Bernd Schurr, Marcos Moreno , Anne Marie Tréhu, John Bedford, Jörn Kummerow, Shaoyang Li, Onno Oncken

Asperities are patches where the fault surfaces stick until they break in earthquakes. Locating asperities and understanding their causes in subduction zones is challenging because they are generally located offshore. We use seismicity, inter- and co-seismic slip, and the residual gravity field to map the asperity responsible for the 2014 M8.1 Iquique Chile earthquake. Until two weeks before the mainshock, seismicity occurred exclusively down-dip of the asperity, until a foreshock series broke first the upper plate and later its updip rim. Together the seismicity formed a ring around the future earthquake’s main slip patch. The asperity correlates both with high inter-seismic locking and a circular gravity low, suggesting that it is controlled by geologic structure. Most features of the spatiotemporal seismicity pattern can be explained by a mechanical model in which a single strong asperity is stressed by slab pull.