Imaging of Active Fault Zone in the Dead Sea Rift
The Arava valley formed as a result of extensive tectonic activity along the Dead Sea Rift (DSR) from the Miocene to the present. While the continuous long-term tectonic activity is evident in abundant geological expressions, the neotectonic activity in the Evrona fault zone, southern Arava valley, is not obvious on the surface. Furthermore, surficial inspection may give the impression that recent tectonic activity on this prominent transform plate-boundary hardly generates significant geologic structures.
However, a unique Ground Penetrating Radar (GPR) imaging to a depth of about 25 meters below the surface, conducted in the framework of this thesis, reveals dense arrays of subsurface discontinuities across the Evrona fault zone. Hundreds of discontinuities per kilometer length of GPR profile, which can be interpreted as sets of fractures and faults with a variety of depths of penetration and spatial orientations, have been detected. Several of these faults can be correlated and verified with fault scarps and lineaments that are exposed on the surface or logged in trenches, and evident in the ancient agricultural farm of Evrona. GPR measurements along part of the “foggara” subsurface irrigation system in the Evrona farm show vertical variations (displacements) and negative local gradients, indicating that water cannot flow through the tunnels at the present time. Combined with archaeological findings, this evidence points to neotectonic activity that destroyed the foggara system and, as a result, the vitality of the farm.
The exceptional resolution afforded by GPR imaging also reveals that, within fault clusters, the density of the faults increases as a function of depth within the first 15 m, and the apparent dips of the faults slope toward each other, suggesting that they merge at a depth of a few tens of meters. Shallow Seismic Reflection (SSR) observations conducted at the same locations generally identify a parent fault below such fault clusters, leading to their interpretation as “flower structures” characteristic of strike-slip faults.
The relation between the interpreted geophysical images and their tectonic surficial expression, as revealed by aerial photographs and geomorphological mapping, forms the basis for a new concept of faulting mechanism. According to both seismic and radar observations, a typical single active fault extending toward the surface through the top hundred meters of alluvium changes its characteristics abruptly as it reaches a depth range of about 35 – 20 m. It then proceeds towards the surface in a series of splays. As a result of this mechanism, the tectonic displacement that accumulates along a plane of a parent fault is dispersed towards the surface. Linear Shear Strength Decrease (LSSD), damage distribution and fault propagation modelling of the relations between the structure and distribution of the faults and the mechanical properties of the alluvial materials of Evrona emphasize the splay mechanism and other phenomena revealed by the GPR images.
Analysis of the faults detected by GPR in Evrona helps develop a method of evaluating the relative level of tectonic activity of an active fault zone. According to this approach, the neotectonic history of the Evrona fault zone during the last 45,000 y is recorded. The GPR record reveals a peak in tectonic activity within 18,000 – 27,000 ybp. This peak wave of tectonic activity is about 37% higher than the average tectonic activity of the whole period. The level of tectonic activity in the recent period recorded by GPR (0 – 9,000 ybp) is about 70% of the average tectonic activity and about 50% of the peak wave. These values and the decreasing trend in the tectonic activity since the peak wave suggest that the most recent period is tectonically silent.
* For a detailed description of the main part of this topic please download the following paper (5.05 MB, PDF format): Development of recent faulting in the southern Dead Sea Rift according to GPR imaging – Basson et al., 2002.
Basson, U., Ben-Avraham,; Z., Garfunkel, Z., and Lyakhovsky, V., 2002. Development of recent faulting in the southern Dead Sea Rift according to GPR imaging, European Geophysical Society (EGS) Stephan Mueller Special Publication Series, Vol. 2, 1–23, 2002.