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Free keywords:
anisotropy, crustal structure, Dead Sea, Middle East, S waves, transform faults, 2-D modelling
DDC:
550 - Earth sciences
Abstract:
An analysis of the shear (S) waves recorded during the wide-angle reflection/refraction (WRR) experiment as part of the DESERT project crossing the Dead Sea Transform (DST) reveals average crustal S-wave velocities of 3.3-3.5 km s-1 beneath the WRR profile. Together with average crustal P-wave velocities of 5.8-6.1 km s-1 from an already published study this provides average crustal Poisson's ratios of 0.26-0.27 (Vp/Vs= 1.76-1.78) below the profile. The top two layers consisting predominantly of sedimentary rocks have S-wave velocities of 1.8-2.7 km s-1 and Poisson's ratios of 0.25-0.31 (Vp/Vs= 1.73-1.91). Beneath these two layers the seismic basement has average S-wave velocities of around 3.6 km s-1 east of the DST and about 3.7 km s-1 west of the DST and Poisson's ratios of 0.24-0.25 (Vp/Vs= 1.71-1.73). The lower crust has an average S-wave velocity of about 3.75 km s-1 and an average Poisson's ratio of around 0.27 (Vp/Vs= 1.78). No Sn phase refracted through the uppermost mantle was observed. The results provide for the first time information from controlled source data on the crustal S-wave velocity structure for the region west of the DST in Israel and Palestine and agree with earlier results for the region east of the DST in the Jordanian highlands. A shear wave splitting study using SKS waves has found evidence for crustal anisotropy beneath the WRR profile while a receiver function study has found evidence for a lower crustal, high S-wave velocity layer east of the DST below the profile. Although no evidence was found in the S-wave data for either feature, the S-wave data are not incompatible with crustal anisotropy being present as the WRR profile only lies 30� off the proposed symmetry axis of the anisotropy where the difference in the two S-wave velocities is still very small. In the case of the lower crustal, high S-wave velocity layer, if the velocity change at the top of this layer comprises a small first-order discontinuity underlain by a 2 km thick transition zone, instead of just a large first-order discontinuity, then both the receiver function data and the WRR data presented here can be satisfied. Finally, the S-wave velocities and Poisson's ratios which have been derived in this study are typical of continental crust and do not require extensional processes to explain them.
