Genetic algorithm inversion of the average 1D crustal structure using local and regional earthquakes

Knowing the best 1Dmodel of the crustal and uppermantle structure is useful not only for routine hypocenter determination, but also for linearized joint inversions of hypocenters and 3D crustal structure, where a good choiceof the initialmodel canbevery important.Here,we tested thecombinationof a simpleGA inversionwith the widely used HYPO71 program to find the best three-layer model (upper crust, lower crust, and upper mantle)byminimizing theoverall P-andS-arrival residuals, using local andregional earthquakes in twoareasof the Brazilian shield. Results from the Tocantins Province (Central Brazil) and the southern border of the S~ ao Francisco craton (SEBrazil) indicated anaverage crustal thicknessof 38and43 km, respectively, consistentwith previous estimates from receiver functions and seismic refraction lines. The GA+HYPO71 inversion produced correct Vp/Vs ratios (1.73 and 1.71, respectively), as expected fromWadati diagrams. Tests with synthetic data showed that themethod is robust for the crustal thickness, Pn velocity, andVp/Vs ratiowhenusing eventswith distanceuptoabout400 km,despite thesmallnumberofeventsavailable (7and22, respectively).Thevelocities of the upper and lower crusts, however, are less well constrained. Interestingly, in the Tocantins Province, the GA+HYPO71 inversion showeda secondary solution (localminimum) for the average crustal thickness, besides the global minimum solution, which was caused by the existence of two distinct domains in the Central Brazil with very different crustal thicknesses. & 2010 Elsevier Ltd. All rights reserved.