Evidence for Deeply Subducted Lower‐Plate Seamounts at the Hikurangi Subduction Margin: Implications for Seismic and Aseismic Behavior
Evidence for Deeply Subducted Lower‐Plate Seamounts at the Hikurangi Subduction Margin: Implications for Seismic and Aseismic Behavior
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DOI：
10.1029/2021jb022866
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发表时间：
2021-07
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影响因子：
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通讯作者：
B. Chow;Y. Kaneko;John Townend
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作者：
B. Chow;Y. Kaneko;John Townend
在许多俯冲带都发现了海山，它们充当海底非均质体，影响巨型逆冲断层的滑动行为。在新西兰北岛近海的希库朗吉俯冲带，在太平洋板块的到来和增生棱柱体的下方发现了海山，但几乎没有具体证据表明海山俯冲到今天的海岸线之外。使用北岛的高分辨率伴随层析成像导出的速度模型，我们确定了东海岸以下的两个高速异常和其中一个异常的实验室内低速区上倾。我们将高速异常解释为以前未识别的深俯冲海山，并将低速区解释为俯冲板中的流体。据推测，海山宽10至30公里，位于板块交界面12至15公里深处。使用点扩散函数的分辨率分析证实这些是分辨率良好的特征。这两座海山的位置与水深特征相吻合，而水深特征的几何形状与海山俯冲的模拟实验和数值模拟所预测的几何形状相一致。推断的海山附近的地震活动和慢滑事件的空间特征与以前的数值模拟预测的海山俯冲对巨型逆冲断层应力和滑动的影响一致。以前也在一个或两个海山观测到异常地球物理特征、磁异常和震群活动。我们建议，永久性断裂的北方希库朗吉上部板块反复海山俯冲可能是负责二分法缓慢滑动行为观察到的大地测量，并部分负责沿着走向变化的板块耦合在希库朗吉俯冲界面。
Seamounts are found at many subduction zones and act as seafloor heterogeneities that affect slip behavior on megathrusts. At the Hikurangi subduction zone offshore the North Island, New Zealand, seamounts have been identified on the incoming Pacific plate and below the accretionary prism, but there is little concrete evidence for seamounts subducted beyond the present‐day coastline. Using a high‐resolution, adjoint tomography‐derived velocity model of the North Island, we identify two high‐velocity anomalies below the East Coast and an intraslab low‐velocity zone up‐dip of one of these anomalies. We interpret the high‐velocity anomalies as previously unidentified, deeply subducted seamounts, and the low‐velocity zone as fluid in the subducting slab. The seamounts are inferred to be 10–30 km wide and on the plate interface at 12–15 km depth. Resolution analysis using point spread functions confirms that these are well‐resolved features. The locations of the two seamounts coincide with bathymetric features whose geometries are consistent with those predicted from analog experiments and numerical simulations of seamount subduction. The spatial characteristics of seismicity and slow slip events near the inferred seamounts agree well with previous numerical modeling predictions of the effects of seamount subduction on megathrust stress and slip. Anomalous geophysical signatures, magnetic anomalies, and swarm seismicity have also been observed previously at one or both seamount locations. We propose that permanent fracturing of the northern Hikurangi upper plate by repeated seamount subduction may be responsible for the dichotomous slow slip behavior observed geodetically, and partly responsible for along‐strike variations in plate coupling on the Hikurangi subduction interface.