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.