Repeated surveys reveal nontectonic exposure of supposedly active normal faults in the central Apennines, Italy

Abstract

We investigate the geomorphic processes that expose bedrock fault surfaces from under their slope‐deposit cover in the central Apennines (Italy). These bedrock fault surfaces are generally located at various heights on mountain fronts above the local base level of glacio‐fluvial valleys and intermountain fluvio‐lacustrine basins and are laterally confined to the extent of related mountain fronts. The process that led to the exposure of fault surfaces has often been exclusively attributed to coseismic earthquake slip and used as proxy for tectonic slip rates and earthquake recurrence estimations. We present the results of monitoring the contact between the exposed fault surfaces and slope deposits at 23 measurement points on 12 different faults over 3.4 year long observation period. We detected either downward or upward movements of the slope deposit with respect to the fault surface between consecutive measurements. During the entire observation period all points, except one, registered a net downward movement in the 2.9–25.6 mm/yr range, resulting in the progressive exposure of the fault surface. During the monitoring period no major earthquakes occurred in the region, demonstrating that the measured exposure process is disconnected from seismic activity. Our results indicate that the fault surface exposure rates are rather due to gravitational and landsliding movements aided by weathering and slope degradation processes. The so far neglected slope degradation and other (sub)surface processes should thus be carefully taken into consideration before attempting to recover fault slip rates using surface gathered data.

Gravity Versus Tectonics: The Case of 2016 Amatrice and Norcia (Central Italy) Earthquakes Surface Coseismic Fractures

“…Thus, we conclude that along steep slopes, the exposure of fault planes, and the generation of fractures in unconsolidated sediments is due to the continuous interaction of structural and geomorphologic elements with gravitational and erosion‐depositional processes. So far this interaction is often overlooked but is proven here to be significant and in many instances even dominant, as recently documented by Kastelic et al () for a number of “nastrini di faglia” (“fault ribbons”) of the central Apennines.…”

Field estimate of paleoseismic slip on a normal fault using the Schmidt hammer and terrestrial LiDAR: Methods and application to the Hebgen fault (Montana, USA)

“…stream modification of the base of the scarp, removal of rock mass via slope processes, biological weathering due to lichen or vegetation cover, remineralization of calcite lower on limestone scarps) (e.g. Kastelic et al, 2017). These assumptions are not always valid and limit the applicability of all scarp dating techniques at some sites.…”

“…Kastelic et al, 2017). In the absence of near-surface creep and significant erosion, bedrock fault scarps can record several meters of cumulative displacement over many earthquake cycles.…”

“…Kastelic et al (2017) observed exposure rates due to non-tectonic erosion or deposition of colluvial material at the base of normal faults in Italy, and concluded that exposure rates could outpace fault slip by orders of magnitude. Kastelic et al (2017) observed exposure rates due to non-tectonic erosion or deposition of colluvial material at the base of normal faults in Italy, and concluded that exposure rates could outpace fault slip by orders of magnitude.…”

“…SCHMIDT HAMMER-DERIVED PALEOSEISMIC SLIP ON A NORMAL FAULT assisted by non-tectonic, continuous or diffusive lowering of the colluvial cover at the base of the scarp. Kastelic et al (2017) observed exposure rates due to non-tectonic erosion or deposition of colluvial material at the base of normal faults in Italy, and concluded that exposure rates could outpace fault slip by orders of magnitude. If this was the only process operating on the Hebgen fault, the linear or power-law models for progressive exposure (Figure 6) could be the most appropriate curve fits.…”