A discrete episode of seismic and aseismic deformation of the Nankai trough subduction zone accretionary prism and incoming Philippine Sea plate
A discrete episode of seismic and aseismic deformation of the Nankai trough subduction zone accretionary prism and incoming Philippine Sea plate
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DOI：
10.1016/j.epsl.2005.11.054
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发表时间：
2006-02-15
影响因子：
5.3
通讯作者：
Kinoshita, M
中科院分区：
文献类型：
--
作者：
Davis, EE;Becker, K;Kinoshita, M
对南开海槽俯冲带幕式变形的新见解来自两个大洋钻探计划钻孔水文观测站(1173站)和南开吸积棱镜向海部分(808站)，以及四国岛(Hi-net)的高灵敏度钻孔加速度计和速度地震仪阵列(Hi-Net)。在每个海上钻孔中监测到的多个水平的流体压力记录了稳定的长期趋势，表明进入板块地点的地壳和沉积部分收缩，以及吸积棱柱脚趾的松弛。从1173点(高达5kPayr(-1))的压力上升速率推算出的应变速率(约10(-6)yr(-1))与如果收敛的弹性应变分布在几十公里宽的区域上所产生的板块收敛所产生的速率相似。与这些趋势相反的是，2003年6月底/7月初观察到的瞬时压力异常表明，进入的板块迅速松弛，外部棱镜收缩。在这些瞬变的同时，一群极低频率的地震在棱镜中更远的陆地上发生，高于目前被锁定的俯冲推力的地震孕育部分的估计向海极限。对于这个群体中的一个事件和另一个事件中的几个事件确定的反向故障机制也表明棱镜的瞬时收缩。我们认为，地震和压力瞬变都是2003年6月26日左右在俯冲推力锁定部分的上倾极限或附近开始的无地震滑移位错的结果。并在大约10天的时间里沿着分隔棱柱和俯冲段的滑脱向海面扩散到增生棱柱脚趾。像这样的变形事件可能会逐渐减轻俯冲推覆过程中局部的应力，并加载邻近地区。除了证明棱镜在两次地震间隔期间远不是不活动的之外，这些观测还可能提供大俯冲地震时应变和水文响应的小幅度模拟。(C)2005 Elsevier B.V.保留所有权利。
New insights into episodic deformation at the Nankai trough subduction zone are provided by data from two Ocean Drilling Program borehole hydrologic observatories drilled into the Philippine Sea plate (Site 1173) and the seaward part of the Nankai accretionary prism off southwestern Japan (Site 808), and from an array of high-sensitivity borehole accelerometer and velocity seismometers on Shikoku Island (Hi-net). Fluid pressures monitored at multiple levels in each of the offshore boreholes document steady secular trends that indicate contraction of the crust and sedimentary section at the incoming plate site, and relaxation of the accretionary prism toe. The rate of strain (ca. 10(-6) yr(-1)) inferred from the rates of pressure rise at Site 1173 (up to 5 kPa yr(-1)) is similar to that which would be produced by plate convergence if convergent elastic strain were distributed over a region a few tens of kin wide. Opposing these trends are transient pressure anomalies observed in late June/early July 2003 that indicate an episode of rapid relaxation of the incoming plate and contraction of the outer prism. Concurrent with these transients, a swarm of very-low-frequency earthquakes occurred farther landward in the prism above the estimated seaward limit of the currently locked seismogenic portion of the subduction thrust. Reverse-fault mechanisms determined for one event of this swarm and several of another also indicate transient contraction of the prism. We suggest that both the earthquakes and the pressure transients are the consequences of an aseismic slip dislocation that initiated on about June 26, 2003, at or near the up-dip limit of the locked portion of the subduction thrust. and propagated seaward over the course of about 10 days to the accretionary prism toe along the decollement separating the prism and underthrust section. Deformational events like this may serve incrementally to relieve stress locally along the subduction thrust and to load neighboring areas. In addition to demonstrating that the prism is far from inactive during interseismic intervals, the observations may also provide a small-amplitude analog for strain and hydrologic response at the time of great subduction earthquakes. (c) 2005 Elsevier B.V. All rights reserved.