Giancanio Sileo

Surface Faulting of the 6 April 2009 Mw 6.3 L’Aquila Earthquake in Central Italy

This paper documents evidence of surface faulting associated with the 6 April 2009 moderate-sized earthquake (ML 5.8, Mw 6.3) in the central Apennines of Italy, which caused major damage to the town of L’Aquila and its surroundings. Coseismic surface ruptures were mapped for a minimum distance of 2.6 km along the Paganica fault, a fault still poorly investigated relative to the other active faults nearby, which bound much wider range fronts. Surface rupture length (SRL) and maximum displacement parameters (2.6 km minimum and 10–15 cm, respectively) are in agreement with what is expected for an Mw 6.3 event in the Italian Apennines tectonic environment. Different viewpoints exist on the amount of SRL and the number of activated faults. We propose a pattern of sympathetic and secondary slip on an array of faults around the master seismogenic structure. Past seismicity and evidence for larger Holocene offsets on this and other capable faults nearby prove that the 2009 event is not a good reference event for assessing the seismic hazard of the region. Nevertheless, the 2009 L’Aquila earthquake once more confirmed the importance of detailed geological studies for a proper seismic hazard assessment, and it clearly illustrates the need to pay attention to moderate events and supposedly minor active faults. Indeed, this type of earthquake is rather frequent in the whole Mediterranean region and is potentially much more destructive than in the past, due to the expanding urban centers and infrastructures inside their epicentral regions and even right above the traces of capable faults.

Progressive offset and surface deformation along a seismogenic blind thrust in the Po Plain foredeep (Southern Alps, Northern Italy)

Here we present, for the first time in the Po Plain foredeep (Northern Italy), the middle to late Pleistocene growth history of an outcropping secondary fold and related faults, whose progressive deformation over an intermediate time window (105 years) is driven by an underlying seismogenic blind thrust. We trenched and logged an outcropping decametric secondary anticline, related to a deeper blind compressional structure, which deforms fluvial sediments and an overlying loess-paleosol sequence. Folded units were dated, using radiocarbon and optically stimulated luminescence methods, to the late Pleistocene–Holocene and tentatively correlated with glacial-interglacial phases occurring during the time interval from marine isotope stage 6 to the present. A multistep retrodeformation of the fold allowed us to calculate uplift rates for this secondary and shallow anticline, varying between 0.02 and 0.1 mm/yr since circa 200 kyr. Trishear forward deformation modeling of the fold indicates that the amplification of the observed fold could be caused by two shallow thrusts formed through a break-backward activation. This generated a decametric surface fold whose most recent growth was associated with bending-moment normal faulting in the crestal and forelimb region. Our observations demonstrate that near-surface compressive tectonics can be caused by blind thrusting, via a complex array of fault and folds: upward strain propagation and generation of shallow low-angle thrust and related folding seem to be mainly due to secondary fold-related faulting, according to an out-of-syncline thrusting mechanism.

Biogeochemical characterization of automicrites building the Cipit Boulders of the Ladinian – Carnian platforms in the Dolomites (northeastern Italy)

We summarize some preliminary results achieved during the investigations conducted for the CARG Project, geological sheet n. 75 Como, i.e. the analysis of the Quaternary evolution of the Monte Olimpino Backthrust and the evaluation of its seismogenic activity. Cross-border field mapping between Ticino (CH) and Lombardia (IT) resulted in the finding of new outcrops (Borgo Vico site, in the north-western sector of the urban area of Como) located along the front of the Monte Olimpino Backthrust, that allowed to recognize evidence for Late Pleistocene reverse surface faulting along this structure.At Borgo Vico, a clastic Tertiary unit, the Villa Olmo Conglomerate, intercalated in the Chiasso Fm. of Early Oligocene age, is thrust over a Late Pleistocene fluvioglacial and glacio-lacustrine sequence (Comerci et alii, 2007).Until now, the Monte Olimpino Backthrust was supposed by most authors to have been active until Tortonian times. Sileo et alii (2007) inferred a Pliocene activity and proposed, based on geomorphic evidence, that fault displacement was still taking place during Pleistocene. However, this is the first time that Pleistocene activity along the Monte Olimpino Backthrust has been documented by unequivocal tectonic offset of late Pleistocene deposits. Paleoseismological analyses are in progress in order to distinguish potential coseismic movement from fault creep during the observed recent displacement.