Shaking Engadine: Prehistoric insights into Lake Silvaplana, SE Switzerland
Presented by Benjamin BELLWALD on 23 Sep 2013 from 17:15 to 19:15
Type: Poster presentation
Session: Poster session
Track: Poster presentations
Lakes in perialpine realms, commonly, comprise high-resolution sedimentary archives, which record climatic, environmental, tectonic and anthropogenic changes and events. In this study, we focus on Lake Silvaplana, using 3.5 kHz single-channel pinger profiles and a 10.5 m long sediment core, in order to improve our knowledge on tectonic and sedimentologic processes and their interactions. Lake Silvaplana is located at 1791 m.a.s.l., it has a surface area of 2.7 km2 (width×length ~1×2 km), a maximum water depth of 78 m, and it is surrounded by up to 4049 m high mountains. Lake Silvaplana is, furthermore, influenced by a prominent fault system, the Engadine Line, along which prehistoric and historic earthquakes have occurred. Generally, the seismic profiles used in this study show a good penetration (up to 90 m below the subsurface) and the acoustic basement (bedrock or moraines) could be identified throughout the lake. The sediment core was taken from the deepest part of the lake using an Uwitec percussion piston-coring system covering the last ~3000 years. The core was analyzed for density, magnetic susceptibility, grain-size, as well as lithology and allowed a precise core-to-seismic correlation. A core chronostratigraphy was established using a combination of 14C AMS dating, gammaspectroscopic measurements of 210Pb and 137Cs radionuclides and inferred sedimentation rates. The seismic data show that the sediment archive above the acoustic basement compromises three different seismic facies: acoustically stratified (Seismic Facies 1, SF1), transparent (SF2) and chaotic (SF3). Laminated draping background deposits (ρ: 1.4-1.5 g/cm3, clay-silt; SF1) are found all over the lake. They are interrupted by mass movement deposits, which were separated into two facies: i) Chaotic wedging bodies (ρ: 1.4-1.5 g/cm3, clay-silt; mass flow, SF3), showing deformed and erosive character, are deposited at the toe of the slopes and ii) transparent onlapping masses, characterized by normal grading (ρ: 1.4-1.8 g/cm3, clay-sand; megaturbidite, SF2), having the deepest part of the lake as sedimentary environment, overlying SF3. All these sediments are filling the lake basin defined by the acoustic basement (SF4). In total ten horizons with synchronously initiated mass-movement deposits have been recognized in Lake Silvaplana. Four of these subaquatic mass movements were also recorded in the core and dated to 1800, 1720, 820 and 600 AD. The ages of the other six events were estimated, using a constant sedimentation rate, to 1200, 1400, 2900, 4800, 7400, 8000 BC. The inferred ages of the mass failures have been compared with the existing historic and prehistoric earthquake catalogues from the region showing that seismic shaking is the most likely triggering mechanism for the latest four events. The most prominent multiple-slide horizon, dated to an age of 600 ± 50 yr AD, was correlated with lacustrine mass movement deposits of other paleoseismic archives (Lake Como and Lake Sils). The strong regional correlation of this event is a further criterion for seismically-induced slope failure. Eventually, this study allows expanding the earthquake catalogue of the Eastern Swiss Alps and the neighboring northern Italian area, providing novel insights for the regional seismic hazard assessment.
Location: GEOMAR East shore
Address: Wischhofstr. 1-3 / D-24148 Kiel