The long precursory phase of most large interplate earthquakes

Many earthquakes are preceded by foreshocks(1,2). However, the mechanisms that generate foreshocks and the reason why they occur before some earthquakes and not others are unknown(3-8). Here we use seismic catalogues from the best instrumented areas of the North Pacific to analyse the foreshock sequences preceding all earthquakes there between 1999 and 2011, of magnitude larger than 6.5 and at depths shallower than 50 km. The data set comprises 31 earthquakes at plate boundaries, and 31 in plate interiors. We find that there is a remarkable contrast between the foreshock sequences of interplate compared with intraplate earthquakes. Most large earthquakes at plate interfaces in the North Pacific were preceded by accelerating seismic activity in the months to days leading up to the mainshock. In contrast, foreshocks are much less frequent in intraplate settings. We suggest that at plate boundaries, the interface between the two plates begins to slowly slip before the interface ruptures in a large earthquake. This relatively long precursory phase could help mitigate earthquake risk at plate boundaries.