The 3-D P- and S-wave velocity models of the upper crust beneath Southwest Iberia are determined by inverting arrival time data from local earthquakes using a seismic tomography method.We used a total of 3085 P- and 2780 S-wave high quality arrival times from 886 local earthquakes recorded by a permanent seismic network,which is operated by the Institute of Meteorology(IM),Lisbon,Portugal.The computed P- and S-wave velocities are used to determine the 3-D distributions of Vp/Vsratio.The 3-D velocity and Vp/Vsratio images display clear lateral heterogeneities in the study area.Significant velocity variations up to 6% are revealed in the upper crust beneath Southwest Iberia.At 4 km depth,both Pand S-wave velocity take average to high values relative to the initial velocity model,while at 12 km,low P-wave velocities are clearly visible along the coast and in the southern parts.High S-wave velocities at 12 km depth are imaged in the central parts,and average values along the coast; although some scattered patches of low and high S-wave velocities are also revealed.The Vp/Vsratio is generally high at depths of 4 and 12 km along the coastal parts with some regions of high Vp/Vsratio in the north at 4 km depth,and low Vp/Vsratio in the central southern parts at a depth of 12 km.The imaged low velocity and high Vp/Vs ratios are related to the thick saturated and unconsolidated sediments covering the region; whereas the high velocity regions are generally associated with the Mesozoic basement rocks.
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