Backprojection of the high-frequency radiation released during the Pisagua (Chile) earthquake (01/04/2014, Mw 8.1) and the Iquique aftershock (03/04/2014, Mw 7.6)

Northern Chile has recently been struck by the Mw 8.1 Pisagua earthquake, which occurred on 01/04/2014 and partially filled the Iquique seismic gap. The Pisagua earthquake has been preceded by intense foreshock activity which started in July 2013 and culminated in a cluster of events in March 2014. We have inferred the rupture dynamics of the mainshock and of its largest (Iquique) aftershock (03/04/2014, Mw 7.6) by backprojecting the high-frequency seismic radiation released during the events and recorded by 310 stations of USArray. The time-evolution of the high-frequency (1-4 Hz) energy radiated during the mainshock shows that the rupture lasted about 80 s, with most of the energy released between 25 s and 50 s from the onset. The cumulative energy emitted during the whole rupture process mainly originated downdip the epicenter just off the coast line, approximately in the latitude range 19.5°-20°S. This region falls at the down-dip side of the co-seismic slip area, similarly to the case of the Maule earthquake (South Chile, 27/02/2010, Mw 8.8) and of other large earthquakes. Differently from the Maule case, most aftershocks not located in the area of large seismic radiation. The time-evolution of the coherent seismic radiation displays an initial low-energy phase (lasting about 20s) during which the source starts to migrate from the nucleation point at the epicenter towards the south-east, activating deeper parts of the subduction interface. After reaching points close to the coast line (after ~30s from the onset), the source moves back towards the epicenter mainly activating in sequence two patches of the interface located around 20°S,70.5°W (F1) and 19.7°S,70.2°W (F2), shortly followed by the activation of points close to the area of the largest co-seismic slip (F3). In the last ~15s of the fracturing (F4), the re-activation of the area releasing energy during F2 is observed. Thus, despite the simple bullseye co-seismic slip pattern, the history of energy radiation is quite scattered, suggesting peculiar and sharply site-dependent frictional properties along this segment of the Chilean subduction interface. The Mw7.6 aftershock displays a similar time-evolution of the radiating source, with an initial low-energy stage, during which the rupture front migrates from the epicenter towards deeper zones. Most energy is released about 25s from the onset, when the rupture front reaches points around 20.5°S,70.0°W.