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    Systematic variation in anisotropy beneath the mantle wedge in the Java-Sumatra subduction system from shear-wave splitting

    Hammond, James O.S. and Wookey, J. and Kaneshima, S. and Inoue, H. and Yamashina, T. and Harjadi, P. (2010) Systematic variation in anisotropy beneath the mantle wedge in the Java-Sumatra subduction system from shear-wave splitting. Physics of the Earth and Planetary Interiors 178 (3/4), pp. 189-201. ISSN 0031-9201.

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    Abstract

    The tectonic context of south-east Asia is dominated by subduction. One such major convergent boundary is the Java-Sunda trench, where the Australian–Indian plates are being subducted beneath the Eurasian plate. We measure shear-wave splitting in local and teleseismic data from 12 broadband stations across Sumatra and Java to study the anisotropic characteristics of this subduction system, which can provide important constraints on dynamical processes involved. Splitting in S-waves from local earthquakes between 75 and 300 km deep show roughly trench parallel fast directions, and with time-lags 0.1–1.3 s (92% ≤0.6 s). Splitting from deeper local events and SKS, however, shows larger time-lags (0.8–2.0 s) and significant variation in fast direction. In order to infer patterns of deformation in the slab we apply a hybrid modelling scheme. We raytrace through an isotropic subduction zone velocity model, obtaining event to station raypaths in the upper mantle. We then apply appropriately rotated olivine elastic constants to various parts of the subduction zone, and predict the shear-wave splitting accrued along the raypath. Finally, we perform grid searches for orientation of deformation, and attempt to minimise the misfit between predicted and observed shear-wave splitting. Splitting from the shallow local events is best explained by anisotropy confined to a 40 km over-riding plate with horizontal, trench parallel deformation. However, in order to explain the larger lag times from SKS and deeper events, we must consider an additional region of seismic anisotropy in or around the slab. The slab geometry in the model is constrained by seismicity and regional tomography models, and many SKS raypaths travel large distances within the slab. Models placing anisotropy in the slab produce smaller misfits than those with anisotropy outside for most stations. There is a strong indication that inferred flow directions are different for sub-Sumatran stations than for sub-Javanese, with >60° change over ∼375 km. The former appear aligned with the subduction plate motion, whereas the latter are closer to perpendicular, parallel to the trench direction. There are significant differences between the slab being subducted beneath Sumatra, and that beneath Java: age of seafloor, maximum depth of seismicity, relative strength of the bulk sound and shear-wave velocity anomaly and location of volcanic front all vary along the trench. We speculate, therefore, that the anisotropy may be a fossilised signature rather than due to contemporary dynamics.

    Metadata

    Item Type: Article
    Keyword(s) / Subject(s): seismic anisotropy, subduction zones, Indonesia, s-wave splitting
    School: Birkbeck Schools and Departments > School of Science > Earth and Planetary Sciences
    Depositing User: Sarah Hall
    Date Deposited: 24 May 2016 09:33
    Last Modified: 17 Nov 2017 15:58
    URI: http://eprints.bbk.ac.uk/id/eprint/15307

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