We investigated the seismic velocity structure of the Hikurangi margin in New Zealand to uncover the physical features of the subduction zone and explore the relationships between microearthquake seismicity, seismic velocity structure, and slow slip events. Using local earthquake tomography with data collected from both temporary ocean bottom seismometers and on-land permanent seismic stations, we used the tomography code TomoFD to iteratively perform a damped least squared inversion of absolute P and S arrival times to obtain relocated hypocenters and generate 3D velocity models for Vp and Vp/Vs. The seismic tomography images show two high Vp/Vs anomalies, one offshore and adjacent to a subducted seamount and the other beneath the North Island of New Zealand. The ∼50-km wide offshore anomaly extends ∼10 km beneath the plate interface and lies directly beneath the area that slipped at least 50 mm during the 2 week-long 2014 slow slip event. High Vp/Vs values may be related to high pore fluid pressures from subducted sediments, and such increases in pore fluid pressures have been suggested to trigger the occurrence of slow slip events in active subduction zones. The second onshore high Vp/Vs anomaly is located in the overlying plate and subducting slab and correlates with areas suggested by other geophysical techniques to be rich in fluids. Our seismic imaging supports interpretations that subduction processes in the Hikurangi margin are highly dependent on physical features such as subducted seamounts and fluid-rich sediments.

中文翻译：

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新西兰Hikurangi俯冲带的地震速度非均质性：重复发生缓慢滑动事件的地区孔隙压力升高

我们调查了新西兰Hikurangi边缘的地震速度结构，以揭示俯冲带的物理特征，并探讨了微地震地震活动性，地震速度结构和慢滑事件之间的关系。使用本地地震层析成像和从临时海底地震仪和陆上永久地震台收集到的数据，我们使用层析成像代码TomoFD来迭代地执行绝对P和S到达时间的阻尼最小二乘反演，以获得重新定位的震源并生成3D速度Vp和Vp / Vs的模型。地震层析成像图像显示了两个高Vp / Vs异常，一个异常在海上，与俯冲海山相邻，另一个异常在新西兰北岛下方。距海岸线约50 km的近海异常延伸至板块界面下方约10 km，并位于2014年为期2周的缓慢滑动事件中至少滑移了50 mm的区域的下方。高的Vp / Vs值可能与俯冲沉积物中的高孔隙流体压力有关，并且已经提出，孔隙流体压力的这种增加会触发活跃俯冲带中缓慢滑动事件的发生。第二个陆上高Vp / Vs异常位于上覆板块和俯冲板块中，并且与其他地球物理技术建议的富含流体的区域相关。我们的地震成像支持这样的解释，即Hikurangi边缘的俯冲过程高度依赖于物理特征，例如俯冲的海山和富含流体的沉积物。