Modeling earthquake-induced seiche processes and subsequent homogenite deposition in lacustrine settings

Earthquakes leave distinct signatures on lake systems, including event deposits that serve as valuable paleoseismological archives. Among these deposits, homogenite layers are commonly associated with lake oscillations, i.e., seiches. Here, we investigate the seiche mechanism and the formation of a homogenite related sediment deposit within a lacustrine environment. This study focuses on the 1822 CE earthquake in the Western European Alps, which triggered subaqueous landslides in Lake Bourget (France). This event caused oscillations in the lake's water, which subsequently resulted in the formation of a homogenite layer in the deep basin. The underlying mechanism is resolved by presenting the first comprehensive numerical model via coupling of coseismic displacement, seismic wave propagation, and mass movement with the tsunami model. The numerical simulations show excellent agreement with the available geological and historical observations. The water disturbances caused by subaqueous landslides generated small tsunami waves with a maximum runup height of approximately 2.5 m. By analyzing the tsunami signals using Fourier spectral analysis and fast iterative filtering, we determined that seismic waves are the primary drivers of seiche, which excite the natural modes of Lake Bourget. Our numerical results confirm that the sediments found in the deep basin originated from one main subaqueous landslide and from tsunami erosion of littoral sands (backwash). However, the seismically induced seiche was solely responsible for keeping the fine-grained sediment cloud in suspension for several days and led to the formation of the homogenite layer (or seiche deposit) with typical grain orientation characteristics. The proposed numerical framework could also be effective in identifying whether landslides or delta collapses (linked to homogenite/megaturbidites) in closed lakes were triggered by seismic or nonseismic sources. This distinction is crucial for reconstructing the history of past earthquakes and associated hazards.