Numerical tests on the seismic visibility of metastable minerals in subduction zones

In current views on the origin of deep-focus earthquakes the presence of me tastable olivine in subducting slabs plays a central role. In this study we construct synthetic slab models to investigate the expression of metastabl e minerals in seismic velocities. In particular, we investigate the visibil ity of different slab mineralogy for seismic traveltime studies. From forwa rd modeling we construct a range of mineralogical models which include phas es with olivine and pyroxene structures. Differentiation of subducted litho sphere with respect to the surrounding mantle is included, and critical tem peratures are used to bound the eventual wedge of metastable minerals. A ca se with pure olivine mantle and slab is also considered. From the mineralog ical slab models we compute seismic P-wave velocities including uncertainty bounds due to a variation range in mineralogical composition, elastic data and slab temperature. As model predictions we compute the traveltime signa tures of different slab models by 2-D ray tracing for a number of earthquak es located at the top and bottom side of slabs. We infer that upper and low er bounds on computed mineralogy lead to significantly distinct seismic vel ocities for slabs with equilibrium or metastable mineralogy. The predicted traveltime signature (including uncertainty bounds) of different slab model s suggests the possibility to discriminate between metastable and equilibri um slab models. Models with a pure olivine mineralogy lead to differences i n traveltime reaching I s. The consideration of non-olivine phases is thus crucial when looking for a weak seismological signal using forward modeling . Further, we conclude from our results that the traveltime signature of sl ab mineralogy strongly depends on earthquake position. Specifically, events at the top side of slabs yield significantly different traveltime signatur es from events located at the bottom side. Top-side events are most discrim inating in traveltimes directly up-dip of the slab, whereas bottom-side eve nts are most discriminating in traveltimes observed directly above the slab , i.e. above the mantle wedge. This leads to the important conclusion that highly accurate event locations and accurate knowledge of 3-D structure out side the slab region are prerequisites for studying slab mineralogy with tr aveltimes. (C) 1999 Elsevier Science B.V. All rights reserved.