Bedrock fault roughness resolves slip increments of large earthquakes: Case studies from Central Italy

While the cyclic nature of tectonic strain build-up and release is at least conceptually well understood (known as the earthquake cycle), it is not clear how variable the corresponding earthquake sizes and the time intervals between them are. Additional observations from past earthquakes are required to better constrain these aspects. In this context, we investigate the roughness of bedrock normal fault scarps along the Mt. Vettore fault and Roccapreturo fault (Italy), motivated by field observations that roughness positively correlates with profile height: higher sections of the fault surface are rougher than lower sections, ostensibly because the scarps were created by consecutive exposure events (e.g., earthquakes), exposing the higher sections for longer periods of time to subaerial weathering processes. Using high-resolution topographic models of the two fault surfaces, we calculate local roughness metrics and determine how they change as a function of profile height. In doing so, we are able to identify step-like changes in fault roughness, suggesting that the studied portions of the Mt. Vettore and Roccapreturo fault scarps were formed respectively by at least 6 and 4 large earthquakes, with slip increments ranging from 0.2 m to 1.7 m and corresponding magnitudes ranging from M6.1 to M6.8. Comparing our results for the Roccapreturo scarp with a previous cosmogenic radionuclide (CRN) investigation at this site indicates that we were able to a) find previously identified earthquakes and b) find additional, previously unresolved ones. It appears that, during the same time interval, fault plane roughness analysis may be a more sensitive indicator of past earthquake activity on carbonate scarps than cosmogenic dating.