Crustal Q in Southern Italy determined from regional earthquakes

Abstract

We estimated the quality factor Q of S waves traveling through the crust in the southern region of Italy. For that purpose 65 regional earthquakes with magnitudes ML 2.0–5.3 and hypocentral distance ranging between 7 and 375 km were analyzed. To estimate Q we first calculated a set of empirical functions that describe the S-wave spectral-amplitude decay with distance for 15 frequencies between 1 and 25 Hz. Within the distance range of the recordings, the average Q can be approximated as Q = 25.7 f 1.4. To separate the effect of upper and lower crust in the total Q, we used the first 50 km of the attenuation functions to estimate Q of the upper crust, and found that Q = 18.8 f 1.7. Then, using the 50–100 km segment of the attenuation functions we estimated that Q = 17.8 f 1.3 for the lower crust. We also found that at high frequencies ( f > 10 Hz) the upper crust Q increases considerable relative to lower crust Q, being a factor of nine greater at 25 Hz. This higher attenuation in the lower crust may be the effect of deep structures as reported by Menardi-Noguera and Rea (2000) that were subjected to brittle–ductile deformation during past tectonic events.