GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 1) First location using the starting model and the observed arrival times. 2) First travel time inversion using the starting model and the travel times with the earthquake positions obtained by the first location. MAIN LOOP (fixed iterations): INTERNAL LOOP (fixed iterations): 3) Update earthquake positions by time residual analysis using the current velocity model and the rays computed by the previous travel time inversion. 4) New travel time inversion using the current model and the travel times from the previous location with the new earthquake positions. Go to point 3. 5) New location using the original observed arrival times, the new velocity model from the previous travel time inversion, and the new earthquake positions from the previous time residual analysis. 6) New travel time inversion using the current model and the travel times from the previous location. Go to point 3. Synthetic example We use a simple 6-layers model with a constant vertical velocity gradient to avoid the influence by other factors (nonlinearity of inversion, errors in ray tracing, poor station coverage, picking indeterminacy). We consider four earthquakes at constant depth spacing and 49 stations covering the space uniformly (Fig. 2). We also assume maximum reliability for all arrivals P and S. We applied the iterative procedure “location + travel time tomography” using a constant velocity model (Vp=4 km/s, Vs= 2.312 km/s, Vp/Vs=1.73) as starting model and considering four different approaches/hypothesis: A. Only iterative procedure. B. Iterative procedure + residual analysis (scheme of Fig.1). C. Only iterative procedure with known origin time. D. Iterative procedure + residual analysis (scheme of Fig. 1) with known origin time.