Zones of inelastic deformation around surface ruptures detected by magnetic fabrics

Abstract

Characterizing the inelastic strain field around co-seismic faults is a key to understanding earthquake rupture processes. We aim to detect zones of inelastic deformation around surface ruptures, using Anisotropy of Magnetic Susceptibility (AMS) analyses. To characterize the paleo-seismic setting we measured 29 normal faults, which cut the late Pleistocene lacustrine Lisan Formation and were associated with shallow (<~12 km) earthquakes at the margins of the Dead Sea Basin. In order to define the geometry of those zones around a representative fault, 220 specimens were sampled in a 2D grid and analyzed by studying the room temperature AMS axes and parameters using spatial IDW (inverse distance weighting) interpolation on a cross section around the fault plane. To separate the total magnetic susceptibility of the Lisan sediments into diamagnetic, paramagnetic and ferromagnetic susceptibilities, we applied the low temperature AMS (LT-AMS) and Anhysteretic Remanent Magnetization (AARM) methods. The structural measurements demonstrate that the co-seismic faulting in the Dead Sea Basin, triggered mainly by E-W extension. The results show that the paramagnetic phase is dominant in both the aragonite-rich and the detritus-rich layers. The zones of inelastic deformation are distributed asymmetrically about the fault plane, with affinity to the observed displacement profile and in agreement with dynamic propagation models that suggest that the rupture propagated towards the surface. The discontinuous distribution of deformation around the fault plane is the result of both the faults single event propagation style and fault plane asperities. These results demonstrate a novel application of AMS analysis for defining the geometry of the inelastic strain field surrounding co-seismic faults, and improving fault propagation models dealing with the inelastic off-fault response.