SHALLOW CRUSTAL MODELS IN THE LISBON AREA FROM EXPLOSION DATA USING BODY AND SURFACE-WAVE ANALYSIS

Knowledge of the shallow velocity structure is very important to under stand the possible site effects which could greatly contribute to the irregular damage distribution observed in Lisbon for the 1755 earthqua ke. Taking advantage of small charge blasts in the Tagus River bed in front of Lisbon, seismic records were obtained in a scattered array ex tending to a 8.5 km distance from the shotpoint. Geological considerat ions allowed the grouping of traces in wide profiles for three zones: West, East and South. The presence of a distinct group of Rayleigh wav es made possible the joint modelling of first arrivals and surface wav es for the West and East zones. The P and S-wave velocity models infer red show the existence of two low velocity layers (in the West zone), which is in agreement with the known geology for the area. The East zo ne structure could not be resolved in detail from refraction data only , but surface wave analysis allow the identification of the first low velocity layer. For the South zone, the study of first arrivals reveal ed a structure similar to the West zone, although velocity values and interface depths are slightly different. The interpretation of these m odels was done according to the age and lithology of the different geo logical formations, The final models obtained represent a significant improvement in detail over previous regional studies for the Tagus Val ley basin (Mendes-Victor et al., 1980) and over the preliminary interp retation of this data presented in Mendes-Victor (1987).