The effects of attenuation and site response on the spectra of microearthquakes in the northeastern Caribbean

abstract The spectra of microearthquakes recorded by a seismic network in the northeastern Caribbean were analyzed to determine the effects of attenuation and site response on these spectra. The ratios of the spectral amplitudes at 5 and 20 Hz were measured for several earthquakes at different distances from a fixed receiver to estimate Q for P and S waves in the crust and upper mantle. The average Q p determined from three stations was 380 for epicentral distances between 40 and 200 km. The average Q s determined from two stations was estimated at 400. Spectra of earthquakes closer than 40 km to one station indicate that a high Q path ( Q > 800) exists for these short distances in the island arc platform. The near equivalence of Q p and Q s observed in this study of the upper lithosphere contrasts with estimates of Anderson et al. (1965) that Q p = 9/4 Q s in the asthenosphere. This finding that Q p is approximately equal to Q s suggests that the attenuation mechanism in the upper lithosphere differs from that in the asthenosphere. The spectra of nearby microearthquakes unaffected by crustal attenuation display corner frequencies which are uniform despite large differences in the seismic moments of the events. The corner frequencies of these spectra appear to be characteristic of the receiver site and insensitive to the location and moment of the earthquakes. At one station, the S -wave corner frequency of a nearby blast was identical to the corner frequency of a magnitude 4.8 earthquake derived from the record of a strong motion accelerograph at the same site.