Imaging Rupture Process of the 2015 Mw 8.3 Illapel Earthquake Using the US Seismic Array

We study the rupture process of the Mw 8.3 Illapel, Chile earthquake that occurred on 16 September 2015 using the US seismic network as a large aperture array. We apply the back-projection technique using two frequency bands, 0.1-0.5 and 0.25-1 Hz. Both frequency bands reveal that this event is characterized by rupture of three patches. The higher frequency band shows an earlier burst of seismic radiation and illuminates a relatively down-dip patch of energy radiation. On the other hand, the lower frequency band shows a more up-dip rupture and matches well with the slip inversion model in other studies. The Illapel earthquake ruptures about 100-km along-strike, and shows 40-km up-dip and 40-km down-dip movement along the subduction megathrust fault. The earthquake first ruptures around the epicenter with a relatively low level of seismic radiation. Then, it propagates northeast along the Juan Femandez Ridge (JFR) to rupture a patch down-dip accompanied by strong higher frequency seismic radiation. Finally, it ruptures to the northwest of the epicenter and terminates south of the Challenger fracture zone (CFZ), releasing a burst of strong lower frequency seismic radiation. Most of the aftershocks are either within or at the edge of the rupture patch, a region characterized by high coupling in central Chile. The rupture is bounded along-strike by two fractures zones to the north and south. The JFR to the south of the rupture zone may have acted as a barrier along-strike, leaving the area south of the mainshock vulnerable for a large damaging earthquake in the near future.