Active fault-related folding in the epicentral area of the December 25, 1222 (Io = IX MCS) Brescia earthquake (Northern Italy) : Seismotectonic implications

A dense grid of petroleum industry seismic reflection profiles, coupled with field mapping, exploratory trenching and geomorphic and structural analysis are used to characterize the Quaternary growth history of the Capriano del Colle Fault System, one of several inferred active buried thrusts that extend across the Po Plain in northern Italy. Shortening is characterized here by a deeply buried south-vergent forethrust and an associated north-vergent backthrust whose upward propagation is expressed by fault-propagation folding near the surface. Structural interpretation based on seismic data suggests that strain is accommodated at very shallow levels by secondary flexural-slip thrusts and reverse faults developed on synclinal flanks that emanate from active axial surfaces. Analysis of syntectonic growth strata document maximum rates of dip-slip of 3.45 ± 0.66 mm/yr (1.6 Myr–1.2 Myr) and 0.47 ± 0.22 mm/yr during a more recent time period (0.89 Myr–present). A quarry excavation at Capriano del Colle allows a preliminary paleoseismologic analysis of coseismic surface faulting and liquefaction exposed near the core of an active mid-Pleistocene to Holocene anticline. These features are interpreted to be generated during strong local earthquakes, consistent with the environmental effects and ground motions of an event similar to the December 25, 1222, Brescia earthquake (Io = IX MCS). This indicates, for the first time, that compressive folds and blind thrusts in the Po Plain are currently accommodating slow rates of modern contraction in an active zone of the Southern Alps that extends from Lake Garda to Varese. We thus argue that earthquakes similar to the December 25, 1222 Brescia event are likely to occur in this region and pose a direct threat to such a densely populated and developed area.