An attenuation study in Southern Italy using local and regional earthquakes recorded by seismic network of Basilicata

We determined a set of empirical functions that describe the spectral amplitude decay of S-waves with distance in Southern Italy. We analyzed 32 earthquakes with magnitudes M-L 2.0-5.4 and hypocentral distances ranging between 12 and 216 km. We obtained attenuation functions for 14 frequencies (1.0<f<20.0 Hz). We compared these functions with average non-parametric attenuation functions reported by Castro et al. (1999) for different regions of Italy, and we observe that at low frequencies (f<5.0 Hz) the spectral amplitudes from earthquakes in Southern Italy decay faster than the average. However, at high frequencies (f>5.0 Hz), the spectral amplitudes are above the average. At higher frequencies (f>10 Hz), the attenuation functions obtained for Southern Italy are slightly above the standard deviation of the average attenuation functions. It is possible that in this frequency range (10-20 Hz) site effects may influence the amplitude decay. In order to quantify the attenuation of the S-waves, we estimated the quality factor Q modeling the empirical attenuation functions using the following parametric form: A(f, r)=10/r(b).e(-pifR/Qbeta); where 1.6 less than or equal to f less than or equal to 10.0 Hz is the frequency band with minimum effect of instrument and site response, rless than or equal to120 km is the distance range where the rate of decay of the spectral amplitudes is approximately constant, R=(r-10) and beta=3.2 km/s. We found that the exponent b=1.0+/-0.2 in the frequency band analyzed and Q shows a frequency dependence that can be approximated by the function Q=32.1 f(1.7).