Source process of two Mw 6.9 aftershocks of the 2015 Mw 8.3 Illapel earthquake

The rupture of the September 16, 2015, Mw 8.3 Illapel earthquake extended over 200 km length along strike, where the southern portion of the rupture area is controlled tectonically by the collision of the Juan Fernandez Ridge at-32.5 degrees S. On November 11th, two large aftershocks occurred in less than an hour, at shallow depths and epicenters closely located. Both events occurred beyond, but nearby, the northern limit of the Illapel rupture. We inverted kinematic rupture models by joint inversion of strong-motion and teleseismic body waves datasets. The Akaike's Bayesian Information Criterion (ABIC) was used to estimate the relative weighting between datasets, as well as the weighting of spatial and temporal constraints. The rupture of the first event propagated updip from the hypocenter, and later in a bilateral mode, with large slip to the south. The slip solution yields a total seismic moment of 4.44 x 1019 N m (Mw 7.0), and peak-slip of 0.82 m. The epicenter of the second event relocates 15 km southwest with respect to the first one. The rupture of the second event propagated mainly southwards and updip to the trench, re-rupturing an area already broken by the first event. The estimated seismic moment is 3.11 x 1019 N m (Mw 6.9), with a maximum slip of 0.59 m. The source durations from their respective moment-rate functions are 35 s, in both models. The spatial slip distribution retrieved for the two events suggests that the upper portion of the megathrust slipped, a region that is usually thought to behave aseismically, controlled by velocity-strengthening friction. We also relocated the earthquake catalog using a double-difference algorithm. The spatial distribution of seismicity shows clusters that are analyzed in different groups, and the most remarkable feature being a strip running parallel -and close by -to the trench region with very low seismicity. The majority of regional moment tensor solutions are thrust faulting mechanisms, and given the centroid depths retrieved, these events are consistent with interplate seismicity.

Automated summary

The Mw 8.3 Illapel earthquake on September 16, 2015, had a rupture length of over 200 km along strike. Two large aftershocks occurred in close proximity and beyond the northern limit of the Illapel rupture. Kinematic rupture models showed that the first event had an updip propagation from the hypocenter and later a bilateral mode with large slip to the south. The second event relocated southwest and re-ruptured an area already broken by the first event. The spatial slip distribution of both events suggested slip in the usually aseismic upper portion of the megathrust.