In this study we used relatively velocity independent methods (Wadati diagrams) to estimate and check the focal depth of the seismic activity in a sector of the Betic Cordilleras-Alboran Sea in Southern Spain. To understand the behaviour of the seismic activity, a total of 335 earthquakes have been analyzed. The most interesting feature found in the analyzed sector is the strong variation of the focal depth. On average, the focal depth distribution in the Granada basin is restricted to h≤20 km. The lower cut-off in the seismic activity, which is interpreted to mark the brittle-ductile transition. This cut-off is coincident with a midcrustal reflector detected in the zone by previous seismic refraction and deep reflection experiments that was interpreted as an extensional detachment. The seismic activity in the M¿laga zone reaches depths up to 100 km. In this case the seismic activity affect to the crust and upper mantle. The deepest activity coincides with low values of heat flow, possibly thus indicating a thicking of the lithosphere towards the W. In the centre of the Albor¿n Sea the activity is shallower and corresponds with a higher heat flow. We think that our results support models that have proposed active delamination towards the West. Such a model may explain the increase in the lithosphere thickness in this direction, which would be congruent with the depth of the earthquakes. ¿ 1997 American Geophysical Union