Rheological control on distributed aftershock activity: insights from the Mw 6.5 Norcia seismic sequence

The spatio-temporal evolution of seismicity is of paramount importance as it provides insights into the brittle deformation of the Earth’s crust. During a seismic sequence, the spatial distribution of aftershocks typically reveals the structural architecture of the fault zone activated during the mainshock. However, within the seismogenic crust, seismicity is not necessarily exclusive of the mainshock rupture plane. In the case of the Mw 5.9 and Mw 6.5 2016-2017 Visso-Norcia (Central Italy) seismic sequence, widespread distributed seismicity (Mw < 4.5) has been recorded down-dip in the hangingwall of the ruptured fault in the depth range of 4-9 km (down-dip hanging-wall seismicity, DHwS). This is the portion of the seismogenic crust where seismic reflection profiles identify the presence of large volumes of Triassic Evaporites, TE, a geological formation composed of anhydrites and dolostones. Field and laboratory observations show that, away from the major brittle faults, TE deformation consists of a background ductile deformation interspersed with brittle processes in the form of distributed failure and folding of the anhydrites associated with boudinage hydro-fracturing and faulting of dolostones. 

In this work we used the DHwS to highlight the seismological evidence of distributed deformation within a layer of the seismogenic crust affected by a background ductile deformation.

Through the construction of a series of seismological cross sections oriented perpendicularly to the strike of the mainshock rupture plane, we identified two main types of DHwS: 

• Diffuse, non-localized seismicity characterized by low variability of daily seismicity rate, and
• Localized seismicity featured by both swarm-like and Omori-like event decay.

In particular, localized seismicity is mostly located south of the mainshock, and illuminates kilometers-long structures with different orientations.

We interpret the occurrence of DHwS as the result of the embrittlement of the evaporitic layer induced by an increase of  strain rate following the Norcia mainshock. This is supported by recent laboratory experiments showing that an increase in strain rate promotes brittle failure and faulting in TE samples, rather than ductile deformation. Furthermore, Coulomb stress changes simulated after the Norcia mainshock suggest an increase in strain rate within rock volumes where DHwS is recorded.

Our results suggest that the aftershock distribution during a seismic sequence can be strongly controlled by the rheology of the lithologies contained within the seismogenic crust.