Abstract
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.
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