Distinguishing Between Tectonic and Non-Tectonic Colluviation on Scarps

Harrison, J.B.J., Amit, R., Enzel, Y.

Institute of Earth Sciences, The Hebrew University of Jerusalem

Jerusalem 91904

Israel Geological Society

Annual Meeting, 1993

An essential part of determining the seismic hazard of an area is to identify the paleoseismicity of existing faults. One of the main techniques used is to identify different faulting events by the deposition of colluvial wedges on the fault scarp. The common assumption in paleoseismic studies is that individual stratigraphic packages in the colluvium, are formed through erosion of the scarp formed by surface rupture related to large seismic events. Dating of the stratigraphic units then leads to an estimate of the timing of surface rupture events. Some of the 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. The degree of soil development on the wedges has been used to identify periods of stability on the scarp slope to constrain the lapse time between depositional events. In a previous study, a large fault scarp in the southern Arava was trenched and a number of different colluvial wedges, separated by buried soils were identified. To determine if all these colluvial wedges are the result of tectonic activity, a terrace riser was trenched in two places. One trench is in close proximity to the fault scarp and the other is approximately 500 m away from the fault. The colluvial stratigraphy on the terrace risers has been described and compared to that found on the fault scarp. A much larger number of colluvial wedges and associated buried soils were found on the fault scarp than on the terrace riser which indicates that many of the wedges are the result of tectonic activity.