Seismic Anisotropy In India: Lithospheric vs Asthenospheric Origin
Abstract
Due to the paucity of available seismological data in the public domain, the structure of the Indian lithosphere is little known. Seismic anisotropy underneath India and Sri Lanka is investigated here using shear wave splitting measurements on core refracted phases. Seismic anisotropy can provide insights into the lithospheric structure and the possible crust/upper mantle mechanical coupling. No such study has yet been done across the whole Indian sub-continent. As a result, some authors have extrapolated to the entire plate results obtained at very sparsely located stations and inferred the northern marker of the Indian plate underthrusting Eurasia, based on unconstrained data. We introduce a very comprehensive dataset recorded at 21 seismological stations deployed from the Himalaya to Sri Lanka. An unprecedented number of null results have been computed and we stress the too often neglected importance of this type of results. The 90° ambiguity inherent to the definition of a null prevents any interpretation without the addition of non-null results, but the added piece of information they carry is very valuable. The consistency in terms of the orientation of the two potential polarization planes of the fast S-wave (phi) is striking across the entire Indian plate. One possible orientation of phi closely follows the trend of the NUVEL1A Indian plate motion (NNE-SSW) with respect to a fixed Eurasian plate. By addition of a reasonable amount of non- null results, the NNE-SSW orientation of phi is confirmed under the majority of the stations spread from Sri Lanka to the northern part of the Dharwar craton. Towards the Himalaya and the Shillong plateau, the orientation of phi rotates clockwise to become EW. The transition zone between those two orientations might be located in the Gangetic plain, which is unfortunately not instrumented. The clockwise rotation in the orientation of phi observed between peninsular India and the Himalayan front seems to counterbalance the counterclockwise rotation of the Indian plate colliding with Eurasia. This response in terms of lithospheric deformation suggests a lithospheric dominated source of anisotropy consistent with the internal strain of the lithosphere due to the active deformation of the plate induced by its collision with Eurasia.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.T23F..03H
- Keywords:
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- 7200 SEISMOLOGY;
- 7203 Body waves;
- 7218 Lithosphere (1236);
- 8120 Dynamics of lithosphere and mantle: general (1213);
- 8155 Plate motions: general (3040)