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

Abstract

[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.

Seismic anisotropy tomography: New insight into subduction dynamics

“…Significant anisotropy aligned with the margins of the TVZ was revealed, which may be related to extensive fracturing on the margins as the TVZ actively extends. Within the heavily intruded and underplated TVZ lower crust, the FVD switches to align with the extension direction, which is consistent with other geophysical data suggesting the presence of connected melt in this region (Eberhart-Phillips and Reyners, 2009).…”

“…A Vp anisotropy tomography in the central North Island, New Zealand was determined using local-earthquake arrival-time data (Eberhart-Phillips and Reyners, 2009). The 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.…”

Mapping P-wave anisotropy of the Honshu arc from Japan Trench to the back-arc

“…The trench-parallel anisotropy may be also due to the dextral shearing of the overlying crust, as suggested by EberhartPhillips and Reyners (2009). Our results also show trench-parallel FVD in the upper part of subducting Pacific slab beneath the entire NE Honshu arc. Such a feature is also found in the Hikurangi subduction zone (Eberhart-Phillips and Reyners, 2009). There are at least two possibilities.…”

Mantle flow in subduction systems: The mantle wedge flow field and implications for wedge processes

“…P wave travel time inversions in which parameters describing anisotropy are included represent another technique that can place constraints on wedge anisotropy (as well as anisotropy in other parts of subduction systems). This technique has recently been applied in several different well‐instrumented subduction zones with good raypath coverage in the wedge, including Hikurangi [ Eberhart‐Phillips and Reyners , ], NE Japan [ Wang and Zhao , ], SW Japan [ Wang and Zhao , ], and Alaska [ Tian and Zhao , ]. The constraints on P wave anisotropy thus obtained are complementary to splitting studies and occasionally present conflicting views on wedge anisotropy and processes.…”