Early Stage and Main Ruptures of the 2015 Mw8.3 Illapel, Chile, Megathrust Earthquake: Kinematic Elliptical Inversions and Dynamic Rupture Simulations

Abstract : We apply kinematic and dynamic modeling to the 2015 Mw8.3 Illapel, Chile, earthquake constrained by continuous high-rate GPS and strong motion data. Kinematic inversion by elliptical patches allows us to rapidly outline the ruptured area with different time windows and frequency ranges. The preferred solution indicates that the main large patch in the north is preceded by a small one in the south close to the hypocenter with a time shift no greater than 20 s. The rupture directivity on the main patch suggests that the origin is at depth, away from the initial small hypocentral patch. We then construct dynamic rupture models to be consistent with the geometry obtained from the kinematic inversion. We estimate the fracture energy of the main patch to be ∼7.5 MJ/m2. The initial rupture of the small hypocentral patch does not trigger the main patch due to the gap in fracture energy, and thus another nucleation is set at depth. This can be regarded as a foreshock-mainshock sequence rather than a direct cascade rupture growth. The ruptured area corresponds to the preexisting zone of large interseismic coupling prior to the Illapel earthquake. The historical seismicity of the previous century suggests a possible reconstruction of the asperity map, assuming that every earthquake represents a characteristic seismogenic patch. Therefore, the construction of dynamic ruptures with geodetic and seismological knowledge is possible and useful not only for reproducing known earthquakes but also for providing a physically constrained model for quantitative seismic hazard study.