Joint Inversion of Geodetic and Strong Motion Data for the 2012, Mw 6.1–6.0, May 20th and May 29th, Northern Italy Earthquakes: Source Models and Seismotectonic Interpretation

Abstract

Abstract We present the first rupture models of the two mainshocks of the 2012 northern Italy sequence, determined by jointly inverting seismic and geodetic data. We aim at providing new insights into the mainshocks for which contrasting seismotectonic interpretations are proposed in literature. Sources' geometric parameters were constrained by seismic reflection profiles, 3-D relocations and focal mechanisms of mainshocks/aftershocks. Site-specific velocity profiles were used to model accelerograms affected by strong propagation effects related to the Po basin. Our source models differ significantly from previous ones relying on either seismic or geodetic data. Their comparison against geological sections and aftershock distribution provides new insights about the ruptured thrust faults. The May 20th Mw6.1 mainshock activated the Middle Ferrara thrust-ramp dipping ∼45° SSW-wards, breaking a main eastern slip patch 4–15 km deep in Mesozoic carbonates (maximum slip 0.7–0.8 m) and Paleozoic-Triassic basement rocks, and a small western patch in the basement. The May 29th Mw6.0 mainshock featured two separated asperities along the Mirandola thrustramp dipping ∼42° S-wards: an eastern asperity 4–15 km deep in Mesozoic carbonates and basement rocks (maximum slip 0.7 m) and a deeper western one (7–16 km depth) mainly in the basement (slip peak 0.8 m). On-fault aftershocks were concentrated within the basement and Mesozoic carbonates, devoiding highslip zones. Slip and aftershock distribution was controlled by the rheological transition between Mesozoic carbonates and Cenozoic sediments. Unlike previous thin-skinned tectonic interpretations, our results point to a complex rupture process along moderately dipping (40°–45°) thrust-ramps deeply rooted into the Paleozoic crystalline basement. Plain Language Summary The two M6 mainshocks of the 2012 Italy sequence are the strongest earthquakes ever observed in the Po Plain, a strategic region for the Italian economy. The mainshocks ruptured blind thrust-faults, however their source models and seismotectonic interpretation are still debated because the thrust-system architecture is controversial. Contrasting thick-skinned and thin-skinned tectonic models are proposed. In thick-skinned interpretations, shortening is accommodated by thrust-ramps rooted into the crystalline basement that represent main seismogenic structures, whereas in thin-skinned interpretations, shortening and seismicity are controlled by listric faults splaying out from dècollement levels in the sedimentary crust. A comprehensive analysis of the mainshocks' source represents an opportunity to provide new insights into the seismogenesis in northern Italy and on a broader scale into seismotectonics of thrust-and-fold belts. We get a complete picture of the mainshocks kinematics by jointly inverting, for the first time, seismic and geodetic data, and unravel rupture heterogeneities not resolved by previous studies. By integrating source models with aftershock locations and geological models, we propose a comprehensive seismotectonic interpretation of the sequence. We conclusively identify the ruptured faults that correspond to thrust-ramps rooted into the crystalline basement and evidence the key role played by lithological changes in the rupture process.