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 410 km-discontinuity is uplifted, owing to the exothermic nature of the α → β transition (the equilibrium model), and models in which the 410 km-discontinuity is depressed, owing to inhibited kinetics of the α → β olivine transition (the metastable model). In the latter case the existence of the low-velocity wedge of metastable olivine has a profound influence on the waveforms of deep-focus earthquakes. The wedge acts as a geometrical waveguide, with the energy-focusing effect apparent even after the wavefront 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 over a range of 300 km and so may provide a valuable diagnostic for discriminating between equilibrium and metastable models of subduction. A recent search for such guided energy, using regionally recorded P and S waves from deep events in Tonga, yielded negative results.