Refining rates of active crustal deformation in the upper plate of subduction zones, implied by geologic and geodetic data: The E-dipping West Crati Fault, southern Italy.

In order to investigate crustal deformation within the upper plate of the Ionian Subduction Zone (ISZ) at different time scales, we have (i) mapped and modelled sequence of Late Quaternary raised marine terraces tectonically deformed by the West Crati normal fault, in northern Calabria, and (ii) refined geodetic rates of crustal extension from continuous GNSS measurements. Indeed, this region experienced damaging earthquakes such as the “1184 Valle del Crati” (M 6.7) and the “1638 Crotonese” (M 6.7) events, possibly on the West Crati Fault; however, an in-depth evaluation of the deformation rates inferred from geologic and GNSS data has not yet been performed. Furthermore, fault slip-rates and earthquake recurrence intervals for the understudied West Crati Fault are still debated and poorly-constrained. Raised Late Quaternary marine terraces are preserved on the footwall of the West Crati Fault; however, it is still debated if the “local” effect of the footwall uplift is affecting the “regional” signal of uplift likely related to the deformation associated either with the subduction or mantle upwelling processes. Within the investigated region lying in the northern part of the uplifting Calabrian-Peloritani Arc there are 32 regionally distributed permanent GNSS stations, for 18 of which the coordinate time series are adequately long (at least 4.5 years) to allow the study of the crustal kinematics. The data of these 18 stations are used to geodetically estimate fault slip-rates and then earthquake recurrence intervals for the West Crati Fault, with the aim of at least partially solve the aforementioned problem of the poor constrains. In particular, velocity and strain across this fault, based on reasonable hypotheses about the fault dip and the mechanical properties of the involved material, are computed starting from GNSS data about the surface kinematics.

Our preliminary results show that GIS-based elevations of Middle to Late Pleistocene palaeoshorelines, as well as temporally constant uplift rates, vary along the strike of the West Crati Fault, mapped on its footwall. This suggests that the fault slip-rate governing seismic hazard has also been constant through time, over multiple earthquake cycles. We then suggest that our geodetically-derived fault slip-rate for the West Crati Fault may be a more than reasonable value to be used over longer time scales for an improved seismic hazard approach, allowing to derive new earthquake recurrence intervals. These results thus suggest a significant yet understudied seismic hazard for the investigated area also because the regional extension might be likely accommodated by a few more active faults across-strike in northern Calabria. These facts highlight the importance of mapping crustal deformation within the upper plate above subduction zones to avoid unreliable interpretations relating to the mechanism controlling regional uplift.