Paléosismologie Et Géologie Des Lacs

Géochronique 117, march 2011 1 Paléosismologie et géologie des lacs Flavio S. Anselmetti, Michael Hilbe, Katrin Monecke, Michael Schnellmann Michael Strasser Sediments of alpine lakes as archives of the environment Central Switzerland lies tectonically in an intraplate area. Recurrence rates of strong earthquakes exceed the time span covered by historic chronicles. Consequently, they are not sufficient to document the full range of neotectonic processes and do not allow to assess intensities and recurrence rates of strong earthquakes. However, many lakes are present in the area that act as natural seismographs: their continuous, datable and high-resolution sediment succession allows to extend the earthquake catalogue from instrumental and historic periods to prehistoric times all the way to the end of the last glaciation (i.e. back to ~16'000 years BP), when the modern lakes formed after glacier retreat. These lakes are sensitive archives of various environmental parameters. Their sediments record changes and events, such as climatic, environmental, tectonic and anthropogenic processes, many of them related to natural hazards. Seismic shaking can be recorded in the lake sediments if the local ground acceleration exceeds certain thresholds. For instance, earthquakes can trigger subaquatic slope instabilities (Fig. 1) if intensities (European Macroseismic Scale; EMS-98) larger than ~VII occur (Monecke et al., 2004, Schnellmann et al, 2002, 2005). These mass movements may be tsunamigenic, causing an additional hazard next to the earthquake shaking itself. Geotechnical in situ and laboratory experiments can further provide input data for slope- stability back-analysis to quantify the required ground acceleration to trigger these slope instabilities (Strasser et al., 2007; Stegmann et al., 2007; Strasser et al., in press). Furthermore, earthquakes may cause in situ microdeformations detectable in layered sediments (Fig. 2) above an intensity of VI-VII (Monecke et al., 2004, 2006). As no active fault has been recognized so far in Central Switzerland, the Central Swiss lakes act mainly as passive recorders of seismic shaking so that, analogue to maps of historic reconstructions, maps of paleointensities of individual prehistoric events may be generated. Example of historic calibration: the 1601 Unterwalden earthquake Instrumentally recorded or historic earthquakes offer the opportunity to calibrate the response of the lacustrine sedimentary system to earthquake shaking of known or measured intensity. For Central Switzerland, the M~6.2 (Imax = IX) Unterwalden earthquake of 1601 (Schwarz- Zanetti et al., 2003), the second largest reported earthquake north of the Alps, is used to test the sensitivity of lacustrine sediments to seismic shaking (Monecke et al., 2004, 2006). This earthquake triggered numerous simultaneous subaquatic slides in various lakes that are for instance still evident with sharp slide scars and runout lobes in the bottom morphology of Lake Lucerne (Fig. 1; Hilbe et al. subm.). It also caused microdeformations in laminated sediments (Fig. 2; Monecke et al., 2004) and furthermore triggered subaerial rockslides, some of which entered the lakes. The various forms of subaquatic mass movements in Lake Lucerne induced over 4 m high tsunami waves that were further enhanced by impact waves from the subaerial rockslides (Siegenthaler et al., 1987; Schnellmann et al., 2002). On reflection seismic data, the subaquatic mass movements triggered by this event can be recognized by their transparent-to-chaotic seismic facies contrasting sharply to the well- layered background sediments. They occur on the same stratigraphic level that can be traced throughout the basins (Fig. 3). Historic reconstructions place the epicenter of this M~6.2 Géochronique 117, march 2011 2 earthquake just south of Lake Lucerne, bringing a series of other lakes in the area with I > VII that is required to cause lake sediment deformation (Monecke et al., 2004). The paleoseismic event catalogue for Central Switzerland Features similar to those caused by the Unterwalden earthquake of 1601 can be found throughout the subsurface of the various Central Swiss lakes. In a regional compilation, all paleoseismic information from various lakes is assembled in an earthquake catalogue (Fig. 4). Traces of three larger historic and at least six major earthquakes during the last 16,000 years were found in the sedimentary record of four lakes in Central Switzerland (Monecke et al., 2006, Schnellmann et al., 2006). All this evidence is shown on the same time scale, in order to evaluate the robustness of individual events. Multiple synchronous subaquatic sliding, as confirmed by seismic stratigraphic analysis of the basin fill, is a strong hint to identify a seismic event. In the case of Lake Lucerne, five such events similar in ground acceleration to the 1601 event could be identified in the last 16'000 years with variable intervals (Schnellmann et al., 2002, 2006), many of them also documented by mass movements and microdeformations in other nearby lakes. Surprisingly, three of these earthquakes were also registered in the sediments of Lake Zurich (red-labeled ages in Fig. 4; Strasser et al., 2006, 2008), documenting an overregional impact of these major events. Implications for Alpine neotectonics Next to identifying prehistoric events, the geotechnical considerations and regional paleointensity maps allow to quantify potential earthquake magnitudes as a function of epicentral locations and, in the case of Central Switzerland, to evaluate the implications for alpine neotectonics. The three prehistoric earthquakes that triggered simultaneous multiple subaqueous mass movements in Lake Zurich and Lucerne, ~50 km apart, at 2200 +/- 55, 11,530 +/- 185, and 13,840 +/- 145 cal. yr B.P., were strong enough to cause I=VII in both lakes. The required magnitudes can be calculated over a grid of trial source locations (Bakun and Wentworth, 1997) using an empirical intensity attenuation relation (Fah et al., 2003), a function of epicentral distance and hypocentral depth. The result of this grid-search approach (Fig. 5) document a minimum required magnitude of M=6.5 if all three events were centrally located between the two lakes, whereas values approaching and exceeding M=7 are required if the events were located farther away (Fig. 5). These values also indicate that events occurred, which must have reached magnitudes exceeding the range known from historic records, documenting the usefulness of lake sediments to reconstruct the so far unknown earthquake history of an area. Further considerations provide clues on location of potential epicenters: including negative evidence from Lake Baldegg in the western part of the investigated area, where no lake sediment deformation at these times was documented (Fig. 4), the possible epicentral location may be located further to the east. The magnitude of M=7 for such epicentral locations (Fig. 5) would require rupture lengths of several tens of km, narrowing the candidates for potential active faults. Such earthquakes could, as most likely candidate, be produced by release of accumulated NW-SE compressional stress related to an active basal thrust beneath the Aar massif, along which the central Aar massif is overthrusted onto the autochthonous foreland cover forming a growing fault-propagation fold (Strasser et al., 2006). In any case, this study indicates ongoing Alpine deformation that generated and could potentially generate destructive events of M=6.5-7 in heavily populated regions that have been unaccustomed to seismic activity during historic time. Gé ochronique 117, march 2011 3 Acknowledgments We are grateful to the support of countless helpers during field work and laboratory analysis. Robert Hofmann provided key support for retrieving the long sediment cores in various lakes. The projects were conducted with in cooperation with Arnfried Becker, Domenico Giardini and Judith McKenzie. The research was integrated in the Swiss Seismologic Service. Funding was provided by Swiss National Science Foundation and the Swiss Commission for the Safety of Nuclear Installations. Figure captions Fig. 1: Bathymetric map (shaded relief illuminated from east, colour indicating depth) of Vitznau Basin of Lake Lucerne, with interpretation of morphology (in black): Several meter high headwalls (broken lines) are present especially on the northern lateral slope. Exposed parts of the sliding surface with minor post-event sediment cover (greyed areas) are often found adjacent to the headwalls. Mass-movement deposits and deformed basin sediments are visible as lobes in low-gradient areas below headwalls (hatched). Prehistoric rockslide deposits are expressed by a blocky lake floor near the southern margin (open squares). Fig. 2: Deformation structures with folded and disrupted layers observed in sediment cores from Baldegger See related to the 1601 Unterwalden earthquake (after Monecke et al., 2004). Fig. 3: Seismic section showing repetitive mass-movement deposits (chaotic-to-transparent seismic facies) along seismic stratigraphic time markers indicating a common seismic trigger mechanism (after Schnellmann et al., 2006). Fig. 4: Compilation of paleoseismic data for Central Switzerland from different lakes for the last 16'000 years, modified after Monecke et al., 2006. Vertical bars in various reds indicate events that are interpreted as earthquake-triggered (ages in cal y BP, except for historic events labeled in y AD). Mass movements in Lake Zurich caused by manmade shore construction in the late 19th and 20th century are not shown. Fig. 5: Zones of earthquake epicenters (dotted and dashed lines for deep foreland and shallow northern Alpine source, respectively) of minimum given magnitude sufficient to produce observed landslides in Lake Zurich and Lake Lucerne, identified by grid-search approach. NAF is North Alpine nappe front (after Strasser et al., 2006). .

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