Assessing the seismic activity at the front of the French Alps since the Late Glacial period through lacustrine sediments multi-proxy analyses.

Abstract:

Seismic hazard calculations are based on the assumption that seismicity rates are stable over time. In order to discuss this hypothesis regionally, it is essential to reconstruct long term seismicity.

The junction between the Jura mountains and the Alps is seismically active. Since the 15^th century, more than twenty earthquakes of epicentral intensity greater than VII and several active faults have been identified. In addition, lacustrine sedimentary sequences from Lake Annecy and Lake du Bourget have highlighted the capacity to record recurrent seismic activity. Such 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.

Two sedimentary sequences were sampled in the shallowest and deepest basins of Lake Aiguebelette. We performed multi-proxy sedimentological, geochemical and paleomagnetic analyses combined with seismic profile analyses and radiocarbon dating to study processes of event layer deposition in this lake.The varved facies of the continuous sedimentation allows the assessment of a precise depositional calendar during the Holocene.

In the deepest basin sequence, 33 homogenites were identified through magnetized sediment low field variations (≤0.4 µT), (equivalent to low Isothermal Remanent Magnetization, Demory et al. 2019) and high foliation (>2%) of the Anisotropy of Magnetic Susceptibility. These parameters are usually associated with seiche waves in the water column, induced by seismic activity. Four event layers were recorded in the shallow basin and three of them are synchronous with homogenites depositional times in the deepest basin (3000 ± 100, 6900 ± 100 and 11400 ± 300 cal. BP).

The oldest synchronous event layer is a 1.15-meter-thick deposit with an upward-graded base and an 84 cm-thick homogenite, also identified as 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 which occured at the same period of time: the Late Glacial-Holocene boundary, raising the question of the influence of this large climatic transition on the regional seismic activity in the Alps.