Did the Late Glacial to Holocene climatic transition trigger large earthquakes in the Western Alps?

International audience<p>Seismic hazard calculations are based on the assumption that seismicity rates are stable over time. In a given area, the seismicity recorded through historical archives and seismometers is considered a reliable indicator to model the occurrence of future high magnitude seismic events. But, to discuss this hypothesis regionally, it is essential to reconstruct long term seismicity.</p><p>The junction between the Jura mountains and the Alps is seismically active, as shown by the occurrence of numerous seismic events and the presence of several active faults (De La Taille, 2015). Since the 15<sup>th </sup>century, more than twenty earthquakes of epicentral intensity greater than VII have been identified in this area. In addition, sedimentary sequences from Lake Annecy and Lake du Bourget have highlighted the capacity of these "natural archives" to record recurrent seismic activity (Beck 2009), with a potential major seismic event identified around 9900 cal. BP (Arnaud et al., 2012). Such lacustrine archives are key to better understand 1) the occurrence of major seismic events and 2) the evolution of seismicity rates through time, prior to historical and instrumental records.</p><p>Here, we present two sedimentary sequences of 11 and 16 metres long respectively, sampled in the shallowest and deepest basins of Lake Aiguebelette (altitude: 374 m). We performed sedimentological, geochemical and paleomagnetic analyses combined with seismic profile analyses and radiocarbon dating to study processes of event layer deposition in this lake. Multi-proxy analyses allow a quantitative identification of event layers, contrasting with varved-sedimentation. In the deepest basin sequence, 33 homogenites are identified through variations of the laboratory induced isothermal remanent magnetization of sediments measured with a high-resolution fluxgate scanner (Demory et al., 2019) and high foliation (>2%) of the Anisotropy of Magnetic Susceptibility. These parameters are usually associated with seiche effect induced by seismic activity (Campos et al., 2013). Among these event layers, archived in the deep basin sequence, three of them occured synchronously in the shallow basin (at 3000 ± 100, 6900 ± 100 and 11400 ± 300 cal. BP, respectively).</p><p>The oldest and thickest event layer recorded in Lake Aiguebelette was deposited at the transition between the Late Glacial and Holocene stages. In the deepest basin, this 1.15-meter-thick deposit is composed of an upward-graded base and a 0.84 meter-thick homogenite, which was also identified as a transparent facies on seismic profiles. In Lake Le Bourget, Lake Annecy, and central Swiss perialpine lakes, several seismic profiles analyses show transparent seismic facies interpreted as mass movement deposits occurring at the same period of time: the Late glacial-Holocene transition.</p><p>Did this climatic transition influence the seismic activity in the Alps? If so, the impact of such climatic forcing on seismic hazard assessment should be evaluated.</p><p> </p><p>Arnaud et al (2012). Lake Bourget regional erosion patterns... QSR., 51, 81-92.</p><p>Beck (2009). Late Quaternary lacustrine paleo-seismic... EarthSciRev., 96(4), 327-344.</p><p>Campos et al (2013). Deciphering hemipelagites from homogenites... SedGeol., 292, 1-14.</p><p>De La Taille et al (2015). Impact of active faulting... Tectonophysics, 664, 31-49.</p><p>Demory et al (2019). A new high‐resolution magnetic scanner... Geochem,Geophy,Geosys., 20(7), 3186-3200.</p><p> </p><p><strong>Keywords: </strong>Lake sediment, homogenites, paleo-earthquakes, seismic hazard, French Alps</p><p> </p><p> </p&gt