Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust
Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust
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DOI：
10.1029/2012gl053793
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发表时间：
2012-12-01
影响因子：
5.2
通讯作者：
Saffer, Demian M.
中科院分区：
文献类型：
--
作者：
Kitajima, Hiroko;Saffer, Demian M.
最近的地震反射和海底地震仪(OBS)研究揭示了沿南开俯冲板块边界的大片低地震速度区域，日本西南部近海的巨型金字塔。这些低速带(LVZ‘s)从海沟向陆地延伸约55公里，相当于海底以下10公里的深度。在这里，我们通过结合地球物理测量获得的P波速度和实验室变形测试确定的P波速度、孔隙度和有效平均应力之间的新的受约束的经验关系来估计这些低压区内的现场孔压和应力状态。我们记录了延伸到俯冲带的超孔压为17-87 Mpa，接近55公里，表明捕获的孔隙流体支持沿上板块底部和周围主要断裂带的类似45%-91%的覆岩应力。在LVZ中产生的有效应力被限制在类似于非超压条件下期望值的1/3。这些低的有效应力应该会导致一个机械上较弱且主要是无地震的板块边界。最低有效应力区域与精确定位的甚低频地震重合，首次提供了将这些异常滑动事件与低应力和高孔压联系起来的定量证据。引用：北岛，H.和D.M.Saffer(2012)，沿南开海槽俯冲巨型逆冲的低频地震区域的孔压升高和异常低的应力，地球物理。莱特，39，L23301，DOI：10.1029/2012GL053793。
Recent seismic reflection and ocean bottom seismometer (OBS) studies reveal broad regions of low seismic velocity along the Nankai subduction plate boundary megathrust offshore SW Japan. These low velocity zones (LVZ's) extend similar to 55 km landward from the trench, corresponding to depths of >similar to 10 km below sea floor. Here, we estimate the in-situ pore pressure and stress state within these LVZ's by combining P-wave velocities obtained from the geophysical surveys with new well-constrained empirical relations between P-wave velocity, porosity, and effective mean stress defined by laboratory deformation tests on drill core samples of the incoming oceanic sediment. We document excess pore pressures of 17-87 MPa that extend similar to 55 km into the subduction zone, indicating that trapped pore fluids support similar to 45-91% of the overburden stress along the base of the upper plate and surrounding major fault zones. The resulting effective stresses in the LVZ are limited to similar to 1/3 of the values expected for non-overpressured conditions. These low effective stresses should lead to a mechanically weak and predominantly aseismic plate boundary. The region of lowest effective stress coincides with precisely located very low frequency earthquakes, providing the first quantitative evidence linking these anomalous slip events to low stress and high pore pressure. Citation: Kitajima, H., and D. M. Saffer (2012), Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust, Geophys. Res. Lett., 39, L23301, doi: 10.1029/2012GL053793.