Mantle wedge anisotropy in the Hikurangi subduction zone, central North Island, New Zealand

Abstract

The anisotropic characteristics of the Hikurangi subduction zone in central North Island, New Zealand are studied using shear‐wave splitting measurements from 55 local earthquakes recorded on a 200 km profile of 19 stations. The eastern fore‐arc shows trench‐parallel fast directions and a low average delay time of 0.2 s. The source of anisotropy is confined to the upper 60 km and is likely to be deformation induced within the overriding Australian plate. Central western North Island shows N‐S trending fast anisotropy which may signify the presence of a viscous blanket of mantle material being entrained by the absolute motion of the Australian plate. Between these regions a dramatic switch to extension‐parallel fast directions and delay times of 0.35 s are seen across the back‐arc in the Taupo Volcanic Zone (TVZ). These results suggest asthenospheric flow beneath the overriding plate with the olivine a‐axes oriented in the extension direction down to a maximum depth of 100 km.

Upper mantle anisotropy beneath central and southwest Japan: An insight into subduction-induced mantle flow

“…Those that are sub-parallel to the trench have also been reported in New Zealand (Audoine et al, 2000) and the Shumagin Island section of the Aleutian (Yang et al, 1995). In addition, complex patterns of fast directions have been observed in northeastern (NE) Japan (Okada et al, 1995;Nakajima and Hasegawa, 2004), central Japan and the Kanto area (Fouch and Fischer, 1996;Hiramatsu et al, 1998;Salah et al, 2008), central South America (Polet et al, 2000), south central Alaska (Wiemer et al, 1999), the Lau backarc (Smith et al, 2001), Kamchatka (Levin et al, 2004) and in the north of New Zealand (Morley et al, 2006).…”

The effect of pressure on the elastic properties and seismic anisotropy of diopside and jadeite from atomic scale simulation

“…Such splitting is not insignificant. S-wave splitting from local events around northeastern Japan (Nakajima and Hasegawa, 2004), the New Zealand (Morley et al, 2006) and the Java-Sumatra subduction zones (Hammond et al, 2010) are typically up to 0.5 seconds. Two effects that are excluded from this simple analysis are the increasing garnet component with depth and the corresponding increase in the jadeitic component of the clinopyroxene.…”

Theory and Observations - Seismic Anisotropy

“…In several regions like the Lau Basin of the SW Pacific (Smith et al, 2001), Japan and Kuriles (Nakajima and Hasegawa, 2004;Nakajima et al, 2006), the Ryukyu arc (Long and van der Hilst, 2006), and to a lesser extent in New Zealand (Greve et al, 2008;Morley et al, 2006), networks of seismometers across the back-arc region reveal trench-parallel fast polarization near the trench that rotates to trench-normal in the back-arc extensional region. The same pattern is obtained with core-refracted S waves in the central Apennines of Italy (Amato et al, 1998;Margheriti et al, 1996) and in Japan (Long and van der Hilst, 2005), as can be seen in Figure 18.…”