doi:10.1016/j.epsl.2005.04.019
Copyright © 2005 Elsevier B.V. All rights reserved.
Soft sediment deformation by Kelvin Helmholtz Instability: A case from Dead Sea earthquakes
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Eyal Heifetza, 
, 
, Amotz Agnonb and Shmuel Marcoa
aThe Department of Geophysics and Planetary Sciences, Tel-Aviv University, Israel
bInstitute of Earth Sciences, The Hebrew University of Jerusalem, Israel
Received 24 January 2005;
revised 3 April 2005;
accepted 14 April 2005.
Editor: Dr. V. Courtillot.
Available online 21 June 2005.
Abstract
The standard explanation for soft sediment deformation is associated with overturn of inverted density gradients. However, in many cases, observations do not support this interpretation. Here we suggest an alternative in which stably stratified layers undergo a shear instability during relative sliding via the Kelvin–Helmholtz Instability (KHI) mechanism, triggered by earthquake shaking. Dead Sea sediments have long stood out as a classical and photogenic example for recumbent folding of soft sediment. These billow-like folds are strikingly similar to KHI structures and have been convincingly tied to earthquakes. Our analysis suggests a threshold for ground acceleration increasing with the thickness of the folded layers. The maximum thickness of folded layers (order of decimeters) corresponds to ground accelerations of up to 1 g. Such an acceleration occurs during large earthquakes, recurring in the Dead Sea.
Keywords: Kelvin–Helmholtz Instability; Soft sediment deformation; Paleo-earthquake intensity; Dead Sea basin
Fig. 1. Examples of different geometry of sediment foldings: (1a) linear wavy geometry, (1b) coherent billow vortices, and (1c) turbulent mixed breccia layer (Photos were taken from the Dead Sea region). In (1a) the speculated original condition supporting KHI is illustrated schematically: Two layers, of thickness H, initially horizontal and stably stratified (ρ1 > ρ2), experience an earthquake shaking in the x direction. In response to the shaking, the denser lower layer moves more slowly than the upper one, forming shear at the interface. The interface, located initially at z = 0, was perturbed becoming unstable with a wavy shape.
Fig. 2. An example for the KHI growth rate, normalized by frequency f, as a function of the layer thickness H, and the normalized ground acceleration a / g, as given by (10). Here we take typical values of the Dead Sea sediment composition (c.f. text) of ρm = 2000 kg/m3, and Δρ = 130 kg/m3. The damping coefficient is taken as r = 0.1 Hz. The parabolic solid line marks the threshold for instability (c.f. 8). For onset of linear KHI wave folding (as in Fig. 1a), the growth rates must be in the order of the driving seismic wave frequency (order of 1 Hz). Coherent billows (Fig. 1b) require high growth rates, where fully turbulent mixing, leading to breccia layers (Fig. 1c), requires yet higher growth rates.
Fig. 3. Kelvin Helmholtz billow clouds in the New Zealand summer sky. The clouds are formed along an atmospheric inversion layer (of strong stratification, where temperature increases with height). The inversion layer tends to isolate the surface boundary layer wind from the free atmosphere above it and as a result a shear is formed along the inversion layer. The combination of shear, stratification and humidity provides the conditions for KHI billow clouds to develop. The photo was taken by Mr. Attay Harkabi on January 2001.
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