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 M-w 9.1 Tohoku (Japan) and the 2014 Iquique (Chile) M-w 8.2 megathrust subduction zone earthquakes.