According to US scientists, there is an urgent need to speed up work on a reliable system for predicting potential aftershocks in the days following a strong earthquake has become more urgent. Writing in Nature, researchers said that the magnitude 8.6 earthquake that struck off the coast of Sumatra, Indonesia, on 11 April this year unleashed an unprecedented number of large events as far away as Japan and Mexico. "The number of earthquakes worldwide of more than [magnitude 5.5] increased by a factor of five over a six-day period," Roland Burgmann, a professor in the Department of Earth and Planetary Science at the University of California, United States."No other recorded earthquake has triggered as many large aftershocks around the world. We believe this was because it was the largest 'strike-slip' earthquake (where the two sides of a fault slip horizontally past each other) ever recorded, involving horizontal motions.