We present a high‐resolution three‐dimensional (3‐D) anisotropic P wave velocity (Vp) model in the northern Hikurangi margin offshore Gisborne, New Zealand, constructed by tomographic inversion of over 430,000 first arrivals recorded by a dense grid of ocean bottom seismometers. Since the study area covers a region where shallow slow slip events (SSEs) occur repeatedly and the subduction of a seamount is proposed, it offers an ideal location to link our understanding of structural and hydrogeologic properties at megathrust faults to slip behavior. The Vp model reveals an ~30‐km‐wide, low‐velocity accretionary wedge at the toe of the overriding plate, where previous seismic reflection studies show a series of active thrust faults branching from the plate interface. We find some locations with significant Vp azimuthal anisotropy >5% near the branching faults and the deformation front. This finding suggests that the anisotropy is not ubiquitous and homogeneous within the overriding plate, but more localized in the vicinity of active thrust faults. The fast axes of Vp within the accretionary wedge are mostly oriented to the plate convergence direction, which is interpreted as preferentially oriented cracks in a compressional stress regime associated with plate subduction. We find that the magnitudes of anisotropy are roughly equivalent to values found at oceanic spreading centers, where the extensional stress regime is dominant and the crack density is expected to be higher than subduction zones. This consideration may indicate that additional effects such as fault foliation and clay mineral alignment also contribute to upper plate anisotropy along subduction margins.

中文翻译：

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NZ3D实验得出的北部Hikurangi边缘的三维P波速度结构：断层各向异性的证据

我们在新西兰吉斯本近海的Hikurangi边缘北部，提出了高分辨率的三维（3D）各向异性P波速度（Vp）模型，该模型是通过对密布的海底网格记录的430,000多次首次到达的层析成像反演而构造的地震仪。由于研究区域覆盖浅慢滑动事件（SSE）反复发生的区域，并提出了海底俯冲技术，因此它为将我们对大推力断层的构造和水文地质特性的理解与滑动行为联系在一起提供了理想的位置。该副总裁该模型揭示了上覆板脚趾处约30公里宽的低速增生楔形物，以前的地震反射研究表明，一系列活动的逆冲断层从板面界面分支出来。我们在分支断层和变形锋附近发现了一些Vp方位各向异性明显大于5％的位置。这一发现表明，各向异性在上覆板块内并非普遍存在，而是均匀分布的，而更多地局限于活动逆冲断层附近。Vp的快轴增生楔形内的应力主要朝向板的收敛方向，这被解释为与板俯冲有关的压应力状态下的优先取向的裂缝。我们发现各向异性的大小大致等于在海洋扩张中心发现的值，在海洋扩张中心，拉应力状态占主导地位，并且裂纹密度预计高于俯冲带。这种考虑可能表明，诸如断层裂隙和粘土矿物排列等附加效应也沿俯冲带引起了上板各向异性。