IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS012 Poster presentation 2206
Detailed Structure of Upper Mantle Discontinuities in the Tonga and
Mariana Subduction Zones
Dr. Rigobert Tibi
Earth and Planetary Sciences Washington University
Dougla A. Wiens
Recordings of deep Tonga earthquakes from two arrays of 12 broadbandseismographs each in the Fiji
and Tonga islands are stacked and searched forreflections and conversions from upper mantle
discontinuities in the Tongasubduction zone. To enhance the low-amplitude discontinuity phases s410p,
P660pand S660p, deconvolved seismograms from each event/array pair are aligned onthe maximum
amplitude of the direct P wave and subsequently slant-stacked.For the 410-km discontinuity, the results
show no systematic variations indepth with distance to the cold slab. The 660-km discontinuity varies
between656 and 714 km in depth. For the southern and central parts of the subductionzone, the largest
depths occur in the core of the Tonga slab. For the northernpart, two separate depressions of the 660
are observed. These anomalies areinterpreted as being induced by the active, steeply subducting Tonga
deep zoneand a subhorizontally lying remnant of subducted lithosphere from the fossilVityaz trench,
respectively. Interpreting the deflections of the 660 in termsof local temperatures implies a thermal
anomaly of -800K to -1200K at 660 kmdepth. Except for the southern region where it may thicken, the
width of thedepressed 660 region implies that the Tonga slab seems to penetrate the 660with little
deformation. We use P-to-S converted phases from teleseimic datarecorded at island and ocean bottom
stations in Mariana to investigate theupper mantle structure in the region. Results of the analysis show
evidencefor double seismic discontinuities at the base of the transition zone nearthe Mariana slab. A
shallower discontinuity is imaged at depths of ~650-715 km,and a deeper interface lies at ~740-770 km
depth. The large lateral extent atnear constant depths for both features is consistent with horizontal
interfacesrather than small-scale scatterers. The amplitude ratios of the seismic signalssuggest that the
shear velocity contrasts across the two interfaces arecomparable. These characteristics support the
notion that the discontinuitiesare the results of the phase transformations in olivine (ringwoodite to
post-spinel) and non-olivine component (ilminite to perovskite), respectively,for the pyrolite model of
mantle composition.
Keywords: subduction, 660 km discontinuity, mariana