Geodetic evidence that earthquakes start with precursory slip

The existence of a potentially observable precursory phase of slip on the fault before large earthquakes has been debated for decades. Though observations preceding a handful of large events have been proposed as possible indicators of precursory slip, these observations do not directly precede the earthquakes, are not seen before most events and are commonly observed without being followed by earthquakes. Here we present a global analysis of geodetic measurements prior to large earthquakes, based on high-rate time series recorded before 78 large (Magnitude ≥ 7) seismic events. Our analysis highlights a 2 hour-long exponential signal consistent with acceleration of precursory slip on the fault. Our observation indicates that precursory phases of slip exist and that future improvements in measurement precision could allow to monitor them, making earthquake prediction potentially achievable.