Source-Parameter Estimation after Attenuation Correction through the Use of Q Tomography

Abstract

The measurement of earthquake source parameters is affected by large uncertainties, and different approaches lead to large variability in results. One crucial aspect is the trade-off between attenuation (Q) and corner frequency (fc ) in spectral fitting: The source corner frequency, inversely proportional to the fault size, can be severely masked by attenuation and site effects. In this article, we describe a method to solve the trade-off based on the fit of displacement spectra to find the source characteristics (corner frequency, f c , and the signal moment, Ω0) and the single-station attenuation operator (t ), in addition to the site response. We follow a parametric approach based on the use of 3D Q seismic tomography and a bootstrap-based method for selecting the best spectra fit. The correction of attenu- ation with synthetic values derived by 3D attenuation tomography efficiently deals with the trade-off between source and path terms, leading to small uncertainties in the deter- mination of source unknowns (f c and signal moment, Ω0 ), thus yielding constrained esti- mates of source parameters for low- to medium-magnitude earthquakes. We show an application to the Emilia 2012 seismic sequence, for which we computed the source param- eters for 1240 aftershocks (from an initial dataset of 1748) with local magnitude ranging from 2.0 to 4.7 using the spectral fit from P and S waves. About 80% of stress-drop esti- mations are characterized by relatively low uncertainties (within 20% of the estimated values), with maximum values of about 40% for the remaining 20%. The attenuation cor- rection is effective to determine source parameters for small-magnitude earthquakes; hence, we obtain reliable estimates of source parameters for the entire aftershock sequence. This approach gives the opportunity to infer the mechanical state of a complete fault system by taking advantage of the larger number of low-magnitude events (with respect to the largest ones) that always follow a major earthquake.