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 (Δτ) 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 Δτ = 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). 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.
We investigate dynamic source parameters of the Mw7.1 Puebla‐Morelos intermediate‐depth earthquake (h = 57 km) of 19 September 2017, which devastated Mexico City. Our simple, elliptical source model, coupled with a new Particle Swarm Optimization algorithm, revealed rupture propagation within the subducted Cocos plate, featuring a high stress drop (Δτ = 14.9±5.6 MPa) and a remarkably low radiation efficiency (ηr = 0.16 ± 0.09). Fracture energy was large (G = (1.04 ± 0.3) × 1016 J), producing a slow dissipative rupture (Vr/Vs = 0.34 ± 0.04) with scaling‐consistent radiated energy (Er = (1.8 ± 0.9)·1015 J) and energy‐moment ratio (Er/M0 = 3.2 × 10−5). About 84% of the available potential energy for the dynamic rupture was dissipated in the focal region, likely producing friction‐induced melts in the fault core of 0.2–1.2 cm width due to heat production (700–1200 °C temperature rise). Such source features seem to be a universal signature of intermediate‐depth earthquakes.
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