High Fluid-Pressure Patches Beneath the Decollement: A Potential Source of Slow Earthquakes in the Nankai Trough off Cape Muroto
High Fluid-Pressure Patches Beneath the Decollement: A Potential Source of Slow Earthquakes in the Nankai Trough off Cape Muroto
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滑脱下方的高流体压力斑块：室户岬附近南海海槽慢地震的潜在来源
DOI：
10.1029/2021jb021831
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发表时间：
2021
期刊：
影响因子：
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通讯作者：
Y. Kubo
中科院分区：
文献类型：
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作者：
T. Hirose;Y. Hamada;W. Tanikawa;N. Kamiya;Y. Yamamoto;T. Tsuji;M. Kinoshita;V. B. Heuer;F. Inagaki;Y. Morono;Y. Kubo
孔隙压力在俯冲带地震的发生中起着关键作用。然而，其确定的数量限制非常有限。在这里，我们通过分析国际海洋发现计划 (IODP) 第 370 次远征队室户岬附近南海海槽 C0023A 钻孔钻井泥浆的瞬态上涌流来估算地下孔隙压力。这种向上流动提供了室户角逆冲沉积物中超压含水层的第一个直接证据。为了估计海底以下 950-1,050 米深度的超压含水层中的钻前孔隙压力，我们检查了从附近 IODP 井取回的岩心样品的测量孔隙度；然后，我们通过求解径向扩散方程对各种尺寸的含水层进行建模，继续解释所观察到的上升流流速的时间演变。据观察，渗透率为 10−13m2 时，含水层的初始超孔隙压力比静水压力高约 5–10 MPa，横向尺寸为数百米，厚度为数十米。 C0023 站点孔隙度-深度剖面的超压估计与该地区其他钻探点的超压估计不同，这表明南海海槽的逆冲沉积物中可能存在多个呈斑状分布的超压含水层。由于孔隙压力与维持断层稳定性有关，超压含水层可能是钻井现场周围观察到的慢地震的来源。
Pore pressure plays a key role in the generation of earthquakes in subduction zones. However, quantitative constraints for its determination are quite limited. Here, we estimate the subsurface pore pressure by analyzing the transient upwelling flow of drilling mud from borehole C0023A of the International Ocean Discovery Program (IODP) Expedition 370, in the Nankai Trough off Cape Muroto. This upward flow provided the first direct evidence of an overpressured aquifer in the underthrust sediments off Cape Muroto. To estimate the pre‐drilling pore pressure in the overpressured aquifer around a depth of 950–1,050 m below sea floor, we examined the measured porosities of core samples retrieved from nearby IODP wells; we then proceeded to explain the observed time evolution of the flow rate of the upwelling flow by modeling various sized aquifers through solving a radial diffusion equation. It was observed that for a permeability of 10−13m2, the aquifer possessed an initial excess pore pressure of ∼5–10 MPa above the hydrostatic pressure, with a lateral dimension of several hundred meters and thickness of several tens of meters. The overpressure estimates from the porosity‐depth profile at Site C0023 differ from those at other drill sites in the region, suggesting the possible existence of multiple overpressured aquifers with a patchy distribution in the underthrust sediments of the Nankai Trough. As pore pressure is relevant in maintaining fault stability, the overpressured aquifers may be the source of slow earthquakes that have been observed around the drilling site.