Attenuation and Source Parameters of Shallow Microearthquakes at Mt. Etna Volcano, Italy

We estimated the attenuation laws of high-frequency seismic waves in the shallow crust (depth 5 km) and earthquake source parameters by using a se- lected data set of 320 shallow events (2.6 MD 4.2), recorded at Mt. Etna volcano during the last two flank eruptions occurring in 2001 and 2002-2003. The quality factor (Q) was estimated from spectra of P and S waves for 24 stations of the local permanent network by applying a spectral ratio technique. The results show varia- tions in both QP and QS as a function of frequency, according to the power law Q Q0f n , with n ranging between 0.3 and 1.3 for P waves and between 0.2 and 0.9 for S waves. As typical of volcanic environments, strong azimuthal variations of QP were also found, suggesting the presence of local strong lateral heterogeneities and/or of fluid-filled cracked volumes. After correction for attenuation, we estimated the source parameters (seismic moment, source radius, and stress drop) of a subset of 66 shallow events, under the assumption of a circular dislocation. The estimated seismic moments M0 range from 10 13 to 10 15 N m. The source radii (r) are confined between 100 and 1000 m and stress drop (Dr) ranges between 0.2 MPa and about 4 MPa. Combining the source parameters obtained in this study with those calculated by Pataneet al. (1997) for an old data set of smaller microearthquakes (10 9 M0 10 14 N m) recorded in the same area, we re-evaluated the scaling relationship between seismic moment (M 0 ) and corner frequency ( f c ) for the earthquakes with M 0 ranging between 10 13 and 10 15 N m. We confirm that microearthquakes at Mt. Etna seem not to obey a

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