An inventory of river anomalies in the Po Plain, Northern Italy: evidence for active blind thrust faulting

The Po Plain is a low-relief area characterised by active shortening accommodated by blind thrust faulting. In
this almost flat region depositional rates are similar to tectonic rates and deformation is seldom expressed by
noticeable surface anticlines. We adopted a geomorphological approach based on the detailed analysis of the
drainage network to identify the location of active thrust faults. A total of 36 anomalies represented by sudden
river diversions and shifts in channel pattern were accurately mapped. After comparison with the location of subsurface
buried anticlines and of historical seismicity, these anomalies could be related to a tectonic origin and
included in a database. Their distribution highlights the activity of the buried outer thrust fronts of both the
Southern Alps and the Northern Apennines. Among all the anomalies, we identified one related to the seismogenic
source responsible for the 12 May 1802 earthquake (Me 5.7), which struck the Oglio River Valley near
Soncino (Cremona). We propose that this earthquake was generated by an east-west trending, north-dipping,
blind thrust fault that roots into the Alpine system. If this inference is correct, other faults along the Southern
Alpine margin are potentially seismogenic.