The definition of appropriate places for the development of paleoseismic studies is extremely important in earthquake engineering site investigations. The seismic reflection method is routinely used to locate shallow fault segments where these do not outcrop, like in low slip-rate areas where faults identified in Quaternary sediments have vertical throws less than 2 m. The Lower Tagus Valley (Portugal), covered by 50 m of alluvium sediments, is one of these areas. To find fault segments in this geological environment is a daunting task. Any displacement in the seismic data can be a velocity anomalies and/or statics effect. To illuminate the presence of a fault in Holocene sediments, we acquired an S-wave seismic reflection profile at V. F. de Xira and modelled the response of a fault segment in order to recognise it in our data. The signature of a fault segment can be a change in the shape/attitude of the reflection hyperbolae but reflections from the fault plane are rather weak. These are controlled by fault width and velocity contrast between damaged zone-adjacent sediments. The resemblance between the modelled and field data of V. F. Xira supports the conclusion that the fault affects the Holocene alluvium and is still active.