Open Session - SWISS GEOSCIENCE MEETINGs

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