Frictional Strengths of Subduction Thrust Rocks in the Region of Shallow Slow Earthquakes
Frictional Strengths of Subduction Thrust Rocks in the Region of Shallow Slow Earthquakes
复制标题
DOI：
10.1029/2019jb018888
复制
发表时间：
2020-03
期刊：
影响因子：
--
通讯作者：
N. Phillips;B. Belzer;M. French;C. Rowe;K. Ujiie
中科院分区：
文献类型：
--
作者：
N. Phillips;B. Belzer;M. French;C. Rowe;K. Ujiie
预计地震将在速度减弱的物质中成核;然而，在俯冲孕震区的上倾极限，主要岩性表现出速度增强行为。在日本的Mugi Mélange（南海）现今俯冲的一个类似物中，有两个古地震特征的例子发生在蚀变玄武岩中，这表明它们可能是速度减弱的。在现场变形条件下（Pc = 120 MPa，T = 150 °C），采用三轴锯切构造从混杂岩中剪切蚀变玄武岩和页岩基质，两个孔隙流体因子（λ = 0.36，0.7）。页岩基质表现出0.4至0.5之间的摩擦系数和速度强化行为。蚀变玄武岩表现出Byerlee摩擦和速度弱化行为。在大多数实验中，变形被分割成里德尔剪切，这有一个较低的百分比的碎屑，碎屑粒度分布具有较高的分形维数，和形状因子指示圆碎屑相比，里德尔剪切之间的矩阵。我们假设，地震优先核内蚀变玄武岩在孕震区的上倾极限。更复杂的变形形式（例如，浅层极低频地震）可能是通过将速度弱化蚀变玄武岩混入速度强化页岩基质中而发生的。在基质由页岩组成的俯冲带中，这种复杂的行为仅限于浅深度（T < ~250 °C），高于基质的向上倾斜过渡到速度弱化行为。蚀变玄武岩是一种普遍存在的俯冲物质，未来需要通过一系列俯冲带条件对其行为进行研究，以全面了解俯冲带的力学行为。
Earthquakes are expected to nucleate within velocity‐weakening materials; however, at the updip limit of the subduction seismogenic zone, the principal lithologies exhibit velocity‐strengthening behavior. At an exhumed analogue for present‐day subduction at Nankai (the Mugi Mélange, Japan) two examples of paleoseismic features occur within altered basalts, suggesting that they may be velocity‐weakening. We shear altered basalt and shale matrix from the mélange in the triaxial saw cut configuration at in situ conditions of deformation (Pc = 120 MPa, T = 150 °C) for two pore fluid factors (λ = 0.36, 0.7). The shale matrix exhibits a coefficient of friction between 0.4 and 0.5, and velocity‐strengthening behavior. Altered basalt exhibits Byerlee friction and velocity‐weakening behavior. In most experiments deformation was partitioned into Riedel shears, which have a lower percentage of clasts, clast size distributions with higher fractal dimensions, and shape factors indicating rounder clasts compared to the matrix between Riedel shears. We hypothesize that earthquakes preferentially nucleate within altered basalt at the updip limit of the seismogenic zone. More complex forms of deformation (e.g., shallow very low frequency earthquakes) may occur by mixing velocity‐weakening altered basalt into the velocity‐strengthening shale matrix. In subduction zones where the matrix is composed of shale, this complex behavior is limited to shallow depths (T < ~250 °C), above the updip transition to velocity‐weakening behavior for the matrix. Altered basalt is a ubiquitous subducting material, and future studies on its behavior through a range of subduction zone conditions are required for a full understanding of the mechanical behavior of subduction zones.