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Lithospheric anisotropy on the Kerguelen hotspot track inferred
from Rayleigh wave polarisation anomalies.
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Station PAF.

Fig. 2: Example of an event recorded at PAF: Earthquake 93_216. (after Pettersen and Maupin, Fig. 1, GJI, 149 225-246)

The Java Trench:

Twenty events in the Java Trench were analysed, with surface waves arriving at PAF in the backazimuth sector 27.8 to 63.2. They separate quite clearly into two groups; the events from the northern part of the trench with backazimuth smaller than approximately 34, showing no Rayleigh wave on the transverse component, and the 12 (of 13 altogether) events in the southern part of the trench showing a clear and strong polarisation anomaly.

Concerning the 12 events in the Java Trench, the polarisations show three prominent features: the particle motion is elliptical, clockwise, and with the major axis oriented between the longitudinal axis and 25 away from it (measured clockwise), as shown in the example in Fig. 3. These features are independent of period, although the degree of ellipticity and orientation differ at the three periods: the major axes of the ellipses at 20 s period have a nearly constant orientation, and the degree of ellipticity decreases with increasing backazimuth. The latter also applies to the ellipses at 25 s period. At 33.3 s period the particle motion is almost linear for the first three events (i.e. backazimuth 37.6), then becomes elliptical with minor axis equal to half the major axis at 39.7, and thereafter the degree of ellipticity decreases gradually with increasing backazimuth.

Fig. 3: Particle motion in the horizontal plane for an event in the Java Trench: event 93_216.
(after Pettersen and Maupin, Fig. 4b, GJI,
149 225-246)

Earthquakes in other locations:

Even though most of the events from the Indian-Antarctic Ridge have normal Rayleigh polarisation, one of those with the strongest polarisation anomalies was recorded in this area. The polarisation of that event in the horizontal plane is quite different from what we found for the Java Trench events. The particle motion is still elliptical, but counter-clockwise and with major axes oriented between 20 and 33 as measured counter-clockwise from the longitudinal axis.

In general, the events from the other regions do not show clear polarisation anomalies at PAF.

Stations AIS and CRZF:

At AIS, 4 events show clear evidences of Rayleigh wave polarisation anomalies, and at CRZF, from the 36 events analysed, only 2 show evidence of polarisation anomalies. The surface waves are clearly more normal at these stations than at PAF.

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Universitetet i Oslo - Institutt for geofag / SPICE 2004