Limnological seismites as a record of past earthquakes in the Dead Sea Rift

Limnological seismites are sedimentary layers within lacustrine deposits that have been deformed by earthquake-induced shaking. They are critical indicators of paleoseismic activity, providing a record of past earthquakes. The longest continuous earthquake record on Earth, 220-kyr-long, recovered from the 460-m-long drill core at the depocenter of the Dead Sea, is interpreted based on recognition of such seismites. The identification of seismites in the Dead Sea deep drill core relies on comparison with detailed structural analyses of seismites exposed along extensive outcrops around the Dead Sea. Many of these seismites are juxtaposed as well with syn-depositional faults, and those that were formed in historical times are tightly correlated with historical records and damaged archaeological sites. The key to characterizing earthquakes is understanding the physics of the seismite's deformation mechanisms. Here we focus on one of the robust mechanisms by which shear instability, of the type of Kelvin-Helmholtz instability, deforms stably stratified fluids. We performed state of the art direct numerical simulations of viscoplastic rheology, with nonlinear relations between the stress and the deformation rate tensors, to study the deformation response of the lacustrine sediments to earthquake-imposed shear. First, we measured the viscoplastic rheological characteristics of samples of Dead Sea sediments (aragonite, detritus and their mixture), by using rotary shear. We found substantial expected differences between the ductile detritus and the brittle aragonite. These data were implemented in the numerical simulations to solve the full Navier-Stokes fluid equation. The comparison between the simulations and the in-situ exposed deformations allows us to constrain the intensity and the peak ground acceleration of their generating paleo-earthquakes.