The distribution of intracontinental seismicity in the upper crust is compared with theoretically obtained strength--depth curves based on recent laboratory data from cold and brittle frictional sliding, as well as from high-temperature creep data. The lower crust is seen as a channel of enhanced ductility like jelly in a sandwich. This rheological picture is compatible with the formation of mylonites in the lower crust and the appearance of cataclasites in the rigid upper crust. Sub-horizontal zones of detachment prefer the upper part of the ductile lower crust in the continents resulting in the formation of crustal nappes during compressional phases. The most active fault zones seem to exhibit reduced values of brittle, elastic and inelastic parameters.