Predicting when large earthquakes might occur may be a step closer to reality, thanks to a new study of undersea earthquakes in the eastern Pacific Ocean. The study, reported in today’s Nature, is the first to suggest that small seismic shocks or foreshocks preceding a major earthquake can be used in some cases to predict the main tremors. Scientists from the Woods Hole Oceanographic Institution (WHOI) and the University of Southern California (USC) report that some types of large undersea earthquakes may be predictable on time scales of hours or less. Earthquakes on land are generally not preceded by systematic immediate foreshocks and hence cannot be predicted as easily with the same methods.