During most of the time between large earthquakes at tectonic plate boundaries, surface displacement time series are generally observed to be linear. This linear trend is interpreted as a result of steady stress accumulation at frictionally locked asperities on the fault interface. However, due to the short geodetic record, it is still unknown whether all interseismic periods show similar rates, and whether frictionally locked asperities remain stationary. Here we show that two consecutive interseismic periods at Simeulue Island, Indonesia experienced significantly different displacement rates, which cannot be explained by a sudden reorganization of locked and unlocked regions. Rather, these observations necessitate the occurrence of a 32-year slow-slip event on a shallow, frictionally stable area of the megathrust. We develop a self-consistent numerical model of such events driven by pore-fluid migration during the earthquake cycle. The resulting slow-slip events appear as abrupt velocity changes in geodetic time series. Due to their long-lived nature, we may be missing or mis-modelling these transient phenomena in a number of settings globally; we highlight one such ongoing example at Enggano Island, Indonesia. We provide a method for detecting these slow-slip events that will enable a substantial revision to the earthquake and tsunami hazard and risk for populations living close to these faults.

在构造板块边界的大地震之间的大部分时间里，地表位移时间序列通常被观察到是线性的。这种线性趋势被解释为断层界面上摩擦锁定的凹凸体稳定应力积累的结果。然而，由于大地测量记录较短，是否所有的震间期都显示出相似的速率，以及摩擦锁定的凹凸体是否保持静止仍是未知数。在这里，我们表明印度尼西亚 Simeulue 岛的两个连续地震间期经历了显着不同的位移率，这不能用锁定和解锁区域的突然重组来解释。相反，这些观察结果需要在大逆冲的一个浅层、摩擦稳定的区域发生 32 年的慢滑事件。我们开发了地震周期期间由孔隙流体迁移驱动的此类事件的自洽数值模型。由此产生的慢滑事件表现为大地时间序列中的突然速度变化。由于它们长期存在的性质，我们可能会在全球许多环境中遗漏或错误地模拟这些瞬态现象；我们在印度尼西亚的恩加诺岛重点介绍了一个这样的例子。我们提供了一种检测这些慢滑事件的方法，这将使居住在这些断层附近的人群的地震和海啸危险和风险得到实质性的修正。我们可能在全球的许多环境中遗漏或错误地模拟了这些瞬态现象；我们在印度尼西亚的恩加诺岛重点介绍了一个这样的例子。我们提供了一种检测这些慢滑事件的方法，这将使居住在这些断层附近的人群的地震和海啸危险和风险得到实质性的修正。我们可能在全球的许多环境中遗漏或错误地模拟了这些瞬态现象；我们在印度尼西亚的恩加诺岛重点介绍了一个这样的例子。我们提供了一种检测这些慢滑事件的方法，这将使居住在这些断层附近的人群的地震和海啸危险和风险得到实质性的修正。