An-Najah Staff

As part of the Dead Sea Rift Transect Project (DESERT 2000) we conducted an active seismic experiment to study the small-scale structure of the Wadi Arava fault zone (WAF). This fault is considered the principal active fault in the southern part of the Dead Sea Transform system, which extends over a length of about 1000 km and is characterized by a sinistral movement of 105 km within the last 18 Ma. One of the aims of the project was to generate and observe seismic guided waves in the fault zone. Guided waves are multiple-reflected waves propagating in narrow low-velocity channels. They provide information on properties and geometry of the fault zones itself which are often not obtained by conventional seismic experiments. In April 2000 we placed 12 detonations within or very close to the surface trace of the WAF. The charges consisted of 45 kg of chemical explosives placed in 20 m boreholes. Seismic signals were recorded at 5 densely-spaced linear geophone arrays crossing the fault. The recordings exhibit prominent wave trains emerging from 2 in-fault explosions. We interpret these phases as waves being guided by a fault-zone related low-velocity layer. Observations of these wave trains are confined to certain segments of the receiver lines and occur only for certain shot locations, matching the surface trace of the WAF. They show high energy and monochromatic or weak dispersive behaviour. We model the guided waves by using an analytical solution for the wavefield (Ben-Zion \& Aki, 1990). The model is characterized by a vertical low-velocity layer embedded in two quarter spaces. Although strong trade-offs between the free parameters are present, preliminary calculations suggest that the observations are adequately fit by models with a 10 to 30 m thick vertical layer where the S wave velocity is reduced by approximately 15 to 25 \% relative to the surrounding rock. We relate the vertical low-velocity layer to the damage zone of the WAF. Compared to other major continental shear zones, the damage zone of the shallow part of the WAF at this location seems to be rather narrow. The WAF shows 100 km of cumulative offset, about a fifth of the San Andreas Fault. The damage zone is proportionately thinner, suggesting a scaling between damage zone thickness and offset.