Summary Megathrust earthquakes occur at subduction zones and produce many of the largest magnitude earthquakes on record. The ruptures of megathrust earthquakes have classically been thought to nucleate at asperities, zones on the subduction interface of rheological strength which accumulate strain as the plates move. Imaging such asperities would critically inform seismic hazard analyses, to the benefit of millions who live adjacent to major subduction boundaries. We created a 3D shear wave attenuation (QS) tomography of the 2015 MW 8.3 Illapel, Chile, earthquake rupture region by measuring QS using a body wave S/P spectral ratio method along 3,852 ray paths between 708 aftershocks of the 2015 earthquake and a network of 22 broadband seismometers. We then used these measurements in a 3D nonnegative least-squares regression inversion to determine the subsurface QS structure. We identify high QS anomalies on the Nazca – South America subduction interface in the Illapel region which correlate spatially with areas of high frequency seismic radiation and high coseismic slip from the 2015 earthquake, and we interpret these anomalies as asperities. We also observe elongated bands of alternating high and low QS on the plate interface which match the orientation of normal fault fabrics on the surface of the subducting Nazca plate. We interpret the high QS bands as subducted horsts, which are materially strong and prominent, and behave as asperities; low QS bands are subducted grabens full of more attenuating subducted sediments and underplated continental material. These structures guided the rupture of the Illapel earthquake: nucleation occurred in a region of moderate QS (∼400) within a graben, but propagated to two adjacent high QS ( 1000) asperities which were critically stressed to near-failure, one on a horst directly up dip and another on a horst directly down dip. Both of these asperities subsequently failed, causing a cascading rupture which characterized the great earthquake.
Kelly et al. (Tue,) studied this question.