GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 The use of time residuals to improve the earthquakes positions in depth within the iterative procedure location + tomography. G. Böhm, L. Moratto National Institute of Oceanography and Applied Geophysics - OGS Introduction The hypocenter–velocity problem has been a challenging topic in seismology to locate the seismic events. This problem can be simplified and resolved as a linear approximation to a nonlinear function, but the solutions obtained and the reliability estimates depend on the initial reference model, and systematic errors in the starting model not only affect the velocity model obtained but also distort the resulting error estimates (Kissling et al., 1994). It is worth pointing out that the success of all locating methods depends on the accuracy of the velocity models. Tomography after location is often used to update the velocity field. In some cases, this is done through an iterative process in which location and tomography alternate at each step. However, if the starting model is very different from the real one, tomography, even if applied several times during the iterations, may converge to a local minimum not far from the starting model used for the first location. So, the coupling between the source location, the origin time of the event and the subsurface parameters makes the hypocenter–velocity problem challenging (Thurber 1992). A possible strategy to resolve the hypocenter–velocity problem would be to ignore the event position and velocity structure coupling and proceed with two alternating-direction monoparametric optimizations for velocity and event location (Monteiller et al. 2005). Other options can be adopting iterative methods, which correct jointly the velocity model and the earthquake hypocenters within the kernel inversion (e.g., Crosson, 1976; Thurber, 1983; Michelini, 1991). Joint inversion of the source parameters and the velocity models was also proposed by Pavlis and Booker (1980) and Spencer and Gubbins (1980); the hypocenter–velocity problem can be resolved also as a full-waveform source-focusing problem (Kamei and Lumley 2014; Song et al. 2019; Aghamiry et al. 2020). Recently, Sambolian et al. (2021) approached the location–velocity problem adopting a strategy based on the slope tomography. The main objective of this work is to define a method within the “Location + Travel Time Tomography” iterative procedure that uses the time residuals (i.e., the difference between the computed and the observed picked arrival times) by exploiting the ray paths computed by tomography. Despite similar iterative methods we propose to insert a correction to the earthquake