GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 We used the software NLLoc (NonLinLoc, Lomax et al. 2000) to locate the events and SIRT algorithm (Simultaneous Iterative Reconstruction Technique, Stewart 1991) to invert the travel times. The input grid of NLLoc consisted of nodes spaced 1 km apart in X, Y, and Z directions. We used OCT (Octree) as the search type and EDT_OT_WT as location method (NLLoc, 2017). In travel time inversion we used bent rays computation associated with diving ray paths (Böhm et al. 1999). In all four cases considered, we run the iterative procedure with 10 main iterations and 8 internal iterations (see the scheme in Fig. 1). Fig. 2 – Description of the synthetic model used in this study. a) Plan view of the stations and earthquakes positions. b) Vertical section of the model with the ray paths referred to the computed synthetic travel times. C) Vertical Vp and Vs velocities of the model. Fig. 3 shows the final velocity models obtained from the four cases for both P and S velocities. In the first case (Fig. 3A), where residual analysis is not applied, even after 10 iterations we obtain both velocities and event positions (table in Fig. 4) that are far from the true values (the P velocities seem even to diverge). In contrast, the application of residual analysis in the iterative procedure yields much better results for both velocities and correct event depths (Fig. 3B), although they never reach the true values, probably due to the presence of a local minimum in the minimization process used in NLLoc. However, if we assume that the origin time (T0) is known, the true model is well reached at the end of the iterations (C and D), even if we do not use the residual analysis, which still improves the solution.