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Late Quaternary fault movements in the Mt. Baldo-Lessini Mts sector of the Southalpine area (northern Italy)
- Fabrizio Galadini, Paolo Galli, Augusto Cittadini, Biagio Giaccio
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- Journal:
- Netherlands Journal of Geosciences / Volume 80 / Issue 3-4 / December 2001
- Published online by Cambridge University Press:
- 01 April 2016, pp. 187-208
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Paleoseismological investigations have been performed at Mt. Baldo and in the Lessini Mts. in order to collect quantitative data on the activity of minor faults showing geomorphic evidence of recent activation. The 4.5-km-long, NNE-SSW trending Naole fault was responsible for the formation of a narrow depression at the top of Mt. Baldo, bordered by a continuous bedrock (carbonate) fault scarp to the west. The extensional activity along this minor fault is probably due to gravitational deformations (lateral spreading) in response to the warping of the Mt. Baldo anticline. A 1.5-km-long graben is instead related to the 2.5-km-long, NNW-SSE trending Orsara fault (Lessini Mts.) which was responsible for the formation of bedrock (carbonate) fault scarps. This minor fault is part of a complex structural framework made of few-km-long faults which show evidence of Quaternary activity. Two trenches have been excavated across the Naole fault which showed the occurrence of displacement events subsequent to 17435-16385 BP (cal. age) and probably prior to 5455-5385/5330-5295 BP (cal. age). Two other trenches have been excavated across the Orsara fault whose analysis indicated that the most recent displacement event occurred between 20630-19795 BP and 765-675 BP (cal. age). The upper chronological limits of the displacements give some indications about the minimum elapsed time since the last fault activation (about 5,300 years for the Naole fault and 5-8 centuries for the Orsara fault). Both 1) the maximum expected magnitude of the earthquakes which may originate along the Mt. Baldo thrust and 2) the identification of a main fault responsible for the displacements along the complex net of minor faults affecting the Lessini Mts. are still open questions. As for point 1 although historical earthquakes with magnitude 4.5-5 may be associated with the Mt. Baldo thrust, the investigations carried out in this area did not clarify whether larger magnitude earthquakes may be expected. As for point 2, the cause of the displacements along the Orsara (Lessini Mts.) fault may be related to the activity of a major blind fault (which, however, has never been identified), responsible for the uplift of the Lessini Mts. More generally, the obtained results demonstrate the limits of traditional paleoseismological analyses in Alpine areas whose erosional/depositional activity has been strongly conditioned by the Late Pleistocene glacial history. The lack of units younger than loess and colluvial sediments related to the Last Glacial Maximum makes it impossible to define narrower chronological constraints for the displacements and to estimate the number and size of the displacement events. Moreover, the rebound following the retreat of the thick glacial cover affecting the Alpine area may have induced stresses responsible for higher deformation rates after the Last Glacial Maximum. Higher surficial deformation rates could imply shorter recurrence intervals for faulting episodes and/or larger magnitude earthquakes. Therefore, paleoseismologically inferred data in Alpine areas may not correctly define the fault behaviour related to the present tectonic regime.
Major active faults in Italy: available surficial data
- Fabrizio Galadini, Carlo Meletti, Eutizio Vittori
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- Journal:
- Netherlands Journal of Geosciences / Volume 80 / Issue 3-4 / December 2001
- Published online by Cambridge University Press:
- 01 April 2016, pp. 273-296
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An inventory of the available surficial data on active faults in Italy has been compiled by gathering all the available information on peninsular Italy (project by CNR, National Group for the Defense against Earthquakes), the central-eastern Alps and the Po Plain (EC ‘PALEOSIS’ project). Such information has been summarised in maps (reporting surficial expressions of faults with length L≥11 km) and in a table where fault parameters relevant for seismic hazard assessment (e.g. slip rates, recurrence intervals for surface faulting events, etc..) have been reported. Based on the geological characteristics of the Italian territory, a fault has been considered as active if it shows evidence of Late Pleistocene-Holocene displacements. Active faults in Italy are distributed throughout the entire Apennine chain, in the Sicilian and Calabrian regions and in some Alpine sectors, but knowledge is not homogeneously distributed through the territory. The largest amount of data is related to the central Apennines. In contrast, fault geometries and parameters are less well defined in the southern Apennines, Sicily and Calabria, where investigations have started more recently. Knowledge is sparse in the northern Apeninnes, where data necessary to define fault parameters are lacking and also the chronology of the activity has to be considered cautiously. Abundant blind faulting in the Po Plain hinders the detection of active faults by means of the classical surficial investigations and therefore the present knowledge is limited to the Mantova fault. Blind faults and the peculiar recent geological history of the Alpine areas, which is strongly conditioned by the erosional and depositional activity during and after the last glacial maximum, also hinder the identification of active faults in the central-eastern Alps. Some faults in this Alpine sector are believed to be active, but data on their segmentation are still missing. Available information indicates that Italian active faults are usually characterised by slip rates lower than 1 mm/yr. Recurrence intervals for surface faulting events are longer than 1,000 years in the central and southern Apennines. This review on the Italian active faults represents the first step to produce a map of the major seismic sources in Italy, which in turn will result from the merge of surficial data with seismological and geological subsurficial data. The available knowledge gathered in this paper indicates those areas where data are presently sparse. It should be, therefore, possible to better plan future geomorphological and paleoseismological investigations.