A (micro)physical understanding of the transition from frictional sliding to plastic or viscous flow has long been a challenge for earthquake cycle modeling. We have conducted ring‐shear deformation experiments on layers of simulated calcite fault gouge under conditions close to the frictional‐to‐viscous transition previously established in this material. Constant velocity (v) and v‐stepping tests were performed, at 550°C, employing slip rates covering almost 6 orders of magnitude (0.001–300 μm/s). Steady‐state sliding transitioned from (strong) v‐strengthening, flow‐like behavior to v‐weakening, frictional behavior, at an apparent “critical” velocity (v_cr) of ~0.1 μm/s. Velocity‐stepping tests using v < v_cr showed “semi‐brittle” flow behavior, characterized by high stress sensitivity (“n‐value”) and a transient response resembling classical frictional deformation. For v ≥ v_cr, gouge deformation is localized in a boundary shear band, while for v < v_cr, the gouge is well‐compacted, displaying a progressively homogeneous structure as the slip rate decreases. Using mechanical data and post‐mortem microstructural observations as a basis, we deduced the controlling shear deformation mechanisms and quantitatively reproduced the steady‐state shear strength‐velocity profile using an existing micromechanical model. The same model also reproduces the observed transient responses to v‐steps within both the flow‐like and frictional deformation regimes. We suggest that the flow‐to‐friction transition strongly relies on fault (micro)structure and constitutes a net opening of transient microporosity with increasing shear strain rate at v < v_cr, under normal stress‐dependent or “semi‐brittle” flow conditions. Our findings shed new insights into the microphysics of earthquake rupture nucleation and dynamic propagation in the brittle‐to‐ductile transition zone.

中文翻译：

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模拟方解石泥中的流变摩擦过渡：实验和微物理建模

从摩擦滑动到塑性或粘性流的过渡的（微观）物理理解长期以来一直是地震周期建模的挑战。我们已经在模拟方解石断层泥的层上进行了环-剪切变形实验，其条件接近于该材料先前建立的摩擦-粘稠过渡。在550°C下进行了恒速（v）和v步进测试，滑移率覆盖了几乎6个数量级（0.001–300μm/ s）。稳态滑动从（强）转变v -strengthening，流动状行为v -weakening，摩擦特性，在表观的“关键”速度（v _CR）〜0.1μm/ s。使用v  <  v _cr的速度步进测试显示出“半脆性”流动行为，其特征在于高应力敏感性（“ n值”）和类似于经典摩擦变形的瞬态响应。对于v  ≥  v _CR，圆凿变形局部存在于边界的剪切带，而对于v  <  v _CR，凿子压实得很好，随着滑移率的降低，其逐渐显示出均匀的结构。以机械数据和事后微观结构观察为基础，我们推导了控制剪切变形的机制，并使用现有的微力学模型定量再现了稳态剪切强度-速度分布图。相同的模型还再现了在流动和摩擦变形状态下观察到的对v阶跃的瞬态响应。我们认为，流变摩擦过渡强烈地依赖于断层（微观）结构，并在v  <  v _cr处随着剪切应变率的增加而构成了瞬态微孔的净开口。在正常的应力依赖性或“半脆性”流动条件下。我们的发现为脆性-延性过渡带中的地震破裂成核和动态传播的微观物理学提供了新的见解。