On fault vs. off-fault seismicity: the role of rock vs. fault rheology

Abstract Analysis of seismicity can illuminate active fault zone structures but also pervasive deformation occurring within large volumes of the seismogenic zone. Here we show that, for the Mw 6.5 2016–2017 Central Italy seismic sequence, seismicity not only localizes along SW-dipping normal faults, but also occurs within larger volumes of Triassic Evaporites, TE, composed of alternated anhydrites and dolostones. This off-fault and diffuse microseismicity shows a different frequency-magnitude distribution than on-fault seismicity along the structures hosting the largest events. We interpret that, during the sequence, shear strain-rate increase promoted widespread ductile deformation within TE that light-up with diffuse microseismicity. This interpretation is supported by field and laboratory observations showing that TE background ductile deformation is complex and dominated by distributed failure and folding of the anhydrites associated with boudinage fracturing and faulting of dolostones. Our results indicate that ductile crustal deformation can cause off-fault and diffuse microseismicity, which obeys to different scaling laws than on-fault seismicity on structures characterized by elasto-frictional stick-sip behaviour.