Dynamic source inversion of theM6.5 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 ( V r ), the rupture area, and the stress drop (Δ τ ) may be determined with formal errors of ~30%, ~12%, and ~10%, respectively. In contrast, derived parameters such as the radiated energy ( E r ), the radiation efficiency ( η r ), and the fracture energy ( G ) have larger errors, around ~70%, ~40%, and ~25%, respectively. We applied the method to the M w 6.5 intermediate‐depth (62 km) normal‐faulting earthquake of 11 December 2011 in Guerrero, Mexico. Inferred values of Δ τ = 29.2 ± 6.2 MPa and η r = 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) × 10 14 J), producing a slow rupture process ( V r / V S = 0.47 ± 0.09) despite the relatively high energy radiation ( E r = (0.54 ± 0.31) × 10 15 J) and energy‐moment ratio ( E r / M 0 = 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.