Unusual features in teleseismic receiver function, computed at a single three-component seismic station, reveal the presence of a dipping interface in the shallow crust with a pronounced seismic velocity contrast in the Campania-Lucania region, southern Apennines (Italy). The locus of this finding is the Pergola-Melandro basin, situated between the maximum intensity areas of two of the most destructive earthquakes reported in the Italian seismic catalogue: the M ~ 7.0 Val d'Agri earthquake (1857) and the Ms = 6.9 Irpinia earthquake (1980). This area is not associated with known historical events and for this reason is currently object of investigation as a potential seismic gap, where the probability of future ruptures is higher than in surrounding regions, also for the static stress increase caused by the two above mentioned earthquakes. Through an inversion procedure applied to the computed receiver functions, we are able to satisfactorily constrain the strike, dip, depth, and velocity contrast of this crustal discontinuity, whose geometric characteristics closely match those proposed by some authors for the sources of the two neighboring earthquakes. We argue that the detected interface is the seismogenic source which fills the gap. The findings of this study could open new perspectives both in the study of seismic gap areas and in the identification of historical earthquake sources, offering a new and effective tool to locate, investigate, and constrain buried or unknown faults.