SPICE-UiO homepage > Home

SPICE-Research Training Network - UiO > Polarisation anomalies > The Kerguelen hotspot


Lithospheric anisotropy on the Kerguelen hotspot track inferred
from Rayleigh wave polarisation anomalies.
Page 4 of 6
Page 1 | 2 | 3 | 4 | 5 | 6

Wavefield distortion by the local structure of the Kerguelen Plateau: isotropic and anistropic models.
Continuation of page
3.

S-wave velocity model:


Fig. 5: Shear-wave depth-velocity models used in this study. The solid black line is for the northern plateau, the solid grey line for the central plateau, and the dashed line for the surrounding oceanic basin.
(after Pettersen and Maupin, Fig. 14, GJI,
149 225-246)

Wavefield distortion by local structure - Isotropic.

The distortion of the wavefield when propagating from the oceanic basins onto the plateau has been modelled for the fundamental mode Rayleigh waves at 20, 25, 30 and 40 s period in the regional models described above. A multiple-scattering method for modelling surface wave propagation in 3-D heterogeneous structures developed by Maupin (2001) has been employed.


Fig. 6: Transverse to longitudinal amplitude ratio at periods 20 s (a) and 40 s (b) in our model of the Kerguelen Plateau for Rayleigh waves arriving from the Java Trench. The boundaries between the oceanic basin, the northern and central plateaux are shown by black lines. The plateaux are rotated compared to Fig. 5 such that the negative x-direction now corresponds to a backazimuth of 40 at PAF. The Rayleigh wave is incident from the left of the figure, as indicated by the arrows.
(after Pettersen and Maupin, Fig. 15, GJI,
149 225-246)

The structure of the Kerguelen Plateau is rotated in the horizontal plane in order to simulate a wavefield arriving at PAF with a backazimuth of 40, similar to the waves generated by events in the Java Trench. Since it is necessary to have a denser horizontal sampling of the model at 20 s than at 40 s period, the model used at 20 s is a zoom of the model used at 40 s.

The largest amplitude ratios are observed in the vicinity of the structural boundaries; locally they reach 15 per cent at 20 s and 10 per cent at 40 s period. Away from the boundaries, the transverse component reaches only a few percent of the longitudinal one. A closer examination of the wavefield shows that the polarisation in the vicinity of PAF is slightly elliptic, with the large axis deviated clockwise with respect to the propagation direction, like on the data. Although far too small to explain the data alone, distortions related to the structure of the plateau may contribute to the observed polarisation anomalies.

We conclude that the structural contrasts between the Kerguelen Plateau and the surrounding oceanic basins are not sufficient to explain the strong polarisation anomalies observed at PAF.

Page 4 of 6
Page 1 | 2 | 3 | 4 | 5 | 6


Back to Top | Previous | Contact | Home

University of Oslo
Universitetet i Oslo - Institutt for geofag / SPICE 2004