Evidence for shallow and pervasive seismic anisotropy in the Wellington Region, New Zealand

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Abstract
The shear waves of local earthquakes were recorded during a 5‐month deployment of seven three‐component digital seismographs on the Wellington Peninsula, New Zealand. The seismographs were spaced an average of less than 5 km apart, and over 300 local earthquakes were recorded with phase arrivals within the shear wave window. A significant number (≈ 37%) of the earthquakes recorded showed clear evidence of shear wave splitting: identifiable fast and slow shear wave arrivals with similar pulse shapes. Consistent polarization directions at particular stations were also observed, even when poor signal‐to‐noise ratio or scattering meant that no slow shear wave arrival could be identified. However, there were large station‐to‐station differences in the polarization directions. Correcting for the observed shear wave splitting improved the fit between the measured shear wave polarizations and those calculated assuming a double‐couple focal mechanism. The cause of the observed shear wave splitting is therefore most likely to be seismic anisotropy. Large station‐to‐station differences in the polarization alignments, ranging from 61°±24° to 137°±18°, suggest that most anisotropy is confined to the top 2–3 km of the crust. However, there is evidence from one station for a small amount of pervasive anisotropy; if such a trend existed on the other stations, it could not be identified because of the large scatter in the data points. The measured delay times between split shear waves vary from 0.02 to 0.22 s, with a mean value of 0.1±0.06 s. This translates to a near‐surface shear wave velocity anisotropy of about 10%, with up to 2% pervasive anisotropy possible throughout the crust. This data set indicates that extensive dilatancy anisotropy cannot be the sole cause of crustal seismic anisotropy and that foliations in the rock fabric and the fracture zones of active faults may also be important. There is no evidence for temporal change in the shear wave splitting parameters during the period of the experiment.