Near-Surface Characterization of the Lisbon and Lower Tagus Valley Area, Portugal, for Seismic Hazard Assessment: VS30 and Soil Classification Maps

Abstract

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 inhab- itants. 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 VS30 and soil classification maps for the LTV region using in situ shear-wave velocity (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 (VS30 and soil classification maps) show that lithological changes within each formation prevent simple generalization of geophysical data/inter- pretations based solely on geological mapping. Contrary to previously available 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 simu- lations. 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.