Interseismic creep of carbonate-hosted seismogenic normal faults (central Italy)

It is crucial to comprehend the fault slip behavior in carbonate formations as they are potential sites of earthquake initiation. We studied three carbonate-hosted seismogenic normal faults in the northern Apennines by using (micro)structural and geochemical analyses of fault rocks combined with new seismic coupling estimates. The (upper bound) seismic coupling is 0.75, meaning that at least 25% of the study-area long-term deformation is released aseismically in the upper crust. Microscopy and electron-backscatter diffraction analysis reveal that whereas the localized principal slip zone records seismic slip (as ultracataclastic material, calcite crystallographic preferred orientation - CPO, truncated clasts, and possibly mirror-slip surfaces), the bulk fault rock below behaves differently. Cataclasites in massive limestones deform by cataclastic flow, intergranular pressure solution, and crystal plasticity, with CPO development. Foliated tectonites in micritic limestones deform by pressure solution and frictional sliding, with CPO development. We suggest these mechanisms accommodate on-fault interseismic creep. This is consistent with experimental results reporting velocity-strengthening behavior at low slip rates. Models of fault slip behavior usually assume that (i) seismic vs. aseismic slip take place in separate fault patches, (ii) creep is limited to lithology-controlled weak domains, and (iii) rate-weakening patches are interseismically locked. We herein bring multi-scale clues of co-existing seismic and aseismic sliding along the same fault in limestones at different times during the seismic cycle. Our results imply that on-fault aseismic motion must be added to seismic slip to pair the long-term deformation rates and that creep is not exclusive of phyllosilicate-bearing units. Our work provides new insights into the fault slip behavior in carbonate rocks and may profoundly impact the comprehension of the seismic cycle and fault seismogenic potential.