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First attempt to model numerically seismically-induced soft-sediment deformation structures : a comparison with field examples

Bronikowska Małgorzata, Pisarska-Jamroży Małgorzata, Loon van Tom

Geological Quarterly

2021

Vol. 65, No. 4

s. 216--225

No numerical model has thus far addressed seismites, even though seismites are frequently used for the conditions which have to be fulfilled for the development of seismites have also been estimated only empirically. The present contribution is a first attempt to model numerically the soft-sediment deformation structures caused by the passage of S-waves through near-surface sedimentary layers. The simulations are based on the so-reconstruction of seismic events in the geological past. This is the more remarkable since the boundary called pressure tube model and the iSALE2D program. We modelled a seismic S-wave with six different vertical velocities, ranging from 1.6 to 2.6 m · s-1, passing through sediments with different densities and porosities in a sedimentary succession from the surface down to a depth of 10 m. The modelled soft-sediment deformation structures (load casts, flame structures, injection structures and sedimentary volcanoes) show similar geometries and sizes as those known from laboratory experiments and field studies. The geometry, size and type of these structures depend on the sediment properties and on the initial pressure used as a trigger mechanism, rather than on S-wave velocity. In contrast, the depth of the seismites appears to depend strongly on the S-wave velocity.

Sedimentological distinction in glacigenic sediments between load casts induced by periglacial processes from those induced by seismic shocks

Van Loon Antonius Johannes, Pisarska-Jamroży Małgorzata, Woronko Barbara

Geological Quarterly

2020

Vol. 64, No. 3

626--640

Loading processes and the resulting load structures induced by processes related to periglacial conditions are compared to those induced by seismic shocks. The load structures themselves are relatively easily recognizable but the responsible trigger mechanism is, though depending on the geological context, commonly difficult to establish. Load structures like load casts, pseudonodules, ball-and-pillow structures and flame structures are commonly ascribed to instable density gradients within sediments and to differential loading, but their formation always requires liquefaction. In glacigenic sediments, deformation structures have most commonly been ascribed to periglacial processes (as a type of cryoturbations), but it becomes ever more clear that glacigenic sediments can, particularly during ice-front fluctuations, be affected by faulting-related earthquakes (due to glacio-isostatic adjustment), and the thus triggered seismic shocks may result in deformations, including - most commonly - load structures. We inventory the evidence that may help to distinguish, on the basis of textural and structural features, load structures with a seismic origin from those that result from periglacial processes, taking into account that truly diagnostic criteria do not exist.