The waveguide effect of metastable olivine in slabs

We use a finite difference algorithm to compute P-SV synthetic seismograms for deep earthquakes recorded at regional seismic stations located above a subducting slab. We calculate synthetics for 2-D slab models in which the 4 10 km-discontinuity is uplifted, owing to the exothermic nature of the alph a --> beta transition (the equilibrium model), and models in which the 410 km-discontinuity is depressed, owing to inhibited kinetics of the alpha --> beta olivine transition (the metastable model). In the latter case the exi stence of the low-velocity wedge of metastable olivine has a profound influ ence on the waveforms of deep-focus earthquakes. The wedge acts as a geomet rical waveguide, with the energy-focusing effect apparent even after the wa vefront has traveled the several hundred kilometers from the wedge boundary to the Earth's surface. The guided energy is most pronounced for receivers located near the surface projection of the deep slab but is observable ove r a range of 300 km and so may provide a valuable diagnostic for discrimina ting between equilibrium and metastable models of subduction. A recent sear ch for such guided energy, using regionally recorded P and S waves from dee p events in Tonga, yielded negative results.