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| Titel |
Effect of fold structures on seismic anisotropy in continental crust |
| VerfasserIn |
W. J. Song, S. S. Vel, S. E. Johnson, D. Okaya |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250070155
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| Zusammenfassung |
| Tectonic deformation and metamorphism in the middle and lower crust can produce and
modify seismic anisotropy owing to the development of micro-scale fabrics including
crystallographic preferred orientation and large-scale structures such as folds, domes, faults
and shear zones. Although the impact of the microfabrics on seismic anisotropy is well
known via petrophysical or thin-section-based measurements, there have been few studies on
how the macro-scale structures affect seismic responses. In this study, we investigate the
influence of types and limb angles of cylindrical fold structures on seismic anisotropy
through tensor manipulation. To calculate the velocity of seismic waves propagating through
the fold structures, the elastic stiffness tensor in the Christoffel equation is substituted by
geological effective media (GEM) of the folds. Here the Voigt averaging method is used in
computing the GEM stiffness analytically or numerically. In this case, the GEM
(C*) is decomposed into the product of a structural geometry operator (SGO) and
stiffness of a representative rock that makes up the fold (Crep); C* = SGO Ã Crep.
SGO is an operator that reorients Crep at each point of the fold with respect to a
geographical reference frame and averages the reoriented stiffnesses. As an example of
the representative rock stiffness, we take the stiffness tensor of the Haast schist
of South Island, New Zealand (Okaya & Christensen, 2002), which has intrinsic
P-wave anisotropy (AVP) of 12.7% and S1-wave anisotropy (AVS1) of 16.5%.
While the rock is hexagonal in symmetry, the calculated GEM for the fold structures
range between hexagonal to orthorhombic symmetry, depending on limb angle. The
common types of folds are described via power or trigonometric functions: cuspate,
chevron, sinusoidal, parabolic and box folds. Our results include the velocity behavior
with respect to limb angle and incident angle on specific planes of the folds as
well as the seismic anisotropy as a function of limb angle. The fold with sharper
hinge represents the minimum seismic anisotropy at lower limb angle. For instance,
chevron fold shows minimum AVP (10.4%) and AVS1 (5.7%) at limb angle of 45Ë ,
whereas sinusoidal fold has minimum AVP (9.2%) and AVS1 (4.9%) at 60Ë limb
angle.
References:
Okaya, D. A. & Christensen, N. I. 2002. Anisotropic effects of non-axial seismic
wave propagation in foliated crustal rocks. Geophysical Research Letters, 29,
doi:10.1029/2001GL014285. |
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