Rapid rupture fronts—akin to earthquakes—mediate the transition to frictional motion. Once formed, their singular form, dynamics and arrest are well described by fracture mechanics. Ruptures, however, first need to be created within initially rough frictional interfaces. Hence, static friction coefficients are not well defined, with frictional ruptures nucleating over a wide range of applied forces. A critical open question is, therefore, how the nucleation of rupture fronts actually takes place. Here we experimentally show that rupture fronts are preceded by slow nucleation fronts—self-similar entities not described by fracture mechanics. They emerge from initially rough frictional interfaces at a well-defined stress threshold, evolve at the characteristic velocity and timescales governed by stress levels, and propagate within a frictional interface to form the initial rupture from which fracture mechanics take over. These results are of fundamental importance to questions ranging from earthquake nucleation and prediction to processes governing material failure.

类似于地震的快速破裂前沿介导了向摩擦运动的过渡。一旦形成，它们的奇异形式、动力学和停滞就可以通过断裂力学得到很好的描述。然而，首先需要在最初粗糙的摩擦界面内产生破裂。因此，静摩擦系数没有很好地定义，摩擦破裂在广泛的作用力范围内成核。因此，一个关键的悬而未决的问题是，破裂前沿的成核实际上是如何发生的。在这里，我们通过实验表明，破裂前沿之前是慢成核前沿——断裂力学没有描述的自相似实体。它们以明确的应力阈值从最初的粗糙摩擦界面出现，以特征速度和受应力水平控制的时间尺度演变，并在摩擦界面内传播以形成断裂力学接管的初始断裂。这些结果对于从地震成核和预测到控制材料失效的过程等问题都具有重要意义。