The role of rheology, crustal structures and lithology in the seismicity distribution of the northern Apennines

The Northern Apennines of Italy is a unique area to study active crustal processes due to the availability of high-resolution subsurface geology (deep borehole and seismic profiles) and seismicity (back-ground and seismic sequences) data. In this work we have investigated the relationship between crustal structures and lithologies, rheological profiles and seismicity cut-off by constructing three integrated profiles across the Umbria-Marche Apennines.

At first approximation we observe a good correspondence between the background seismicity cut-off and the modelled brittle ductile transition (BDT): 90% of the seismic activity is located above the transition. In the area characterized by active extension, where the majority of the seismicity is occurring, most of the crustal earthquakes are confined within the brittle layer at depth < 12 km. In areas where the brittle layer is affected by regional structures, we observe an active role played by these structures in driving the seismicity distribution. One example is the region where the gently eastward dipping Altotiberina low-angle normal fault is present, where the seismicity cut-off is completely controlled by this detachment that separates an active hanging-wall from an almost aseismic foot-wall block. At smaller scale also the lithology plays a strong control on the seismicity distribution. We observe that the largest earthquakes of the area, 5.5 < M < 6.0, do not nucleate at the base of the BDT but they occur at the base of the sedimentary cover.

Our work suggests that rheology and therefore the position of the brittle ductile transition exerts a role at regional scale for the occurrence of crustal seismicity, however crustal structures and lithology play the major role at a more local scale and therefore they need to be considered for a better understanding of earthquake distribution within the seismogenic layer.