Robust estimation of temporal resistivity variations: Changes from the 2010 Mexicali, Mw 7.2 earthquake and first results of continuous monitoring

We propose a methodology to estimate resistivity variations in the underground geo-electrical structure by applying a linearized iterative nonlinear least-square inversion scheme using the Marquardt-Levenberg method to stabilize the inversion. The method is tested first with synthetic MT data, then with data registered on a permanent MT station in the Mexicali Valley, Mexico; and finally, with data measured before and after a M w 7.2 earthquake, also in the Mexicali Valley. We propose a strategy to define and apply a reference model for situations where the available constraining information is not enough. Our method seems to produce stable and coherent results, as long as the ground resistivity change is able to produce response differences larger than the data uncertainty.