Current Research in EEPS Seminar: Steven Roecker – Rensselaer Polytechnic Institute
The Nature of the Subduction Wedge in an Erosive Margin: Insights from the Analysis of Aftershocks of the 2015 Mw 8.3 Illapel Earthquake Beneath the Chilean Coastal Range
Aftershocks of large earthquakes generally are presumed to be caused by asperities or other sources of residual strain on the mainshock rupture surface, and analyses of these events typically focus on how strain is released on that surface. Moreover, the rate of aftershock activity is usually significantly higher than that of the ambient background, which allows an extensive dataset to be recorded by a short-term seismic network. Hence, in addition to investigating the mainshock, aftershock surveys provide rare opportunities to illuminate the tectonics of the region in which the mainshock occurred. We recorded aftershocks following the 16 September 2015 M8.3 Illapel earthquake in central Chile for about one year, and used that data to generate subsurface images of the coastal wedge. Surprisingly, we discovered that the bulk of the aftershock activity did not occur on the rupture interface but more likely was located within a nappe-like structure that had been eroded from the western edge of the South American plate and reaccreted to that plate near the subduction point at about 60 km depth. This model explains many of the features we observe in the seismicity and wavespeeds and is consistent with geologic observations of uplift and extension in this part of the Chilean coastal margin, and suggests that the high Andes are not supported by friction along the subduction zone interface as is often assumed.