THE ACCURACY OF SEISMIC ESTIMATES OF DYNAMIC STRAINS - AN EVALUATION USING STRAINMETER AND SEISMOMETER DATA FROM PINYON-FLAT-OBSERVATORY, CALIFORNIA

The dynamic strains associated with seismic waves may play a significa nt role in earthquake triggering, hydrological and magmatic changes, e arthquake damage, and ground failure. We determine how accurately dyna mic strains may be estimated from seismometer data and elastic-wave th eory by comparing such estimated strains with strains measured on a th ree-component long-base strainmeter system at Pinon Flat, California. We quantify the uncertainties and errors through cross-spectral analys is 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 o f 470, 345, and 206 km, respectively). Our analysis indicates that in most cases the phase of the estimated strain matches that of the obser ved strain quite well (to within the uncertainties, which are about +/ -0.1 to +/-0.2 cycles). However, the amplitudes are often systematical ly 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 di rection), which should be zero theoretically; in the worst case, the r ms epsilon(phi phi) Strain exceeds the other nonzero components. These nonzero epsilon(phi phi) strains cannot be caused by deviations of th e surface-wave propagation paths from the expected azimuth or by depar tures from the plane-wave approximation. We believe that distortion of the strain field by topography or material heterogeneities give rise to these complexities.