Fault slip activity at subduction zones is governed by sediment frictional properties, which in turn are affected by diagenetic processes. To study the spatial patterns in frictional properties across the Nankai Trough, SW Japan, and their relations to fault slip activity, we used sediment samples (10%–59% clay mineral content) along the Kumano transect covering a large spatial range from the inputs via the outer prism to the inner prism, including the deepest sample ever recovered to date. We performed laboratory friction experiments under in situ effective normal stresses and seawater-saturated conditions. Our results generally demonstrate that the friction coefficient inversely correlates with the clay mineral content. However, the outer prism sediments show higher friction coefficients than sediments from the other locations for a comparable clay mineral content. All samples show velocity-weakening behavior at low velocities, but the outer prism and the deep inner prism sediments show velocity strengthening at higher velocities. Based on the experimental results combined with a Coulomb wedge model, we propose that the lowest friction coefficient on the décollement occurs beneath the trenchward portion of the outer prism, whereas the minimum friction coefficient of the prism sediment occurs in the landward portion of the outer prism. In addition, the calculated critical nucleation length for slip instability suggests that the décollement beneath the outer prism area is more frictionally unstable than it is beneath the inner prism. This inference is consistent with the spatial distribution of very-low-frequency earthquakes and slow slip events along the shallow Nankai Trough.

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南海海槽熊野断面沉积物摩擦行为的空间格局

俯冲带的断层滑动活动受沉积物摩擦特性的控制，而后者又受成岩过程的影响。为了研究日本西南部南海海槽摩擦特性的空间模式及其与断层滑动活动的关系，我们使用了沿熊野断面覆盖大空间范围的沉积物样本（10%–59% 粘土矿物含量）通过外棱镜到内棱镜，包括迄今为止发现的最深的样本。我们在原位进行了实验室摩擦实验有效的法向应力和海水饱和条件。我们的结果通常表明摩擦系数与粘土矿物含量成反比。然而，对于可比的粘土矿物含量，外棱柱沉积物显示出比来自其他位置的沉积物更高的摩擦系数。所有样品在低速下都表现出速度减弱行为，但外棱柱和深内棱柱沉积物在较高速度下表现出速度增强。结合库仑楔模型的实验结果，我们提出，在外棱柱的开沟部分下方，décollement 的摩擦系数最低，而棱柱沉积物的最小摩擦系数出现在外棱柱的向陆部分。 . 此外，计算出的滑动不稳定性临界成核长度表明，外棱柱区域下方的碰撞比内棱柱下方的摩擦更不稳定。这一推论与南海浅海槽的极低频地震和慢滑移事件的空间分布是一致的。