The aftershocks and the now found “silent” afterslip are key to understand the processes occurring after strong earthquakes. The GPS data in combination with seismological data allowed for the first time a comparative analysis: Are aftershocks triggered solely by stress transfer from the main shock or are additional mechanisms active? “Our results suggest, that the classic view of the stress relaxation due to aftershocks are too simple” says Jonathan Bedford from GFZ to the new observation: “Areas with large stress transfer do not correlate with aftershocks in all magnitude classes as hitherto assumed and stress shadows show surprisingly high seismic activity.”

Setting up a Creepmeter Station in Southern Central Chile. (Credit: GFZ)

A conclusion is that local processes which are not detectable at the surface by GPS monitoring along the plate interface have a significant effect on the local stress field. Pressurized fluids in the crust and mantle could be the agent here. As suspected previously, the main and aftershocks might have generated permeabilities in the source region which are explored by hydrous fluids. This effects the local stress field triggering aftershocks rather independently from the large scale, main shock induced stress transfer. The present study provides evidences for such a mechanism. Volume (3D) seismic tomography which is sensitive to fluid pressure changes in combination with GPS monitoring will allow to better monitor the evolution of such processes.

The main shock was due to a rupture of the interface between the Nasca and the South American plates. Aftershocks are associated with hazards as they can be of similar size as the main shock and, in contrast to the latter, much shallower in the crust.