HR: 1340h
AN: T13A-0444    [Abstracts]
TI: Deep Source Anisotropy Revealed From Back-Azimuthal Variation of Shear-Wave Splitting in Southwest Ireland
AU: * Do, V
EM: vcd@cp.dias.ie
AF: School of Cosmic Physics, Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin, Dublin 2 Ireland
AU: Readman, P W
EM: pr@cp.dias.ie
AF: School of Cosmic Physics, Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin, Dublin 2 Ireland
AU: O'Reilly, B M
EM: bor@cp.dias.ie
AF: School of Cosmic Physics, Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin, Dublin 2 Ireland
AB: Observation of large splitting in core shear wave phases such as SKS/SKKS are usually attributed to upper mantle anisotropy due to either a fossilized deformation imprint in the lithosphere or the influence of asthenospheric flow, or to a combination of both. Here we report splitting results from southwest Ireland that show a strong back-azimuthal dependence of fast direction which suggests the influence of a deeper source of anisotropy (i.e. sub-lithospheric, e.g. the transition zone or D"?). Shear-wave splitting measurements were made on data from two permanent stations (5-10 year deployments) and from up to 23 mobile stations (up to 2 year deployments), using a selection from 200 shallow and deep focus earthquakes with epicentral distances within the range 90°-130° and with magnitude Mb above 6.0, with most of the high quality splitting analyses from earthquakes in South America and the East Asia region. Results show an average delay time of 1.2 seconds which is generally considered to require an accumulative anisotropic layer thickness of 80-100 km. Events from East Asia generally show more variation in fast polarization directions but with a dominant trend closely related to the Caledonian/Variscan tectonic fabric indicating that this may be preserved in the mantle lithosphere. However, events from South America give a consistently more northerly fast-polarisation direction, which does not appear to carry the signature of sub-continental lithospheric deformation history. There is no observable alignment of the fast polarization direction with the absolute plate motion direction so there is no direct correlation with mantle flow. Preliminary modelling based on two-layer anisotropy with a contribution from both mantle flow and possible lithospheric mantle deformation does not fit the observed splitting pattern. Hence, a more complex mechanism and origin for the observed anisotropy is required to explain our results.
DE: 7218 Lithosphere (1236)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Tectonophysics [T]
MN: Fall Meeting 2005