Quantification of stress accumulation and release during subduction zone seismic cycles requires an understanding of the distribution of fault slip during earthquakes. Reconstructions of slip are typically constrained to a single, known fault plane. Yet, slip has been shown to occur on multiple faults within the subducting plate1 owing to stress triggering2, resulting in phenomena such as earthquake doublets3. However, rapid stress triggering from the plate interface to faults in the overriding plate has not been documented. Here we analyse seismic data from the magnitude 7.1 Araucania earthquake that occurred in the Chilean subduction zone in 2011. We find that the earthquake, which was reported as a single event in global moment tensor solutions4,5, was instead composed of two ruptures on two separate faults. Within 12 s a thrust earthquake on the plate interface triggered a second large rupture on a normal fault 30 km away in the overriding plate. This configuration of partitioned rupture is consistent with normal-faulting mechanisms in the ensuing aftershock sequence. We conclude that plate interface rupture can trigger almost instantaneous slip in the overriding plate of a subduction zone. This shallow upper-plate rupture may be masked from teleseismic data, posing a challenge for real-time tsunami warning systems.
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We are grateful to all field crews from partner organizations who participated in the deployment and servicing of seismic instruments used in this study. We thank J. Zahradník and E. Sokos for their assistance in setting up the ISOLA code. S.P.H. is financially supported by a NERC studentship (NE/J50015X/1).
The authors declare no competing financial interests.
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Hicks, S., Rietbrock, A. Seismic slip on an upper-plate normal fault during a large subduction megathrust rupture. Nature Geosci 8, 955–960 (2015). https://doi.org/10.1038/ngeo2585
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