Earthquake focal depths, effective elastic thickness, and the strength of the continental lithosphere

Almost all earthquakes on the continents are confined within a crustal laye r that varies in thickness(T-s) from about 10 to 40 km, and are not in the mantle. Variations in T-s correlate with variations in the effective elasti c thickness (T-e), both of them having similar values, although T-e is usua lly the smaller of the two. These observations suggest that the lower crust , at. least in some places, is stronger than the mantle beneath the Moho, c ontrary to most models of continental rheology. Thus the strength of the co ntinental lithosphere is likely to be contained within the seismogenic laye r, variations in the thickness of this strong layer determining the heights of the mountain ranges it can support. The aseismic nature of the continen tal mantle and the lower crustal seismicity beneath some shields are probab ly related to their water contents.