The 2015 Mw 8.3 Illapel, Chile, earthquake: direction-reversed along-dip rupture with localized water reverberation

Abstract

The kinematic rupture process of the 2015 Illapel, Chile, earthquake is investigated based on a joint inversion of teleseismic, Interferometric Synthetic Aperture Radar, Global Positioning System, and tsunami data, as well as backprojection (BP) techniques. The coseismic slip area is determined to be similar to 100 x 100 km along strike and along dip, with a peak slip of similar to 7.0 m located similar to 80 km to the north-northeast of the epicenter. The total seismic moment is estimated to be 2.5 x 10(21) N.m (M-w 8.2). The rupture kinematics is featured by unilateral propagation along the strike and reverse rupture along the dip. In our model, the rupture bifurcates to up-dip and down-dip between 20 and 40 s. The downward rupture branch reaches the down-dip border of the rupture area at 30-40 s, reverses its propagation direction, and migrates to the trench between 80 and 100 s. This is also revealed in the BP results in a wide frequency range. The aftershocks on the plate interface are complementary to the coseismic rupture area. Normal-faulting aftershocks are observed in the out-trench area, and its along-strike extent is consistent with that of the near-trench rupture. To determine the source duration of the main event, we compare the teleseismic waveforms of the mainshock and aftershocks at different depths. Coda waves after 100 s are observed in both the mainshock and shallow aftershock waveforms, with similar duration, relative amplitude, and characteristic period (16 s). This is consistent with a theoretical water-reverberation-phase period near the trench. Therefore, it suggests that the coda waves likely originate from water reverberation generated by shallow rupture, instead of from a prolonged source duration.