Rupture process of the April 24, 2017, Mw 6.9 Valparaíso earthquake from the joint inversion of teleseismic body waves and near-field data

Abstract

The central Chilean margin (32°–33°S) is characterized by the subduction of the Juan Fernández Ridge (JFR) beneath the continental South American plate. The JFR corresponds to a hotspot track composed by seamounts typically 3–3.5 km high above the surrounding seafloor, with a ridge-trench collision zone underlying the prominent Valparaiso Forearc Basin (VFB). This region has been affected by several large and mega earthquakes, where the last event corresponds to a complex seismic sequence that took place at the southern edge of VFB in April 2017. The spatio/temporal distribution of the seismic events is characterized by a predominant southeast migration of the seismicity. An Mw 6.9 earthquake triggered two days after the sequence started and occurred at the northern end of the rupture area of the 1985 Mw 8.0 Valparaiso earthquake. We compute the kinematic rupture process of the 2017 Mw 6.9 Valparaiso earthquake from the joint inversion of teleseismic body waves and near-field data. The Akaike’s Bayesian Information Criterion was used to objectively estimate both, the relative weighting between datasets and the weighting of spatial and temporal constraints used as a priori information. The coseismic slip is distributed over an area of dimensions ∼35 × 10 km2, with a maximum slip of 1.5 m. The rupture propagated unilaterally downdip. The source duration from the moment-rate solution is ∼20 s, with a total seismic moment of 3.05 × 1019 Nm (Mw 6.9). The analysis of the seismicity shows that most of the events occurred along the plate interface, foreshock clustered northern from the mainshock epicenter and the aftershocks occurred to the southeast, at a deeper location. The inverted regional moment tensors show similar faulting mechanism than the mainshock. The seismic sequence started two days before the mainshock and lasted for about two weeks, and a migration pattern of the seismicity was observed. The rupture of the 2017 Mw 6.9 earthquake nucleated where the San Antonio seamount (belonging to the JFR) is subducting, and propagated downwards along a zone that presents high interseismic coupling. The complex seismic sequence might be explained by an aseismic slip transient in the zone and the influence of the downdip migration of fluids from the accretionary prism along the subduction channel. The erosive and tunneling effect left by the sudden slip of the subducting seamount might provide the cavity for downdip migration of fluids and subsequent swarm seismicity.