The Use of Soils to Determine Paleoseismicity of Faults

in the Arava Rift, Israel

J. Bruce J. Harrison, Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801, E-mail: Bruce@prism.nmt.edu.

R. Amit, Institute of Earth Sciences, Hebrew University of Jerusalem, Givat Ram Campus, Jerusalem, Israel 91904.

Y. Enzel, Institute of Earth Sciences, Hebrew University of Jerusalem, Givat Ram Campus, Jerusalem, Israel.

One of the main techniques used to determine paleoseismicity of a fault is to identify different faulting events by the deposition of colluvial wedges on the down-thrown block of the fault scarp. The assumption in paleoseismic studies is that individual stratigraphic packages in the colluvium are formed through erosion of the scarp produced by surface rupture. Dating of the stratigraphic units then leads to an estimate of the timing of surface rupture events. Limitations of this approach are the difficulty of locating datable material and the possibility that some of the stratigraphic units in the colluvium may result from slope processes other than tectonic-related activity. A large fault scarp in the southern Arava was trenched and a number of colluvial wedges, separated by buried soils were identified. To determine if all these colluvial wedges are the result of tectonic activity and to evaluate if the soils on the slope could be used to constrain the ages of colluviation, we also trenched an adjacent terrace riser. Colluviation on the terrace riser is presumed to develop in the absence of tectonic activity. The soils on the slope were compared to that on the upper and lower terrace treads to determine the influence of slope processes on pedogenesis in this environment. More colluvial wedges and associated buried soils were found on the fault scarp than the terrace riser indicating that the wedges are tectonic in origin. The soils on the slopes are strongly influenced by slope processes and can only provide a maximum estimate of the age of the associated colluvial wedge.