abstract

Initial P-wave pulse widths (first break to first zero crossing) from quarry blasts within a narrow magnitude range were measured at epicentral distances of 5 to 100 km on seismograms from the USGS central California array. Pulse width is related to spectral shape and assuming a constant source should change as a function of travel time (distance) and anelastic properties. In this geologically complex region no simple systematic change with distance was observed but an excellent correlation with station surface geology and fault proximity was shown. Consistently shorter pulse widths were associated with stations on the granitic Salinian side of the San Andreas fault while larger and more varying pulse widths were associated with station on the Franciscan side and along the San Andreas fault itself. Stations along or near the Calaveras and Sargent faults showed shorter pulse widths than those along the San Andreas. When a second quarry source was used similar pulse widths were measured at most stations. Those stations that had significantly different pulse widths could be associated with propagation paths through anomalous features (a metamorphosed limestone pendant and a deep sedimentary trough). Preliminary quantitative estimates of attenuation using a simple model of Gladwin and Stacey (1974) and Ramana and Rao (1974) include a granitic basement Q greater than 200 (possibly less than 800), and a Q of 19 to 28 for the upper crust associated with the Franciscan Formation assuming a previously estimated Q of 40 to 250 for the upper crust associated with the Gabilan granite. Pulse width measurements are a promising method for measuring variations in anelastic properties that are not affected by scattering or lateral refractions.

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