Two-stage composite megathrust rupture of the 2015 M_w8.4 Illapel, Chile, earthquake identified by spectral-element inversion of teleseismic waves

The M_w8.4 Illapel earthquake occurred on 16 September was the largest global event in 2015. This earthquake was not unexpected because the hypocenter was located in a seismic gap of the Peru-Chile subduction zone. However, the source model derived from 3-D spectral-element inversion of teleseismic waves reveals a distinct two-stage rupture process with completely different slip characteristics as a composite megathrust event. The two stages were temporally separated. Rupture in the first stage, with a moment magnitude of M_w8.32, built up energetically from the deeper locked zone and propagated in the updip direction toward the trench. Subsequently, the rupture of the second stage, with a magnitude of M_w8.08, mainly occurred in the shallow subduction zone with atypical repeating slip behavior. The unique spatial-temporal rupture evolution presented in this source model is key to further in-depth studies of earthquake physics and source dynamics in subduction systems.