Results of analysis of gravimetric and seismological data for the island arc-trench systems and adjacent regions of the Northwest Pacific are briefly summarized. Gravimetric data indicate anomalously dense zones beneath the trenches, where densities 0.1 to 0.15 g/cm3 greater than normal occur to depths of the order of 100 km, while beneath the island arcs the Upper Mantle appears to be less dense than normal by −0.05 to −0.1 g/cm3. In general the gravimetric data agree well with seismic data, which suggest that the dipping high-velocity layer is not a continuation of the oceanic lithospheric plate, from which it differs in physical properties. Earthquake foci distribution also does not support the concept of a subsiding lithospheric plate. The main earthquake focal plane transects the surface not in the deep trenches, but on their steep slopes, and a second earthquake focal plane dips towards the ocean. Available data agree better with theories of differentiation of the mantle at large depths and with discontinuous movement of lighter components due to difference in lithostatic pressure.