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Seismic Anisotropy Research in New Zealand
On this page:
- What is seismic anisotropy?
- Tectonic setting of NZ
- Broadscale SKS measurements
- Studies by region
- Resources
What is seismic anisotropy?
In an anisotropic medium the elastic parameters vary as a function of orientation.
Seismic anisotropy occurs when elastic waves vibrating or travelling in one direction
travel faster than another.
In recent years a number of studies have been undertaken to measure seismic anisotropy
in New Zealand. Here we provide a summary of these studies, the majority of which use the
shear wave splitting technique.
When shear waves travel in an anisotropic medium, the component polarised
parallel to the fast direction begins to lead the orthogonal component.
The difference in speed causes the waves to separate; this phenomenon is called shear wave
splitting. We are able to measure the polarisation direction of the fast wave (phi) and the delay
time between the fast and slow waves (dt). This gives us information about deformation in
the Earth.
(Image source: E. Garnero, 2001. Motivated from the famous Crampin [1981] figure).
For further information on seismic anisotropy and shear wave splitting see the following review paper:
Savage, M.K. (1999)
Seismic anisotropy and mantle deformation: What have we learned from shear wave splitting?, Rev. Geophys., 37, 65-106.
 (0.9 MB PDF)
Tectonic setting of New Zealand
Information about the tectonic setting of New Zealand can be found here (still to come)
Broadscale SKS measurements
The teleseismic phase most commonly used in shear wave splitting studies is SKS
(the "K" means that it travels through the liquid outer core as a P wave).
The main advantage in using SKS is that since it undergoes a P-to-S conversion as it emerges from
the liquid outer core, any splitting must have occurred between the recording station and the core-mantle
boundary.
A number of SKS splitting studies have been undertaken in New Zealand on a
broadscale (with large station spacing). For more information on these studies, as
well as mantle xenolith results, click on the image below.
Studies by region
For information about the anisotropy studies undertaken in particular regions of New Zealand,
please click on the corresponding region name listed to the right of the map.
A text only version is given beneath the map.
Text only version:
Resources
Papers
Audoine E., M.K. Savage and K. Gledhill (2000)
Seismic anisotropy from local earthquakes in the transition region from a subduction to a
strike-slip plate boundary, New Zealand, J. Geophys. Res., 105, 8013-8033.
http://www.agu.org/pubs/crossref/2000/1999JB900444.shtml
Audoine E., M.K. Savage and K. Gledhill (2004)
Anisotropic structure under a back-arc spreading region, the Taupo Volcanic Zone,
New Zealand, J. Geophys. Res., in press.
Duclos M., M.K. Savage, A. Tommasi, and K.R. Gledhill (2004)
Mantle tectonics beneath New Zealand inferred from SKS splitting and petrophysics,
J. Geophys. Res., submitted.
Gerst A. and M. K. Savage (2004)
Seismic anisotropy beneath Ruapehu Volcano: A possible eruption
forecasting tool,
Science, in press.
Gledhill, K.R. (1990)
A shear-wave polarization study in the Wellington region New Zealand,
Geophys. Res. Lett., 17(9), 1319-1322,
http://www.agu.org/pubs/crossref/1990/90GL01498.shtml
Gledhill K.R. (1991a)
Evidence for shallow and pervasive seismic anisotropy in the Wellington region, New Zealand,
J. Geophys. Res., 96, 21,503-21,516.
http://www.agu.org/pubs/crossref/1991/91JB02049.shtml
Gledhill K.R. (1993a)
Shear waves recorded on close-spaced seismographs: I. Shear wave splitting results,
Can. J. Expl. Geophys., 29, 285-298.
Gledhill K.R. (1993b)
Shear waves recorded on close-spaced seismographs:
II. The complex anisotropic structure of the Wellington peninsula, New Zealand,
Can. J. Expl. Geophys., 29, 299-314.
Gledhill K., and D. Gubbins (1996)
SKS splitting and the seismic anisotropy of the mantle beneath the Hikurangi subduction zone,
New Zealand, Phys. Earth Planet. Inter., 95, 227-236.
Gledhill K., and G. Stuart (1996)
Seismic anisotropy in the fore-arc region of the Hikurangi subduction zone, New Zealand,
Phys. Earth Planet. Inter., 95, 211-225.
Klosko, E.R., F.T. Wu, H.J. Anderson, D. Eberhart-Phillips, T.V. McEvilly, E. Audoine,
M.K. Savage, and K.R. Gledhill (1999)
Upper mantle anisotropy in the New Zealand region,Geophys. Res. Lett., 26, 1497-1500.
http://www.agu.org/pubs/crossref/1999/1999GL900273.shtml
Marson-Pidgeon K. and M.K. Savage (1997)
Frequency-dependent anisotropy in Wellington, New Zealand,
Geophys. Res. Lett., 24, 3297-3300.
http://www.agu.org/pubs/crossref/1997/97GL03274.shtml
Marson-Pidgeon, K., M.K. Savage, K. Gledhill, and G. Stuart (1999)
Seismic anisotropy beneath the lower half of the North Island, New Zealand,
J. Geophys. Res., 104, 20,277-20,286.
http://www.agu.org/pubs/crossref/1999/1999JB900212.shtml
Marson-Pidgeon K. and M.K. Savage (2004)
Modelling shear wave splitting observations from Wellington, New Zealand,
Geophys. J. Int., 157, 853-864, doi:10.1111/j.1365-246X.2004.02274.x.
(0.4 MB PDF)
Marson-Pidgeon K., and M.K. Savage (2004)
Shear-wave splitting variations across an array in the southern North Island, New Zealand,
Geophys. Res. Lett., in press.
Matcham I., M.K. Savage and K.R. Gledhill (2000)
Distribution of seismic anisotropy in the subduction zone beneath the Wellington region, New Zealand,
Geophys. J. Int., 140, 1-10.
Miller V. and M. Savage (2001)
Changes in seismic anisotropy after volcanic eruptions: Evidence from Mount Ruapehu,
Science, 293, 2231-2233.
Pulford A., M. Savage, and T. Stern (2003)
Absent anisotropy: The paradox of the Southern Alps orogen,
Geophys. Res. Lett., 30(20), 2051, doi:10.1029/2003GL017758.
http://www.agu.org/pubs/crossref/2003/2003GL017758.shtml
Savage, M.K. (1999)
Seismic anisotropy and mantle deformation: What have we learned from shear wave splitting?, Rev. Geophys., 37, 65-106.
(0.9 MB PDF)
Savage M.K., K.M. Fischer, and C.E. Hall (2004)
Strain modelling, seismic anisotropy and coupling at strike-slip boundaries:
application in New Zealand and the San Andreas fault,
in Vertical Coupling and Decoupling in the Lithosphere, 227, 9-39,
Geological Society, London, Special Publications.
Scherwath M., A. Melhuish, T. Stern, and P. Molnar (2002)
Pn anisotropy and distributed upper mantle deformation associated with a continental transform fault,
Geophys. Res. Lett., 29(8), doi:10.1029/2001GL014179.
http://www.agu.org/pubs/crossref/2002/2001GL014179.shtml
Theses
Audoine E.L. (2002)
Upper mantle and crustal seismic anisotropy across the Pacific-Australian plate boundary,
New Zealand, Ph.D. thesis, 215 pp., Victoria University of Wellington, New Zealand.
Baldock, G. (2004)
High resolution crustal and upper mantle structure adjacent to
a continental transform, South Island, New Zealand,
M.Sc. thesis, 116 pp., Victoria University of Wellington, New Zealand.
Balfour N. (2004)
Stress and crustal anisotropy in Marlborough, New Zealand: Frictional-strength of faults
and structure-controlled anisotropy,
M.Sc. thesis, 136 pp., Victoria University of Wellington, New Zealand.
Gerst A. (2003)
Temporal changes in seismic anisotropy as a new eruption forecasting tool?,
M.Sc. thesis, 184 pp., Victoria University of Wellington, New Zealand.
Gledhill K.R. (1991b)
Shear-wave splitting and seismic anisotropy in the Wellington region, New Zealand,
Ph.D. thesis, Victoria University of Wellington, New Zealand.
Hofmann S.D. (2002)
Seismic anisotropy in the crust and mantle: A study at the western edge of the central
volcanic region, New Zealand, M.Sc. thesis, 134 pp., Victoria University of Wellington,
New Zealand.
Kohler N. (2003)
SKS wave splitting beneath the Marlborough fault zone, South Island, New Zealand,
Grad. Dip. Sci. thesis, 54 pp., Victoria University of Wellington, New Zealand.
Marson K. (1996)
Seismic anisotropy in the Wellington region: shear-wave splitting results,
Honours project, 32 pp., Victoria University of Wellington, New Zealand.
Marson K. (1997)
Seismic anisotropy beneath the lower half of the North Island, New Zealand,
M.Sc. thesis, 72 pp., Victoria University of Wellington, New Zealand.
Matcham I. (1997)
Seismic anisotropy in the Wellington region from local events recorded at the IRIS station SNZO,
Honours project, 44 pp., Victoria University of Wellington, New Zealand.
Miller V. (2000)
Possible methods for monitoring Mount Ruapehu volcano: Shear wave splitting and
ground deformations using GPS, M.Sc. thesis, Victoria University of Wellington, New Zealand.
Scherwath, M. (2001)
Lithospheric structure and deformation in an oblique continental
collision zone, South Island, New Zealand, Ph.D. thesis, Victoria University of Wellington, New Zealand.
Posters
Marson-Pidgeon K., and M.K. Savage (2004)
A synthesis of anisotropy measurements in New Zealand,
presented at the 2004 Joint Assembly, 17-21 May, Montreal, Canada.
Page 1 (1.5 MB PDF)
Page 2 (0.6 MB PDF)
Other relevant publications
Brisbourne A.M., and G.W. Stuart (1998)
Shear-wave velocity structure beneath North Island, New Zealand, from Rayleigh-wave interstation phase velocities,
Geophys. J. Int., 133, 175-184.
Brisbourne A., G. Stuart, and J.-M. Kendall (1999)
Anisotropic structure of the Hikurangi subduction zone, New Zealand - integrated interpretation
of surface-wave and body-wave observations,
Geophys. J. Int., 137, 214-230.
Eberhart-Phillips D., and C.M. Henderson (2004)
Including anisotropy in 3-D velocity inversion and application to Marlborough, New Zealand,
Geophys. J. Int., 156, 237-254.
Little T., M.K. Savage, and B. Tikoff (2002)
Relationship between crustal finite strain and seismic anisotropy in the mantle, Pacific-Australia
plate boundary zone, South Island, New Zealand,
Geophys. J. Int., 151, 106-116
Molnar P., H.J. Anderson, E. Audoine, D. Eberhart-Phillips, K.R. Gledhill, E.R. Klosko, T.V. McEvilly,
D. Okaya, M.K. Savage, T. Stern, and F.T. Wu (1999)
Continuous deformation versus faulting through the continental lithosphere of New Zealand,
Science, 286, 516-519.
Moore M., P. England, and B. Parsons (2002)
Relation between surface velocity field and shear wave splitting in the South Island of New Zealand,
J. Geophys. Res., 107, (B9) 2198.
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