Free surface effects and rupture dynamics : insights from the 2019 Mw=8 northern Peru intraslab earthquake

The 2019/05/26 Northern Peru earthquake (Mw=8) is a major intermediate-depth earthquake that occurred close to the eastern edge of the Nazca slab flat area. We analyze its rupture process using high-frequency back-projection and seismo-geodetic broadband inversion. Both imaging techniques provide a very consistent image of the peculiar space-time rupture process of this earthquake : its 60-second long rupture is characterized both by a main northward propagation (resulting in a rupture extent of almost 200km in this direction) and by a reactivation phase of the hypocentral area, particularly active 35s to 50s after origin time.Given the depth of this earthquake (125-140km), the reactivation time window coincides with the arrival time of the surface-reflected elastic wavefield. Computed values of the dynamic Coulomb stresses associated with this wavefield are of the order of ten to several tens of kPa, in a range of values where dynamic triggering has already been observed. The reactivation phase of the Peru earthquake may thus originate from fault areas that were brought close to rupture by the initial rupture front before being triggered by stress increments provided by the reflected wavefield. Source time function complexity observed for other large intermediate-depth earthquakes further suggests that such a mechanism is not an isolated case.