The signature and mechanics of earthquake ruptures along shallow creeping faults in poorly lithified sediments
The signature and mechanics of earthquake ruptures along shallow creeping faults in poorly lithified sediments
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不良岩化沉积物沿浅层爬行断层地震破裂的特征和机制
作者：
Balsamo F
地震滑动发生在沿着浅蠕动断层在不良石化沉积物代表了一个未解决的悖论，主要是由于我们对蠕动断层的力学认识不足，缺乏记录的地质证据表明如何同震破裂叠印蠕变在近地表条件。在这里，我们描述了地震破裂传播的签名沿着浅蠕动断层影响松散的弧前沉积物。对变形带为主的断层带的野外观测表明，断层核部有广泛的片理化碎裂岩，局部覆盖着由薄的（0.5-1.5 cm）黑色断层泥层（此处为黑色断层泥）装饰的尖锐滑动面。与片理化碎裂岩相比，黑色断层泥的粒度、孔隙度和渗透率要低得多。此外，由于地震滑动期间的摩擦加热，它们具有与高温（180-200 °C）相容的独特矿物组合。在亚震滑动速率（≤0.1 m/s）下对宿主沉积物进行的实验室实验也产生了叶状碎裂岩，显示出高残余摩擦（µf= 0.65）和应变硬化行为。在地震（1 m/s）滑动速率下进行的实验中产生了黑色泥，由于动态弱化，显示出低残余摩擦（µf= 0.3）。我们的研究结果表明，黑色泥代表了一个潜在的诊断标志，地震断层浅蠕动断层。这些发现有助于理解地壳浅部断层活动性与地震活动性的时空划分。
Seismic slip episodically occurring along shallow creeping faults in poorly lithified sediments represents an unsolved paradox, largely due to our poor understanding of the mechanics governing creeping faults and the lack of documented geological evidence showing how coseismic rupturing overprints creep in near-surface conditions. Here we describe the signature of seismic ruptures propagating along shallow creeping faults affecting unconsolidated forearc sediments. Field observations of deformation band–dominated fault zones show widespread foliated cataclasites in fault cores, locally overprinted by sharp slip surfaces decorated by thin (0.5–1.5 cm) black gouge layers (herein, black gouge). Compared to foliated cataclasites, black gouges have much lower grain size, porosity, and permeability. Moreover, they are characterized by distinct mineralogical assemblages compatible with high temperatures (180–200 °C) due to frictional heating during seismic slip. Foliated cataclasites were also produced by laboratory experiments performed on host sediments at subseismic slip rates (≤0.1 m/s), displaying high residual friction (µf= 0.65) and strain-hardening behavior. Black gouges were produced during experiments performed at seismic (1 m/s) slip rates, displaying low residual friction (µf= 0.3) due to dynamic weakening. Our results show that black gouges represent a potential diagnostic marker for seismic faulting in shallow creeping faults. These findings can help understanding the time-space partitioning between aseismic and seismic behavior of faults at shallow crustal levels.
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