SEISMIC SOURCE PARAMETERS FOR MICROEARTHQUAKES OF THE GRANADA BASIN (SOUTHERN SPAIN)

Ninety-five microearthquakes of the Granada basin (southern Spain) wit h duration magnitudes ranging from 1.3 to 3.5 (moment magnitude from 0 .9 to 2.5) have been spectrally analyzed with digital recordings from ten stations of the Andalusian Seismic Network. The coda-Q quality fac tor, Q(c) (assumed to be frequency dependent) was used instead of Q(be ta) for the path correction and the Q alpha adopted was equal to (9/4) Q(c). The kappa parameter for site attenuation correction had values in the range of 0.02-0.05. Fourier displacement spectra were calculate d for P- and S-waves and analyzed with respect to the source model of Brune (1970, 1971). Spectral parameters were automatically determined by the method of Snoke (1987). The seismic moment ranged from 10(17) t o 10(20) dyne cm, with average M(0)(P) to M(0)(S) ratio of 1.02, and t he source radii spanned from 0.14 to 0.40 km, with average r(P) to r(S ) ratio of 1.3. The seismic energy values ranged from 1 x 10(3) to 9 x 10(7) J, with P- to S-energy ratio from 1 to 10 for about 85% of the events, whereas the static stress drop varied from 0.02 to 2.6 bar. Th e scaling relation between the seismic moment and the stress drop indi cates a decrease in stress drop with decreasing seismic moment. Appare nt stress is linearly correlated with stress drop and have on average similar values, and this points out to a partial stress drop.