The Lower Tagus Valley (LTV) region includes the metropolitan area of Lisbon and has the highest population density in Portugal, with about 3.5 million inhabitants. The LTV has been struck by several historical earthquakes that caused significant economic and human losses, and therefore, earthquake damage mitigation is of great importance. The present research was directed toward preparation of the first detailed V S30 VS30 and soil classification maps for the LTV region using in situ shear‐wave velocity (⁠V S VS ⁠) measurements. These maps were built using P‐ and S‐wave seismic velocities in the shallowest surface, obtained mostly from seismic refraction and a few crosshole datasets, together with lithostratigraphic studies and analyses of boreholes drilled for water supply and geotechnical investigations. Borehole data were used to confirm layer thicknesses and lithologies, and to overcome the limitations of traditional refraction interpretation. Our results (⁠V S30 VS30 and soil classification maps) show that lithological changes within each formation prevent simple generalization of geophysical data/interpretations based solely on geological mapping. Contrary to previously available V S30 VS30 maps based on proxies or gross geological generalizations, different classes are obtained inside the Holocene alluvial sediments and the Miocene units, for instance. Certain areas with Miocene outcropping, such as the district capital of Santarém, unexpectedly fall into a moderate risk class, albeit showing hard‐rock outcrops. Though there is scope for further improvements in the future, the maps presented results from the first rigorous near‐surface characterization campaign undertaken in the region. Velocity information assembled in this work can be further used to correct earthquake records from a number of seismological stations and to update velocity models used in ground‐motion simulations. Furthermore, seismic refraction interpretation was compared among different acquisition geometries for seismic noise measurements at three geologically distinct sites to evaluate the use of these techniques for future S‐wave data acquisition. @en

Expressions of ductile, soft-sediment deformations induced by ground movements due to past earthquakes are difficult to recognize in near-surface soils. We have carried out shallow S-wave reflection studies in a seismically active area located northeast of metropolitan Lisbon, Portugal. Identifying shallow disturbed zones and hidden fault segments in this area is important but quite difficult because of small vertical slips due to earthquakes, the Holocene alluvial cover hiding the fault segments, and a high rate of surficial sedimentation. We have performed S-wave reflection profiling at two sites — Vila Franca Xira and Castanheira de Ribatejo. We detected different but interrelated evidence of soft-sediment deformation in the seismic data. This evidence includes sharp lateral changes in the S-wave velocity field; changes in the reflection horizons in stacked sections; aligned diffractions in unmigrated sections; discontinuities in common-offset gathers; and discontinuities, backscattered, and diffracted arrivals in common-source gathers. Though not equally clear everywhere, this evidence is recognizable at many locations where earthquake-motion-induced disturbed zones are interpreted. To confirm these interpretations, we have performed synthetic modeling of a seismic wavefield using the same acquisition geometry as in the field experiments, and with multiple disturbed zones present as vertical emplacements through horizontally lying soil layers. The modeling results resemble the observations in field data. It is possible to confirm the signatures of soft-sediment deformation in the shallow S-wave reflection data. The approach that we used will be useful in many seismically active, soilcovered areas in the world. @en