The early stage of the postseismic phase is characterized by a large deformation rate. Its analysis is thus key to decipher the role played by different mechanisms (afterslip and viscoelasticity) at various time scales. Here, we process GPS data to obtain 30-s kinematic position time series recording the surface deformation following the M_w 9.0 Tohoku-Oki megathrust earthquake (2011), and combine them with static solutions over 9 years. We analyze the temporal evolution of the time series and use these observations to image the postseismic slip. We find that the first month of deformation following Tohoku-Oki can be explained by an afterslip mechanism, that exhibits an “Omori-like” decay, with a p-value around 0.75 almost everywhere with the exception of a small region around Ibaraki prefecture where p ∼ 1 is observed. This p < 1 indicates that the postseismic displacements do not increase logarithmically with time as predicted by rate-and-state rheology. Instead, we argue that early afterslip is associated with a transient brittle creep mechanism. We use numerical simulations to show that an exponent of p < 1 can be explained by a combination of thermal activation of local slips and elastic interactions. Over longer time scales, an additional mechanism is required to explain the observed deformation signal, and the transient brittle creep mechanism is combined with viscoelastic relaxation modeled by a Newtonian flow. The spatial analysis reveals two distinct afterslip regions, a major one on the North, associated with a p-value around 0.75, and a smaller one close to the Ibaraki aftershock, associated with p ∼ 1.

中文翻译：

---

瞬态脆性蠕变机制解释了 2011 年 Tohoku-Oki 大推力地震的早期震后阶段：高速 GPS 解决方案的观测

震后阶段早期的特点是变形率大。因此，它的分析对于破译不同机制（后滑和粘弹性）在不同时间尺度上所起的作用至关重要。在这里，我们处理 GPS 数据以获得 30 秒运动学位置时间序列，记录M _w 9.0 Tohoku-Oki 大逆冲地震（2011 年）后的地表变形，并将它们与 9 年的静态解结合起来。我们分析了时间序列的时间演变，并使用这些观察结果对震后滑动进行了成像。我们发现 Tohoku-Oki 之后的第一个月变形可以用后滑机制来解释，这种机制表现出“大森式”衰减，p- 值几乎在所有地方都在 0.75 左右，茨城县周围的一个小区域除外，那里观察到p  ∼ 1。这个p  < 1 表明地震后位移不会像速率和状态流变学预测的那样随时间以对数方式增加。相反，我们认为早期后滑与瞬态脆性蠕变机制有关。我们使用数值模拟来表明p的指数 < 1 可以通过局部滑移的热激活和弹性相互作用的组合来解释。在更长的时间尺度上，需要额外的机制来解释观察到的变形信号，并且瞬态脆性蠕变机制与牛顿流动模拟的粘弹性松弛相结合。空间分析揭示了两个不同的后滑区，一个位于北部的主要后滑区，其p值约为 0.75，另一个靠近茨城县余震的较小区，与p  ∼ 1 相关。