Open Session - SWISS GEOSCIENCE MEETINGs
Open Session - SWISS GEOSCIENCE MEETINGs
Open Session - SWISS GEOSCIENCE MEETINGs
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1.13<br />
Subduction Zone anisotropic Patterns Induced by Faulting and Hydration<br />
of the Slab<br />
Faccenda Manuele*, Burlini Luigi**, Gerya Taras V.*, Mainprice David***<br />
* Institute of Geophysics, ETH Zürich, Schafmattstr. 30, 8093 Zürich, Switzerland (faccenda@erdw.ethz.ch)<br />
** Institute of Geology, ETH Zürich, Leonhardstr. 19, 8092 Zürich, Switzerland<br />
*** Geosciences Montpellier, Université Montpellier II and CNRS, Montpellier, France<br />
Normal faulting and hydration of the subducting oceanic plate is widely observed at the outer rise of subduction zones.<br />
In order to investigate these processes, we performed 2D numerical models of a spontaneously bending oceanic plate using<br />
I2ELVIS code that account for visco-elasto-plastic rheologies. At the outer rise, bending-related slab faulting occurs and provides<br />
a pathway for water percolation in the slab. Faults generally deep trenchward, but antithetic faults are also common.<br />
As the slab subducts, serpentinized faults acquire a sub-vertical position. The upper part of the slab is, hence, formed by a<br />
sub-vertical layering of rocks with different elastic properties (dry and hydrated portions) that, for long wavelength SKS<br />
waves, appears as transverse isotropic body with a horizontal axis of symmetry (SPO). Furthermore, hydration of ultramafic<br />
rocks leads to the formation of sheet silicates such as lizardite, antigorite, talc and chlorite. Such hydrous minerals are highly<br />
anisotropic (80-100%) and tend to orient parallel to the shear zone walls. Hence, along the hydrated normal faults these<br />
minerals will tend to align parallel to the faults (LPO).<br />
Calculations of the slab anisotropy lead to ≈1 second time delay for a 30-40 km thick anisotropic layer, depending on the<br />
aspect ratio of the hydrated fractures. Indeed, in medium to very old slabs, the vertical distance between the upper and lower<br />
planes of the Double Benioff Zone is around 30-45 km.<br />
The spatial distribution of fault sets at the outer rise is consistent with the common trench-parallel orientation of the fast<br />
SKS component measured in the forearc. We also found that in areas where the fast SKS component is normal to the trench,<br />
fault sets are oriented at high angle respect to the trench. Therefore, slab anisotropy produced by localized hydration of the<br />
slab can significantly contribute to the observed splitting patterns at subduction zones.<br />
1.1<br />
Finite element method versus finite difference method: Numerical<br />
accuracy study for two different applications<br />
Frehner Marcel*, Deubelbeiss Yolanda*, Schmalholz Stefan M.*,<br />
Kaus Boris J.P.*, Saenger Erik H.**,***, Steeb Holger***<br />
* Department of Earth Science, ETH Zurich, 8092 Zurich, Switzerland, marcel.frehner@erdw.ethz.ch, yolanda.deubelbeiss@erdw.ethz.ch<br />
** Spectraseis AG, 8005 Zurich, Switzerland<br />
*** Multi Scale Mechanics, TS, CTW, University of Twente, 7500 AE Enschede, The Netherlands<br />
Despite its wide-spread use in geology and geophysics, it is unclear whether there is a difference in accuracy between finite<br />
element (FEM) and finite difference methods (FDM). For this reason, we here compare the accuracy of the two methods for<br />
two different geophysical problems. In both cases an analytical solution is available that provides the exact solution to the<br />
respective problem and allows quantifying the numerical errors of the different methods.<br />
The first study considers two-dimensional scattering of elastic waves in a medium containing a circular heterogeneity.<br />
Different combinations of the FDM and the FEM are used to approximate both time and space derivatives of the elastodynamic<br />
wave equation. The different numerical algorithms are compared for simulations of an incident plane P-wave that is<br />
scattered by a mechanically weak circular inclusion whereby the diameter of the inclusion is of the same order than the Pwave’s<br />
wavelength. Staircase-like spatial discretization of the inclusion’s circular shape with the FDM using a rectangular<br />
grid provides accurate velocity and displacement fields close to the inclusion boundary only for very high spatial resolutions.<br />
Implicit time integration based on either the FDM or the FEM does not provide computational advantages compared to explicit<br />
schemes. The best numerical algorithm in terms of accuracy and computation time for the investigated scattering<br />
problem consists of a FEM in space using an unstructured mesh combined with an explicit FDM in time.<br />
23<br />
Symposium 1: Structural Geology, Tectonics and Geodynamics