Seismic anisotropy in South Island, New Zealand

Savage, Martha K.; Duclos, M.; Marson-Pidgeon, Katrina

doi:10.1029/175gm06

Cited by 12 publications

(16 citation statements)

References 86 publications

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“…The bars with black circles denote

{\bar{ϕ}}_{b}

with standard error (SE) >25°. Note gray color symbols (see the left‐top legend) represent SH max [ Townend et al ., ], SKS

ϕ

[ Zietlow et al ., ] and P n anisotropy [ Savage et al ., ] measurements from previous studies. (c) Cross section (E‐W) of the ray‐traced ray paths (blue lines) through AK135 velocity model across the plate‐boundary.…”

Section: Splitting Parameters From Obs Datamentioning

confidence: 99%

S‐wave splitting in the offshore South Island, New Zealand: Insights into plate‐boundary deformation

Karalliyadda

Savage

Sheehan

et al. 2015

Geochem Geophys Geosyst

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Local and regional S-wave splitting in the offshore South Island of the New Zealand plateboundary zone provides constraints on the spatial and depth extent of the anisotropic structure with an enhanced resolution relative to land-based and SKS studies. The combined analysis of offshore and land measurements using splitting tomography suggests plate-boundary shear dominates in the central and northern South Island. The width of this shear zone in the central South Island is about 200 km, but is complicated by stress-controlled anisotropy at shallow levels. In northern South Island, a broader (>200 km) zone of plate-boundary parallel anisotropy is associated with the transitional faulting between the Alpine fault and Hikurangi subduction and the Hikurangi subduction zone itself. These results suggest S-phases of deep events ( 90 km) in the central South Island are sensitive to plate-boundary derived NE-SW aligned anisotropic media in the upper-lithosphere, supporting a ''thin viscous sheet'' deformation model.

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“…The bars with black circles denote

{\bar{ϕ}}_{b}

with standard error (SE) >25°. Note gray color symbols (see the left‐top legend) represent SH max [ Townend et al ., ], SKS

ϕ

Section: Splitting Parameters From Obs Datamentioning

confidence: 99%

S‐wave splitting in the offshore South Island, New Zealand: Insights into plate‐boundary deformation

Karalliyadda

Savage

Sheehan

et al. 2015

Geochem Geophys Geosyst

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“…Previous studies have demonstrated marked seismic anisotropy beneath the South Island of New Zealand. Shear wave splitting of core phases, like SKS , is large, and the orientations of the faster quasi‐ S waves are nearly parallel to the orientation of relative plate motion beneath the center of the island [e.g., Duclos et al ., ; Klosko et al ., ; Savage et al ., ; Zietlow et al ., ]. Consistent with shear wave splitting, speeds of Pn , the P phase that propagates nearly horizontally in the uppermost mantle, also vary markedly with propagation direction, with the fastest Pn propagating roughly parallel to relative plate motion [ Baldock and Stern , ; Bourguignon et al ., ; Melhuish et al ., ; Scherwath et al ., ].…”

Section: Introductionmentioning

confidence: 98%

Pn anisotropy beneath the South Island of New Zealand and implications for distributed deformation in continental lithosphere

Collins

Molnár

2014

JGR Solid Earth

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Pn travel times from regional earthquakes recorded both by stations on New Zealand and by ocean bottom seismographs deployed offshore indicate anisotropy in the uppermost mantle beneath the region. The largest anisotropy of~8% (±2%, 1σ) lies beneath the deforming part of the South Island to just off its West Coast, a zone roughly 100-200 km wide. The fastest propagation is aligned N60°E (±3°), essentially parallel to the largely strike-slip relative plate motion since 20 Ma, also~N60°E. The magnitude of anisotropy decreases abruptly northwest and southeast of this zone, and on the southeast side of the island, the orientation of fastest propagation is between N32°W and N-S. The~N60°E orientation of fast propagation is consistent with finite strain within the uppermost part of the mantle lithosphere if the measured 850 km of displacement of the Pacific plate past the Australia plate is spread over a region with a width of 100-200 km. The agreement of this orientation of fast propagation with the orientation or relative plate motion suggests the possibility of but does not require some dynamic recrystallization in rock as cold as 500-800°C, where Peierls creep seems to be the likely deformation mechanism. Such a strain distribution matches deformation of a thin viscous sheet that obeys a constitutive relationship of the form ε e τ n , whereε is the average strain rate and τ is the operative deviatoric stress, with an average value of n ≈ 3-10. Presumably, the NW-SE fast propagation in the region southeast of the island results from strain that precedes the Cenozoic deformation that has shaped the island.

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“…But we have also made some preliminary studies in nonvolcanic areas of New Zealand and have found it to work equally well. With changes in filters we expect that it could readily be modified for teleseismic events, because the original Teanby code has already been modified for teleseismic analysis [Savage et al, 2007].…”

Section: Recommendationsmentioning

confidence: 99%

“…For some studies in which there were many local earthquakes close to a volcano, we used a maximum of 0.4 s for the local events [Johnson et al, 2010;Savage et al, 2008Savage et al, , 2010. For SKS measurements, 4 to 6 s may be used [Greve et al, 2008;Savage et al, 2007].…”

Section: Basic Splitting Measurement Techniquementioning

confidence: 99%

“…Using different filters would make this method applicable to broad classes of data beyond the application to local S waves described herein. For example, the Teanby codes can be used on teleseismic data to study mantle anisotropy [Greve et al, 2008;Savage et al, 2007] and straightforward modifications of the codes described herein could be used to create automatic SKS measurements. If short-period data are used, the long-period filters should be modified since they will effectively be cut off at higher frequencies than expected, so some of the resulting filters will give nearly identical responses.…”

Section: Filteringmentioning

confidence: 99%

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Automatic measurement of shear wave splitting and applications to time varying anisotropy at Mount Ruapehu volcano, New Zealand

et al. 2010

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[1] We present an automatic shear wave splitting measurement tool for local earthquakes, with the sole manual step of choosing an S arrival time. We apply the technique to three data sets recorded on Mount Ruapehu volcano in New Zealand that have previously been determined to have fast polarizations that vary in time and with earthquake depth. The technique uses an eigenvalue minimization technique, applied over multiple measurement windows. The dominant period of each waveform sets minimum and maximum window lengths. Cluster analysis determines the best solution among all the windows, and quality grading criteria assess the results automatically. When the same filters are used for events determined to be high quality from manual studies, the automatic technique returns virtually identical results. Scatter increases when the automatic technique chooses the best filter, but the average automatic results remain consistent with the manual results. When the automatic technique is used on sets that include data previously judged as poor quality, some stations yield distributions of fast polarizations that include peaks that were not present in previously published results. The difference may stem from two factors: automatic grading lets through some measurements that independent analysts consider poor quality, but also unconscious bias in the manual selection process may downgrade measurements that do not fit expectations. Nonetheless, the new objective analysis confirms changes in the average fast polarizations between 1994 and 2002 and between shallow and deep events. Therefore, this new technique is valuable for objective assessment of anisotropy and its variation in time.

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Seismic anisotropy in South Island, New Zealand

Cited by 12 publications

References 86 publications

S‐wave splitting in the offshore South Island, New Zealand: Insights into plate‐boundary deformation

S‐wave splitting in the offshore South Island, New Zealand: Insights into plate‐boundary deformation

Pn anisotropy beneath the South Island of New Zealand and implications for distributed deformation in continental lithosphere

Automatic measurement of shear wave splitting and applications to time varying anisotropy at Mount Ruapehu volcano, New Zealand

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