Upper mantle anisotropy in the S.W. Pacific from earthquake travel-time analysis

Galea, Pauline

doi:10.1016/0031-9201(93)90015-2

Cited by 16 publications

(20 citation statements)

References 28 publications

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“…This average fast polarization also agrees reasonably well with the direction of maximum P velocity (N62øE) inferred by Galea [1993] Gledhill and Stuart [ 1996] found a maximum delay time of 0.2 s for shallow events, which translates to a pervasive crustal shear wave velocity anisotropy of-•4%. We will assume that this same crustal anisotropy exists beneath all our stations.…”

Section: Resultssupporting

confidence: 88%

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Seismic anisotropy beneath the lower half of the North Island, New Zealand

Marson-Pidgeon¹,

Savage²,

Gledhill³

et al. 1999

J. Geophys. Res.

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Abstract. Teleseismic ScS and SKS events recorded on nine broadband seismograph stations have been used to investigate seismic anisotropy beneath the lower half of the North Island, New Zealand. This area lies above the Hikurangi subduction zone, and the array provides ray paths which sample the mantle both above and below the slab. Shear wave splitting measurements give similar fast polarizations and delay times at each station. The average SKS fast polarization is approximately NE-SW, subparallel to the strike of subduction and the major geological features, with an average SKS delay time of 1.6 + 0.1 s. This lack of.variation in splitting parameters suggests that similar fast polarizations are found in both the mantle wedge and the subslab mantle. The anisotropy in the lithospheric portion of the mantle wedge is most likely caused by the preferred orientation of olivine due to the shear deformation associated with oblique convergence. Any anisotropy in the slab is probably due to fossil mineral alignment. Anisotropy in the asthenosphere is most likely caused by the preferred orientation of olivine due to asthenospheric flow. The similar NE-SW fast polarizations found in the asthenosphere both above and below the slab suggest that the mantle flow is in a trench-parallel direction in both regions. IntroductionShear wave splitting of teleseismic phases such as ScS and SKS is now a common method used to investigate mantle anisotropy, which can in turn be related to deformation of the upper mantle [e.g., Silver, 1996]. This phenomenon occurs when a shear wave enters an anisotropic medium, upon which it is split into two orthogonally polarized waves which travel with different velocities [e.g., Crampin, 1981 ]. Shear wave splitting measurements can be characterized by two parameters; the polarization direction of the fast shear wave, q•, can be related to the symmetry of the anisotropic system, and the time separation between the two waves, fit, can be related to the strength of anisotropy and the path length through the anisotropic material. Thus, if the cause of the anisotropy is known, the splitting parameters can be related to the deformation and tectonic structure of a region. For example, mantle anisotropy is usually assumed to be due to straininduced lattice-preferred orientation of olivine. The polarization direction of the fast shear wave is then assumed to align parallel to the mantle flow direction, if it is in the form of

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Section: Resultssupporting

confidence: 88%

“…The average NE-SW fast polarization direction is subparallel to the strike of the trench and the geological structures in the lower North Island, such as the mountain ranges, active faults, and major shear systems, and also agrees with the fast P velocity axis found in the southwest Pacific by Galea [1993] …”

Section: Fast Polarization Directionssupporting

confidence: 75%

Seismic anisotropy beneath the lower half of the North Island, New Zealand

Marson-Pidgeon¹,

Savage²,

Gledhill³

et al. 1999

J. Geophys. Res.

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“…Trench‐parallel (30°) fast anisotropy and 4.4 ± 0.9% was determined within the slab from local earthquakes beneath Wellington [ Matcham et al , 2000]. Similar directions were also determined for the within‐slab mantle from nearby regions from P waves [ Galea , 1993; Chadwick , 1997] and for the nearby unsubducted oceanic mantle from surface waves [ Shearer and Orcutt , 1986; Yu and Park , 1994]. We therefore consider that trench‐parallel anisotropy within the slab is likely, due to fossil anisotropy from the formation of the oceanic lithosphere.…”

Section: Discussionmentioning

confidence: 84%

Anisotropic structure under a back arc spreading region, the Taupo Volcanic Zone, New Zealand

2004

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We investigate anisotropy surrounding a continental back arc spreading region, the Central Volcanic Region (CVR), in the North Island of New Zealand and present a simple method for spatial averaging to examine heterogeneous anisotropy. S phases from local earthquakes yield consistent trench‐parallel fast directions (ϕ) in the southern, compressional region but varied directions in the northern, extensional region. In the forearc east of the CVR, ϕ is nearly trench‐parallel, suggesting shear in the mantle or trench‐parallel flow. In the western CVR, local phases from 50 to 80 km depth give ϕ parallel to extension (120°), and phases from 150 to 220 km depth yield similar ϕ (150°). These results suggest asthenospheric flow with olivine a axes oriented in the extension direction down to 100–150 km depth. In the eastern CVR, local phases show mixed ϕ, suggesting that it is a transition between the western CVR and the forearc. Trench‐parallel ϕ for shallow local events within the CVR may be caused by fluid‐filled cracks. Trench‐parallel ϕ for deeper local events and for SKS phases can be explained by standard olivine fabrics developed by lithospheric shearing, by trench‐parallel flow, and by fossil anisotropy in the subducting oceanic lithosphere. Alternatively, in the CVR they could be caused in part by hydrous minerals at the slab‐mantle upper interface, which could cause olivine a axes to orient parallel to the trench due to a maximum shear stress oriented perpendicular to the trench along the slab. We suggest that trench‐perpendicular flow in the extensional region helps to drive fluids away from the slab, while in the compressional region, trench‐parallel flow fails to distribute the fluids, explaining the location of changes in geophysical properties.

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“…Thus crustal anisotropic mechanisms such as distributed faults [ Faccenda et al , 2008] cannot explain the extensive trench‐parallel anisotropy. For the upper mantle of the Pacific plate, Galea [1993] inferred fast N62E anisotropy using Hikurangi and Kermadec slab earthquakes to 100‐km depth, with an unexplained increase in anisotropy with earthquake depth. This observation could be related to relatively high anisotropy in the subducted portion of the Pacific plate compared to the unsubducted plate.…”

Section: Discussionmentioning

confidence: 99%

Three‐dimensional distribution of seismic anisotropy in the Hikurangi subduction zone beneath the central North Island, New Zealand

Eberhart‐Phillips¹,

Reyners²

2009

J. Geophys. Res.

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[1] We use earthquake arrival time data in the central North Island, New Zealand, in inversion for 3-D Vp azimuthal anisotropy. It is parameterized with an isotropic component and two azimuthal anisotropy parameters for each node, and local earthquake shear wave splitting observations define the initial anisotropy model. Our results suggest that mantle flow, either within the mantle wedge or below the subducted slab, is not the primary source of anisotropy in the Hikurangi subduction zone. The largest region of anisotropy that we image is within the subducted slab, where anisotropy is consistently trench-parallel and 5-9% magnitude from 32 to 185 km depth. Bending-induced yielding in the slab offshore and as it passes through the trench provides an explanation for this slab anisotropy. Beneath the Taupo Volcanic Zone (TVZ), there is 1-4% trench-normal anisotropy within 30 km of the surface of the slab, within a region entrained with motion of the slab. In contrast, only very weak (0-2%) anisotropy is imaged in the region of high-temperature partial melt arising from active corner flow in the mantle wedge. In the crust geologic structure is important, with the largest crustal anisotropy (14%) related to schist. We image significant anisotropy aligned with the margins of the TVZ, related to extensive fracturing on the margins as the TVZ actively extends. Within the heavily intruded and underplated TVZ lower crust, the orientation of anisotropy switches to align with the extension direction. This orientation is consistent with other geophysical data suggesting the presence of connected melt in this region.

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Upper mantle anisotropy in the S.W. Pacific from earthquake travel-time analysis

Cited by 16 publications

References 28 publications

Seismic anisotropy beneath the lower half of the North Island, New Zealand

Seismic anisotropy beneath the lower half of the North Island, New Zealand

Anisotropic structure under a back arc spreading region, the Taupo Volcanic Zone, New Zealand

Three‐dimensional distribution of seismic anisotropy in the Hikurangi subduction zone beneath the central North Island, New Zealand

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