalar moment, M-0, and duration, T, potentially provides key constraints on the physics governing fault slip. The prevailing interpretation of M-0-T observations proposes different scaling for fast (earthquakes) and slow (mostly aseismic) slip populations and thus fundamentally different driving mechanisms. We show that a single model of slip events within bounded slip zones may explain nearly all fast and slow slip M-0-T observations, and both slip populations have a change in scaling, where the slip area growth changes from 2-D when too small to sense the boundaries to 1-D when large enough to be bounded. We present new fast and slow slip M-0-T observations that sample the change in scaling in each population, which are consistent with our interpretation. We suggest that a continuous but bimodal distribution of slip modes exists and M-0-T observations alone may not imply a fundamental difference between fast and slow slip.

Gomberg, J, Agnew D.  1996.  The accuracy of seismic estimates of dynamic strains: An evaluation using strainmeter and seismometer data from Pinon Flat Observatory, California. Bulletin of the Seismological Society of America. 86:212-220. AbstractWebsite

The dynamic strains associated with seismic waves may play a significant role in earthquake triggering, hydrological and magmatic changes, earthquake damage, and ground failure. We determine how accurately dynamic strains may be estimated from seismometer data and elastic-wave theory by comparing such estimated strains with strains measured on a three-component long-base strainmeter system at Pinon Flat, California. We quantify the uncertainties and errors through cross-spectral analysis of data from three regional earthquakes (the M(0) = 4 x 10(17) N-m St. George, Utah; M(0) = 4 X 10(17) N-m Little Skull Mountain, Nevada; and M(0) 1 x 10(19) N-m Northridge, California, events at distances of 470, 345, and 206 km, respectively). Our analysis indicates that in most cases the phase of the estimated strain matches that of the observed strain quite well (to within the uncertainties, which are about +/-0.1 to +/-0.2 cycles). However, the amplitudes are often systematically off, at levels exceeding the uncertainties (about 20%); in one case, the predicted strain amplitudes are nearly twice those observed. We also observe significant epsilon(phi phi) strains (phi = tangential direction), which should be zero theoretically; in the worst case, the rms epsilon(phi phi) Strain exceeds the other nonzero components. These nonzero epsilon(phi phi) strains cannot be caused by deviations of the surface-wave propagation paths from the expected azimuth or by departures from the plane-wave approximation. We believe that distortion of the strain field by topography or material heterogeneities give rise to these complexities.