Spatio‐Temporal Complexity of Aftershocks in the Apennines Controlled by Permeability Dynamics and Decarbonization.

In 2016, a series of large normal faulting earthquakes in the Apennines filled the seismic gap between the 1997 Colfiorito and 2009 L'Aquila earthquakes. These earthquakes, known as the Amatrice‐Visso‐Norcia (AVN) sequence, spawned hundreds of thousand of aftershocks in the first year, extending about 60 km along strike. The Colfiorito and L'Aquila aftershocks showed a significant high fluid‐pressure component in their aftershock genesis, and here we show evidence that the AVN aftershocks also include a significant high pressure CO2 component. This CO2 is both deeply‐derived, and internally generated by thermal decomposition. Using a simple model of non‐linear diffusion with a source term, we compare model results with the observed cumulative number of aftershocks, and also spatial comparisons between the calculated fluid pressure and the hypocenters of about 35,000 well‐located events. The good comparisons between model and observations provide evidence that the AVN sequence includes a contribution (additional fluid source) from co‐seismic decarbonization. Our results suggest that internal fluid generation through devolitization or decarbonization is responsible for non‐Omori type aftershock behavior such as observed for the AVN sequence and for the 2011 Mw 9.1 Tohoku (Japan) and the 2014 Iquique (Chile) Mw 8.2 megathrust subduction zone earthquakes. Plain Language Summary: The 2016 Amatrice‐Visso‐Norcia (AVN) earthquake sequence (Italian Apennines) spawned hundreds of thousands of aftershocks in the first year. In this work, we show using a physical and numerical model that the aftershocks are generated by high pressure CO2 thermally decomposed from the carbonates (decarbonization), in addition to deeply‐derived CO2 already shown to be a major contributing factor in Apenninian earthquake sequences. Model results also suggest that aftershock sequences that differ from the standard Omori‐type decay rate (e.g., The 2016 AVN‐sequence, the 2011 Tohoku earthquake, Japan and the 2014 Iquique earthquake, Chile) indicate the internal generation of fluids from either dehydration or de‐carbonization. Key Points: Amatrice‐Visso‐Norcia earthquakes sequence modeled as a fluid‐pressure diffusion process with a source termModel shows that this sequence is driven by deeply‐derived and co‐seismically generated CO2 through thermal decompositionGood spatial and temporal agreement for all depth profiles between modeled and well‐located aftershocks of about 35,000 events [ABSTRACT FROM AUTHOR]