Body-wave Attenuation in the Region of Garda, Italy

Abstract

We analyzed the spectral amplitude decay with hypocentral distance of P and S waves generated by 76 small magnitude earthquakes (ML 0.9-3.8) located in the Garda region, Central-Eastern Alps, Italy. These events were recorded by 18 stations with velocity sensors, in a distance range between 8 and 120 km. We calculated nonparametric attenuation functions (NAF) and estimated the quality factor Q of both body waves at 17 different frequencies between 2 and 25 Hz. Assuming a homogeneous model we found that the Q frequency dependence of P and S can be approximated with the functions QP=65 f 0.9 and QS=160 f 0.6, respectively. At 2 Hz the QS / QP ratio reaches the highest value of 2.8. At higher frequencies QS/QP varies between 0.7 and 1.7, suggesting that for that frequency band scattering may be an important attenuation mechanism in the region of Garda. To explore the variation of Q with depth, we estimated Q at short (r ≤ 30 km) and intermediate (35-90 km) distance paths. We found that in the shallow crust P waves attenuate more than S (1.3 < QS / QP < 2.5). Moreover, P waves traveling along paths in the lower crust (depths approximately greater than 30 km) attenuate more than S-waves. To quantify the observed variability of Q with depth we considered a 3-layer model and inverted the NAF to estimate Q in each layer. We found that in the crust Q increases with depth. However, in the upper mantle (~40-50 km depth) Q decreases and in particular the high frequency QS (f>9 Hz) has values similar to those estimated for the shallow layer of the crust.