The Tsunami Risk in Lake Como (Italy)
by
Daniela Fanetti
Dipartimento di Scienze Chimiche e Ambientali, Università dell'Insubria, Via Vallegio 11, 22100, Como, Italia
Coauthors: Vezzoli Luigina

Lake Como is the deepest lake of the Alps (-425 m below lake level) and with its surface area of 142 km2 the third largest along the southern alpine margin lakes. Its drainage basin is in a central position within the Alpine chain and corresponds to one of the major morphological pathways between northern and southern Europe.

Pursuant to new limno-geological investigations, that combine a bathymetric survey (Multibeam Simrad 3000) with a high-resolution seismic reflection study (single-channel 3.5 kHz) and a coring campaign (gravity short-corer), the sedimentary subsurface of the L ake Como western branch is characterized to the average sediment depth of 50 m. The recorded data show that the western branch of the lake can be subdivided in several sub-basins, that have different characteristics from the morphological, physical and sedimentological point of view. In particular, two areas of great interest are a) the Tremezzo-Bellagio plateau area at the northern end of the Como branch and b) the Argegno deep basin.

The plateau area, located at the northern tip of the Como branch, has a water depth of about 140 m and separates the Como branch from the rest of Lake Como. The high-resolution seismic analyses of this area illustrate that the base of the sedimentary cover is composed of glacial deposits. The morphology of these glacial deposits influenced the subsequent lacustrine sedimentation. The depressions between the glacial deposits were first infilled by onlapping pelagic and laminated sediments locally intercalated with confined mass-flow deposits. In the following phase, pelagic well-layered sediments composed of acoustically finely laminated mud with rare sandy layers draped the glacial deposits. They are also interbedded with sub-acqueous mass-flow deposits.

The interpretation of the bathymetric data highlights the presence of two major scarps along the south-western slope of the plateau. That scarps are clearly mappable also on the seismic data, where reflection truncation testifies the scarps recent activity. Besides, at the base-of-slope, remoulded sediments occur, which show that the south-western slope of the plateau is quite unstable.

The Argegno sub-basin is characterized by an over 10 km long area with water depths of over 400 m, comprising the lake's deepest point with a water depth of 425 m. This sub-basin shows different depocenters, which are filled with onlapping, acoustically well-layered pelagic-type sediments and intercalated turbidite deposits. In particular, two mega-turbidite deposits were detected on the high-resolution seismic lines, each with a considerable volume in the order of 10*106 m3. The estimated ages of these events are extrapolated from isotopic analyses (137Cs, 14C) and sedimentation rates obtained from the short cores. According to the distributions and the thicknesses of these two mega-turbidite deposits, a source area located at the northern tip of the Como branch can be proposed. Based on the morphological and sedimentological characteristics of the area, we suggest that these mega-turbidites resulted from the development of sub-lacustrine landslides along the south-western slope of the plateau area. Possible trigger mechanisms leading to these catastrophic events include overloading of slope deposits, major flood events, significant lake-level change and earthquake shaking.

These megaturbidite events registered in the Lake Como have an extraordinary volume and thickness for the lacustrine environment and represent a major natural hazard. Underwater mass movements in fact, can provoke tsunami and seiches, water oscillations that can cause elevated damage along the lake shore. In the case of Lake Como, the mass movements registered in the sedimentary sequence had their source at the northern limit of the western branch (plateau area). The movement of the flows were longitudinal along the branch and the involved sediments filled the topographic pre-existing depression in the southern part. The mass movements probably had also a consequence on the water column (generation of surface waves). In the central part of the branch, where the water column is of about 400 m, the water movements may have been of less relevance compared to what happened towards the end of the branch near the city of Como, where the water depth is significantly lower (few dozen of meters as average water depth) and where tsunami or seiche waves are likely to form higher amplitudes. In addition,the gently inclinded subaquatic slopes of the Como area, that favour higher wave runup, plus the gentle coastal morphology of the valley plain towards the south both increase the potential hazard since a wave may inundate a large area of relatively low elevation.

Date received: July 15, 2005

Copyright © 2005 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caqy-39.