DOUBLE SEISMIC ZONE IN DOWNGOING SLABS AND THE VISCOSITY OF THE MESOSPHERE

The seismic zone beneath several island arcs between about 100 and 200km depth consists of an upper zone having down-dip compression and a lower zone having down-dip tension. Several numerical models of the Aleutian arc were computed to test the hypothesis that these double seismic zones are due to sagging of the slab under its own weight. This sagging occurs because the asthenosphere (between about 100 and 200km) provides little support or resistance to the slab, which is supported from below by a more viscous mesosphere and from above by the lithosphere. The viscosity of the mesosphere was constrained to the interval between 0.25 x 1022P by noting that the slab would have mainly down-dip compression at high viscosities and mainly down-dip tension at lower viscosities. The deviatoric stress in the slab and the fault plane between the slab and the island arc is about 200-300bars (expressed at shear stress). The models were calibrated to the observed depth and gravity anomalies in the trench. -Author