Imaging the source region of recent megathrust earthquakes along the Chile subduction zone: A summary of results from recent experiments

Abstract

In addition to the largest documented earthquake, the Mw 9.5 Valdivia earthquake of 1960, the Chilean subduction zone has experienced three great (>Mw 8) earthquakes since 2010 for which detailed models of deformation prior to, during and after the earthquake are available. The 1960 and 2010 Mw 8.8 Maule earthquake affected the south-central Chilean margin, where the trench is sediment filled, whereas the 2014 Mw 8.2 Iquique and 2015 Mw 8.3 Illapel earthquakes occurred farther north where there is little sediment in the trench. We summarize recent results from three marine seismological projects designed to elucidate the impact that geologic structure may have had on slip during these events. Because analysis of these datasets is ongoing, this report should be considered as an interim summary rather than a comprehensive synthesis. None-the-less, some patterns can be discerned. That we can resolve 5 m of margin uplift by differencing swath bathymetric data highlights the value of obtaining ground truth marine geophysical data from potential seismic gaps. Controlled source seismic imaging indicates that subduction of nearly all incoming sediment in the sediment-filled section of the trench is a widespread characteristic of the subduction zone segment that ruptured in the two largest events and is likely a generalizable characteristic that can be used to anticipate whether a subduction zone is likely to rupture in great tsunamigenic earthquakes. While this is not a new idea, our data provide additional insights into plate boundary evolution at depth in this scenario. We also find that subduction of anomalously rough basement topography ∼2 million years ago may have led to indentation, large scale internal deformation, and erosion of the overlying wedge north of the 2010 rupture zone and that the process of wedge recovery may have resulted in an abrupt offset along strike of the plate boundary that served as a barrier to slip propagation in 2010. In general, structures generating wedge disruption may no longer be detectable following their subduction deep beneath the forearc wedge, but their passage can be inferred from residual structures mapped within the wedge, and their impact on slip during large earthquakes may be long lasting. We anticipate that further analysis of the data from these experiments, and from future experiments that build on these results, will further elucidate the relationship between geologic structure and slip such that images of subsurface structure become an additional tool in the toolbox of observations used to evaluate future hazard.