Morphologic Dating of Fault Scarps and Terrace Risers in the Southern Arava, Israel: Comparison to Other Age-Dating Techniques and Implications for Paleoseismicity

Yehouda Enzel, Rivka Amit, J. Bruce J. Harrison, and Naomi Porat

Isr. J. Earth Sci.; 43: 91-103

Fault scarps and terrace risers in the southern Arava, Israel, were analyzed to determine the applicability of morphologic dating through a diffusion modeling approach (a) as an estimator of age and (b) as a relative age dating tool. The relative effect on the modeling results of estimating both the initial scarp angle and the regional diffusion coefficient was also evaluated. Using diffusion modeling we were able to differentiate between slopes that varied in age, based on soil-geomorphic studies, by only a few thousands of years. The modeling results indicate that the specifically studied fault scarp is between 50,000 and 17,000 years old. The large range of the estimated age results from the range of possible diffusion coefficients. The model also predicted early and middle Holocene ages for two terrace risers, which are younger than the fault scarp according to field stratigraphic relationships. The morphologic age determined by the modeling agrees well with the correlative age from earlier soil and geomorphologic studies and with recently acquired optically stimulated luminescence (OSL) dates. Combining the OSL dates with the morphologic ages allowed us to calculate a diffusion coefficient of 2 X 10^-4 to 3 X 10^-4 for these scarps. The agreement between the ages, which were independently estimated by all three methods, makes it possible to estimate (a) the recurrence interval of faulting along this segment of the fault with some confidence and (b) the time since faulting and intensity of the seismic activity in the region as seen from the long-term perspective. Assuming that the displacement and earthquake magnitude are correlated, the magnitudes associated with the studied fault are all smaller than 7.5 and appear to change from higher magnitudes (about 7) during the latest Pleistocene, to lower magnitudes (about 6) during the Holocene. Scarp diffusion modeling provides acceptable estimates of ages of faulting events in a region where other age dating methods (numeric or correlative) are non-existent or rare, time-consuming, or very expensive. In addition, the ability of the modeling to distinguish between scarps of different ages provides us with a tool to group the numerous fault scarps in the southern Arava according to their morphologic ages and to help in tectonic analysis of that region.