Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

308 PATA Days 2024 Figure 1. Examples of soft sediment deformation from outcrops within the Dead Sea Basin A: A typical exposure of asymmetric folds and thrusts in the lake deposits of the Lisan Formati deformation in different layers, the lower one is more enriched by aragonite whereas the upper B: Linear folds. C: Fold and thrust structures of various sizes. D: Folded folds. E: An examp folded layers. F: A breccia layer with upward-finning fragments, some of which sti G: Paleoseismic records that corroborate each other in the Dead Sea Basin. Figure situ fo structu sequen Dead b, Typ depoce from demon Quanti accele the f thickne 12th International INQUA Meeting on Paleoseismology, Active Tectonics and Archaeoseismology (PATA), October 6 th -11 th , 2024, Los Andes, Chile re 3. Photo shows an example of slumps with alternating vergence of folds (blue arrows to the left, reds to the right). Yel ws mark the event horizon. White arrows mark a truncation surface where the folds are eroded. The truncation surface is over breccia layer. Right: Schematic cartoons summarizing the interpreted evolution of slump and tsunami/seiche-related depo op and Marco, 2012). } upward-finning breccia ion Postseismic section 10 cm nating vergence, truncation, cap breccia EH Peratzim to have been deformed and eroded by tsunami and seiche waves. 7.3. How do tsunami and seiche waves facilitate deformation of sediments? It has long been recognised that one of the principal mechanisms to reduce the shear strength of sediments and thereby facilitate their deformation is to increase pore fl uid pressure (e.g. Maltman, 1994b and references therein). It is therefore notable that pressure pulses associated with small 1 – 2 m tsunami waves on the sea bed are approximately double those of swell and storm waves of similar size (Bryant, 2001 p. 38). The relative pressure pulse of tsunami waves compared to storm waves actually increases with water depth, with a 1 m tsunami wave creating a greater pressure pulse than 4 m storm waves at water depths of 100 m as envisaged for the Lisan Formation at Peratzim (Bryant, 2001 p.39). In addition, it has been calculated that a 5 m high tsunami wave will generate a pressure pulse capable of causing compacted muds to fail on the sea bed at water depths of 100 m (Bryant, 2001 p. 39). However, one of the most signi fi cant factors is that pressure increases associated with shearing of the sea-bed by tsunami may last for several minutes (Bryant, 2001 p.40) enough to facilitate liquefaction (e.g. Fig. 7f). The shear between the water body and the lakebed would create folds tha evolve, increase in size, with their upper parts ultimately becom- ing unstable (Heifetz et al. 2005; Wetzler et al. 2010). Thus, it has been theor tically shown that tsunami wa es in deeper water (~100 m) settings not only effectively increase fl uid pressure to en- courage further weakening of sediment over a period of time, but also simultaneously increase shear stress along the sediment water interface that we suggest would facilitate folding. 7.4. Does seismicity or slumping trigger tsunami and seiche waves? Submarine slides and slumps may displace signi fi cant volumes of material and thereby induce tsunami in their own right, which are typically smaller than those created by seismic displacement of the sea-bed (Bardet et al., 2003). Within the Lisan Formation, it is unlikely that the downslope slump itself caused a tsunami because a) the palaeoslope was very gentle with dips of b 1° to drive slumping, Fig. 11. Schematic sedimentary and structural log summarising features associated with slumping. Pre-slump beds beneath the basal detachment remain undeformed, whilst beds within the overlying primary slump deposits display downslope verging folds and thrusts (1). The upper parts may form a reworked slump deposit with an ini- tial incoming seiche/tsunami wave creating upslope verging structures (2), that are succeeded and reworked by a secondary backwash associated with outgoing fl ow that may become locally erosive (3). Sediment that has been carried in suspension is deposited during post slump quiescence to create graded beds that in fi ll local topogra- phy and mark the top of the slump and seiche event (4). Normal deposition resumes resulting in undeformed beds that overlie the slump (5). a) b) c) d) Fig. 12. Schematic cartoons summarising the sequential evolution of slump and tsuna- mi/seiche related deposits. a) Fault movement (marked by an earthquake) causes off- set of the sea fl oor that displaces the overlying water column thereby generating a tsunami and seiche waves. The same earthquake triggers downslope slumping of sed- iments in adjacent areas. b) The tsunami and seiche wave arrive immediately after slumping and move towards the shore, resulting in upslope directed shearing and reworking of initial slump folds. c) The outgoing seiche or tsunami backwash causes downslope directed shearing of reworked sediments, which may locally become ero- sive. d) As the seiche waves dissipate, sediment is deposited from suspension resulting in a graded bed that caps the underlying deformed and locally scoured slumped unit and also in fi lls local topography. 104 G.I. Alsop, S. Marco / Sedimentary Geology 261 – 262 (2012) 90 – 107 In addition to earthquake history, the deformed layers, particularly the slump sheets, provide natural analogs for large-scale tectonic settings. For example, the structures of orogen-size fold and thrust belts, where the high- viscosity crustal blocks move slowly, are very similar to the low viscosity-high velocity sub-aqueous slumps. The similarity includes even small details such as fold duplexes (Alsop et al., 2020), back-thrusts and upslope-verging back thrusts (Fig. 4) (Alsop et al., 2017b), and slide stacking, which results in repeated stratigraphic sequences (Alsop et al., 2024). Fig.2: Numerical simulatio on in situ folded layer and i traclast breccia structures in the Dead Sea sedimentary sequences. a, Typical structures from Dead Sea onshore outcrops (Fig. 1b). b, Typical structures from Dead Sea depocenter Core 5017-1. c, Snapshots from the numerical simulations demonstrating the four structures. d, Quantitative estimation of the accelerations that are needed to initiate the four structures with different thicknesses. Fig. 3: Photo shows an example of slumps with alternating vergence of folds (blue arrows to the left, reds to the right). Yellow arrows mark the event horizon. White arrows mark a truncation surface where the folds are eroded. The truncation surface is overlain by a breccia layer. Right: Schematic cartoons summarizing the interpreted evolution of slump and tsunami/ seiche-related deposits (Alsop and Marco, 2012).