Seismic precursors revealed by statistical physics methods are important for earthquake forecasting and helpful for understanding fault behaviour. One of the precursors, earthquake migration, has been observed in simulated models and rock experiments, as well as anecdotal observations for large earthquakes along major fault zones. However, there has been no systematic methodology for investigating earthquake migration in nature, and only a small number of studies have observed earthquake migration at large scales. In this work, we show that migration of small earthquakes towards the epicentre before large earthquakes can be visualized. We present a method for detecting small earthquake migration in the spatiotemporal domain, and retrospectively apply it to two recent examples of major earthquakes in Taiwan. We observe that anomalous seismicity occurs increasingly close to the epicentre as the occurrence time is approached. At that point, the bulk of the seismicity concentrates on the epicentre, accumulating stress on the rupture zone and leading to the full rupture. Our findings imply a new type of large-scale pre-seismic earthquake migration pattern that is complementary to the well-studied patterns of source-region activation and quiescence.
