During the 2011 Tohoku-oki earthquake, extremely extensive coseismic slip ruptured shallow parts of the Japan Trench subduction zone and breached the sea floor1, 2. This part of the subduction zone also hosts slow slip events (SSE)3, 4. The fault thus seems to have a propensity for slip instability or quasi-instability that is unexpected on the shallow portions of important fault zones. Here we use laboratory experiments to slowly shear samples of rock recovered from the Tohoku-oki earthquake fault zone as part of the Japan Trench Fast Drilling Project. We find that infrequent perturbations in rock strength appear spontaneously as long-term SSE when the samples are sheared at a constant rate of about 8.5 cm yr−1, equivalent to the plate-convergence rate. The shear strength of the rock drops by 3 to 6%, or 50 kPa to 120 kPa, over about 2 to 4 h. Slip during these events reaches peak velocities of up to 25 cm yr−1, similar to SSE observed in several circum-Pacific subduction zones. Furthermore, the sheared samples exhibit the full spectrum of fault-slip behaviours, from fast unstable slip to slow steady creep, which can explain the wide range of slip styles observed in the Japan Trench. We suggest that the occurrence of SSE at shallow depths may help identify fault segments that are frictionally unstable and susceptible to large coseismic slip propagation.
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机译：在2011年的东北冲地震中，极度广泛的同震滑动使日本海沟俯冲带的浅层破裂并破坏了海底1、2。俯冲带的这一部分还发生了慢滑事件（SSE）3、4。在重要断层带的浅层部分似乎有意想不到的滑移不稳定或准不稳定倾向。在这里，我们作为日本Trench快速钻井项目的一部分，使用实验室实验来缓慢剪切从东北冲木地震断裂带中回收的岩石样品。我们发现，当以大约8.5 cm yr-1的恒定速率剪切样本时（相当于板块收敛速度），岩石强度的不稳定性会像长期SSE一样自发出现。岩石的抗剪强度在约2至4 h的时间内下降了3％至6％，或从50 kPa降至120 kPa。在这些事件中滑移达到最高速度为25 cm yr-1，类似于在几个环太平洋俯冲带观测到的SSE。此外，从快速不稳定滑移到缓慢稳定蠕变，剪切样品表现出了完整的断层滑动特性，这可以解释在日本海沟中观察到的各种滑动类型。我们认为，SSE在浅深度的发生可能有助于识别摩擦不稳定且易于发生大地震同跳的断层段。
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