Details of seismic refraction modeling. The seismic refraction data was modeled interactively (Song and ten Brink, 2004) using forward and inverse travel-time ray tracing routines (Zelt and Smith, 1992). The preferred model is that which minimizes the travel time difference between observed and calculated arrivals. Figure S1 shows the observed (in black) and calculated (in red) travel times for each shot. The lack of calculated arrivals propagating to the right from shot (b), is due to the fact that two-point ray tracing algorithms are only valid for smooth velocity structures (Cerveny, 1987). The velocity structure near this shotpoint, being located <1 km west of the Dead Sea Transform Fault, has a sharp lateral velocity discontinuity. Model runs, which produce good fit to these arrivals east of the shotpoints, required modifications to smooth the near-surface velocity structure (layers 1 and 2) in the vicinity of the shotpoint. Figure S2 shows the ray coverage of the model. Note that the sediments and upper crust are well covered by both diving waves and wide-angle reflections, but the lower crust is covered mostly by wide-angle reflections. This implies good lateral resolution of lower crust velocity, but poor vertical resolution in that layer. Therefore, data coverage is satisfactory to resolve the presence or absence of lateral variations in P wave velocity in the lower crust under the Dead Sea basin, but is not satisfactory to resolve the overall velocity gradient (6.8-7.0 km/s) of this layer. References: Cerveny, V., Ray-tracing algorithms in three-dimensional laterally-varying layered structures, D. Reidel, Norwell, MA, 1987. Song, J., and ten Brink, U.S., RayGUI2.0 - A graphical user interface for interactive forward and inversion ray tracing, USGS Open-file report 2004-1426, 2005. Zelt, C.A. and Smith, R.B., Seismic traveltime inversion for 2-D crustal velocity structure, Geophys. J. Int., 108, 16-34, 1992 Acknowledgements: We thank D. Lizarraldi for providing software to calculate to the bottoming points.