Seismic Ground Motion and Boulder Formation Along the Margins of the Dead Sea Fault System, Southern Israel

Abstract:

Rockfall ages in tectonically active regions provide information regarding frequency and magnitude of earthquakes. Such rockfalls have been dated using several methods including lichonometry modeling, radiocarbon and optically stimulated luminescence dating of material buried beneath boulders, and the concentration of cosmogenic nuclides in exposed boulder faces. In the hyper-arid environment of the Dead Sea fault (DSF), southern Israel, rockfalls are most probably triggered by earthquakes. We dated rockfalls along the western margin of the DSF using terrestrial cosmogenic nuclides (TCN). In Timna, where the exposure history of the boulders is relatively simple, we recognize three discrete rockfall events, at 31 ka, 15ka, and 4.5ka. However, the ages of the majority of the boulders concentrate at ~4.5 ka. In Shehoret, where boulder exposure ages are more complicated, samples were collected from simultaneously exposed conjugate boulders and cliff surfaces. Pre-rockfall inheritance and post-rockfall production rates of TCN cannot be evaluated in a straightforward way. We developed a numerical code that suggests most-likely rockfall ages between 3.6±0.8 and 4.7±0.7 ka. Our ages agree with dated earthquakes determined in various paleoseismic studies along the entire length of the DSF and support the observation of intensive earthquake activity around 4-5 ka. Our rockfall record, however, under represents the regional earthquake record derived from historic and paleoseismic records. This under representation implies that either the ongoing development of detachment cracks prior to triggering event is slower than the earthquake cycle or that most boulder piles are formed only during big and rare earthquakes.