Geophysical Research Abstracts Vol. 12, EGU2010-12912, 2010 EGU General Assembly 2010 © Author(s) 2010 Latest Pleistocene to Holocene thrust faulting paleoearthquakes at Monte Netto (Brescia, Italy): lessons learned from the Middle Ages seismic events in the Po Plain Alessandro Maria Michetti (1), Andrea Berlusconi (1), Franz Livio (1), Giancanio Sileo (1), and Andrea Zerboni (2) (1) Università dell’Insubria, Dipartimento di Scienze Chimiche e Ambientali, Como, Italy (alessandro.michetti@uninsubria.it, +0312396630), (2) Università degli Studi di Milano, Dipartimento di Scienze della Terra, Via Mangiagalli, Milano The seismicity of the Po Plain in Northern Italy is characterized by two strong Middle Ages earthquakes, the 1117, I˚ X MCS Verona, and the December 25, 1222, I˚ IX-X Brescia, events (Magri & Molin, 1986; Guidoboni, 2000; Galli 2005; Stucchi et al., 2008). Historical reports from these events describe relevant coseismic environmental effects, such as drainage changes, ground rupture and landslides. Due to the difficult interpretation of intensity data from such old seismic events, considerable uncertainty exists about their source parameters, and therefore about their causative tectonic structures. In a recent review, Stucchi et al. (2008) concluded that “the historical data do not significantly help to constrain the assessment of the seismogenic potential of the area, which remains one of the most unknown, although potentially dangerous, seismic areas of the Italian region”. This issue needs therefore to be addressed by using the archaeological and geological evidence of past earthquakes, that is, archeoseismology and paleoseismology. Earthquake damage to archaeological sites in the study area have been described by Galli (2005) and Stucchi et al. (2008). Here we focus on new paleoseismological evidence, and in particular on the first observation of Holocene paleoseismic surface faulting in the Po Plain identified at the Monte Netto site (Livio et al., 2009), located ca. 10 km S of Brescia, in the area where the highest damage from the Christmas 1222 earthquake have been recorded (Guidoboni & Comastri, 2005). Monte Netto is a small hill, ca. 30 m higher than the surrounding piedmont plain, which represent the top of a growing fault-related fold belonging to the Quaternary frontal sector of the Southern Alps; the causative deep structure is a N-verging back thrust, well imaged in the industrial seismic reflection profiles kindly made available by ENI E&P. New trenching investigations have been conducted at the Cava Danesi of Monte Netto in October 2009, focused on the 1:10 scale analysis of the upper part of the 7 m high mid-Pleistocene to Holocene stratigraphic section exposed along the quarry walls. In particular, we excavated a 3 m deep trench across the graben that affects the crest of a decametric anticline due to paleoseismic bending-moment faulting; evidence of paleoseismicity is also provided by the observation of paleoliquefaction features near the graben. The trench walls allowed to identify 3 discrete events of graben reactivation. Based on available OSL and AMS dating of stratigraphic units sampled on the main quarry walls, these 3 paleoearthquakes occurred between ca. 45 kyr B.P. and ca. 5 kyr B.P.; new dating is in progress from samples collected in the October 2009 trench. The post 5 kyr B.P. stratigraphy is missing at the trench site, due to plowing and reworking. The paleoseismic deformation observed at Monte Netto is suitable to constrain the magnitude of the causative earthquakes. At a local scale, the November 24, 2004, Ml 5.2 Salò, earthquake, occurred ca. 40 km NE of Monte Netto, produced rockfalls and fractures in the Lake Garda shores, but no fault displacement at the surface (Michetti et al., 2005). The only well known surface faulting earthquakes in the Southern Alps is the May 6, 1976, Ms 6.5 Friuli event; secondary surface ruptures have been observed for a length of few km and with maximum offset of few decimeters (Bosi et al., 1976). Using the global database of surface faulting events (e.g., Wells and Coppersmith, 1994; Yeats et al., 1996), the smallest thrust faulting earthquake known to be associated with secondary tectonic ruptures at the ground surface is the October 29, 1989, Mw 6.0 Mount Chenoua, Algeria, event. On the other hand, stronger shallow crustal compressional earthquakes such as the 1980 El Asnam, 1988 Spitak, and 1999 Taiwan events are invariably associated with tens of km of primary tectonic scarps, with maximum surface displacement