A Holocene alpine seismic chronicle from Lake Aiguebelette (NW French Alps)

Abstract. Lake sediments are valuable archives and can help construct a chronology of event deposits induced by seismic events. Such a chronology can be used to better understand the recurrence times between seismic events over longer periods than those covered by historical seismicity catalogs. However, only a few studies in lake palaeoseismology have focused on areas with moderate seismicity. This study aims to improve the catalog of paleoseismological archives in the front of the Western Alps. In this part, new multi-proxy data from the sedimentary archives of Lake Aiguebelette (France) allow the identification of 32 homogenites (thickness ≥ 0.5 cm) interpreted as of coseismic origin over the Holocene. An age model based on short-lived radionuclides, paleomagnetic data and radiocarbon ages constrains the chronology of sedimentary deposits in the deep basin of Lake Aiguebelette. Among these homogenites, several were deposited at time intervals compatible with historical seismic events. To correlate the historical seismic events likely to have generated the event deposits identified in the sedimentary sequences of the deep basin of Lake Aiguebelette, the Earthquake Sensitivity Threshold Index (ESTI) method is used. Historical seismicity catalogs with uncertainties about intensities and epicenter coordinates for earthquakes make correlations to event deposits difficult. To better understand which seismic events may have been archived, a relative comparison was conducted between the pseudospectral acceleration (PSA) values calculated for each event in the FCAT-17 seismic catalog and for two distinct frequencies. Based on this PSA approach, for higher frequencies (5 Hz), the contribution of nearby and moderate events is significantly stronger than that of strong and distant events in the lake sequence of Aiguebelette. Thus, the chronicle established based on the event deposits archived in Lake Aiguebelette sediment is interpreted as representative of local events (epicentral distance to the lake < 50 km). Recurrence intervals between the deposition of event layers do not follow a specific distribution (log-normal, Weibull, gamma or exponential) but might be a combination of several distributions. This suggests possible coexistence of several processes over the Holocene, impacting the evolution of the seismicity in this area.