Crustal shaping of high frequency ground motion in the Washington-Oregon, northern and central California regions

Large data sets of vertical and horizontal seismograms from the Pacific Northwest Seismic Network, Northern California Seismic Network, and Berkeley Digital Seismic Network were used to study the high frequency (0.25-16 Hz) ground motion characteristics in Washington-Oregon, northern, and central California. I have split the data set of central California into eastern and western parts to investigate regional variations of ground motion parameters. I performed regressions of Fourier velocity spectra and peak filtered ground velocities to obtain the propagation, excitation, and site terms.; The regression results for the propagation term show a simpler from of geometrical spreading in central east California compared to other regions. For northern California this term shows a rapid decrease of amplitude with distance especially for frequencies higher than 4 Hz. The propagation term for the Pacific Northwest and central west California show a typical trend of attenuation with some effects of supercritical reflections.; For the Pacific Northwest, the best model that fits the observation is expressed by Q(f) = 280 f 0.55. The geometrical spreading is r-1.2 for a distance range of 40 to 70 km. For other distance ranges the decrease of amplitude is less rapid. The central east California and central west California results are parameterized with Q(f) = 280 f0.50 and Q(f) = 250 f0.30 respectively. The northern California results are not easy to model and require using a frequency dependent eta and geometrical spreading. The average model for two frequency ranges is Q(f) = 210 f0.50. The geometrical spreading effect for the frequencies higher than 4 Hz is very strong at distances less than 70 km.; The excitation terms were modeled using the Brune's source model. An average stress drop (Deltasigma) of 30 bars was obtained for the Pacific Northwest. The northern California average value for stress drop is 90 bars. The observations of central east California were modeled with a stress drop of 45 bars while the best fit for the western part required Deltasigma = 125 bars.; The wide range of the values obtained for Q, g(r), and Deltasigma, indicates that the ground motion parameters of a specific region may not be used for another region. For example, using the stress drops of central California may overestimate the ground motion level predictions in the Pacific Northwest.; Regional attenuation relationships can be developed using small earthquake data which are available in many regions. These relationships reflect the regional wave propagation characteristics and give more accurate results in seismic hazard analysis.