Dynamic source inversion of the M6.5 intermediate‐depth Zumpango earthquake in central Mexico: A parallel genetic algorithm

We introduce a method for imaging the earthquake source dynamics from the inversion of ground motion records based on a parallel genetic algorithm. The source model follows an elliptical patch approach and uses the staggered-grid split-node method to simulate the earthquake dynamics. A statistical analysis is used to estimate errors in both inverted and derived source parameters. Synthetic inversion tests reveal that the average rupture speed (Vr), the rupture area, and the stress drop (Δτ) may be determined with formal errors of ~30%, ~12%, and ~10%, respectively. In contrast, derived parameters such as the radiated energy (Er), the radiation efficiency (ηr), and the fracture energy (G) have larger errors, around ~70%, ~40%, and ~25%, respectively. We applied the method to the Mw 6.5 intermediate-depth (62 km) normal-faulting earthquake of 11 December 2011 in Guerrero, Mexico. Inferred values of Δτ = 29.2 ± 6.2 MPa and ηr = 0.26 ± 0.1 are significantly higher and lower, respectively, than those of typical subduction thrust events. Fracture energy is large so that more than 73% of the available potential energy for the dynamic process of faulting was deposited in the focal region (i.e., G = (14.4 ± 3.5) × 1014J), producing a slow rupture process (Vr/VS = 0.47 ± 0.09) despite the relatively high energy radiation (Er = (0.54 ± 0.31) × 1015 J) and energy-moment ratio (Er/M0 = 5.7 × 10− 5). It is interesting to point out that such a slow and inefficient rupture along with the large stress drop in a small focal region are features also observed in both the 1994 deep Bolivian earthquake and the seismicity of the intermediate-depth Bucaramanga nest.