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Lithospheric anisotropy on the Kerguelen hotspot track inferred
from Rayleigh wave polarisation anomalies.
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In the far-field, the Rayleigh waves are polarised in the vertical and longitudinal directions and the Love waves in the transverse direction in laterally homogeneous structures. But some recordings of surface waves present significant polarisation anomalies. These deviations may be ascribed to lateral heterogeneities in the structure, anisotropy, or both. This study focuses on Rayleigh fundamental mode polarisation anomalies in the South Indian Ocean on the Kerguelen Plateau.

Photo from the PAF scientific base, located on Kerguelen Island, in the South Indian Ocean.
(photo taken by Jean-Yves Thore (2003) and provided by Chantal Condis, EOST Strasbourg)

The Kerguelen Islands are located in the southern part of the Indian Ocean, on the Antarctic Plate. They are French overseas territory. The Kerguelen Islands have been created by a hotspot which also created the Kerguelen Plateau around.

Data selection and dispersion analysis:

45 earthquakes between 1993 and 1998 were analysed. The recordings were made at the three Geoscope stations PAF, AIS and CRZF of events at epicentral distances between 30 and 90. The polarisation of the fundamental mode Rayleigh wave was analysed by performing group velocity analysis simultaneously on the 3 components of the seismograms. The analysed periods were in the range 15 s to 120 s, in order to cover Rayleigh wave fundamental mode, overtones and Love waves. Most of the analysed events show that the Rayleigh fundamental mode has a significant transverse component. The largest polarisation anomalies occur for events in the Java Trench (with backazimuth between 37 and 64) and to the North (at backazimuth around 350).

Fig. 1: Localisation of the stations PAF, AIS and CRZF; Epicentres and wave paths for the events recorded at PAF. Black line indicates observed polarisation anomaly, grey line indicates uncertain data and dashed line indicates no anomaly.
(after Pettersen and Maupin, Fig. 2, GJI, 149 225-246)

We will now first analyse the data from the stations PAF, AIS and CRZF.

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Universitetet i Oslo - Institutt for geofag / SPICE 2004