Long-lived shallow slow-slip events on the Sunda megathrust

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.