Constraining nodal plane orientation and Q from a pulse width method. Application to the Amatrice-Visso-Norcia Sequence (2016-17)

Summary A new inversion technique, aimed at inferring the fault plane orientation and a frequency-independent quality factor of P-waves (QP) at each recording site, from the inversion of rise times ( τ ) and pulse widths ( Δ T ) of a small magnitude earthquake recorded on a regional scale, has been developed. The proposed method enables us to obtain a misfit between data and their theoretical estimates smaller than the mean error of the data, which was the main limitation of previous approaches based on the assumption of a homogeneous Q. To this end, the theoretical equations describing the relations among data and model parameters, inferred in previous studies, have been adapted to the case of a heterogeneous QP-structure. For each of the two nodal planes inferred from focal mechanisms, an iterative two-step inversion technique has been implemented. At the first step, fault radius L and a background homogeneous Qback are inferred using a grid-search method. At the second step, the residuals between data and their theoretical estimates are used to obtain a new perturbed dataset and to compute a QP correction at each station. Finally, the best-fitting nodal plane is selected via an F-Test at a fixed level of significance. The application of the method to a dataset of 21 earthquakes (3 ≤ ML ≤ 4) of the 2016 Amatrice–Visso–Norcia (AVN) sequence in Central Italy, allows to reduce the misfit between data and model to values less than the average error on data for all the studied events and enables the fault plane discrimination for a subset of 13 events. Inferred fault plane orientations indicate that studied events can be related to a complex system of synthetic and antithetic normal faults generally oriented along the strike of the Apennines (NNW-SSE); this consideration has been confirmed by previous works. Site-QP inferred values are in the same range as those obtained in previous studies in the same area. The comparison with S-wave attenuation studies indicate that QP\<\ QS for the AVN sequence, in agreement with the results obtained from the analysis of the 1997 Umbria-Marche seismic sequence, occurred in a limited region of the same area of the AVN sequence.