Multi-seismic cycle velocity and strain fields for an active normal fault system, central Italy

Vertical offsets of Late Pleistocene–Holocene deposits and landforms are combined with slip-direction indicators on dipping active normal faults in central Italy to produce velocity and strain-rate fields averaged since 12–18 ka for a 5 by 5 km grid. Maximum strain-rates of 0.078 ± 0.0156 ppm/yr, or 0.052 ± 0.01 ppm/yr across a distance of 63,639 m imply maximum NE–SW extension rates of 5.0 ± 1.0 mm/yr or 3.3 ± 0.66 mm/yr depending on whether rates are averaged over 12 ka or 18 ka. The high spatial resolution reveals that strain-rates vary by up to 0.04 ± 0.008 or 0.06 ± 0.012 ppm/yr over a distance of only c. 7 km along the strike of the fault system, again depending on the timescale chosen. For the first time, these data allow a comparison of rates derived over time periods that would contain numerous seismic cycles with those measured with GPS that represent either interseismic strain, or displacements due to a single large earthquake with some interseismic strain. Overall, the rates compare reasonably well with those measured geodetically or derived from seismic moment summations (< 0.12 ppm/yr and generally in the range of 0.10–0.06 ppm/yr; 6.36–3.82 mm/yr). However, the present wide spacing of geodetic sites (30–40 km) means that the small-scale spatial variations in strain-rate since 12–18 ka have not yet been resolved using GPS. This makes it difficult to compare strain measured geodetically with that released during historical earthquakes to assess whether excess strain will be released in an impending earthquake.