Enhanced seismicity in the early post-glacial period: Evidence from the post-Wurm sediments of Lake Annecy, northwestern Alps

The sediments of the two largest post-Riss and post-Wurm lakes, in the northwestern Alps and southern Jura, contain possible records of seismo-tectonic instability: earthquake triggering of gravity reworking, and direct stratification disturbance. In Lake Annecy, the lacustrine s.s. (non-proglacial) part of a 40 meters-thick rhythmic sediment pile has been investigated for this purpose by CLIMASILAC coring. For the last 15,000 to 20,000 years, 50 sedimentary ''events'' have been detected, with the most frequent time-recurrence intervals from 1 to 5 centuries. These events appear to have been concentrated during the deglaciation period. Thus, different factors could be responsible for the subsequent decrease: change of sediment composition and properties, modification of slope instability related to the latter and to modification in the catchment area, and decrease of regional seismic activity. Following the third possibility, the period of high reworking frequency, coeval with the main glacial melting, should be related to a glacio-isostatic rebound. On the other hand, the scarce ''events'' registered for the Holocene could represent the record of a longer term (tectonic) seismicity. Copyright (C) 1996 Elsevier Science