AFTER the Landers earthquake, which shook southern California in 1992, seismologists were confused by a series of aftershocks in distant locations, such as Long Valley, more than 400km away. Alan Linde, at the Carnegie Institution of Washington DC, reporting in Nature, thinks he has found the reason for them. As pressure waves from an earthquake's epicentre spread outwards they become weaker. But, weak though they are, they may still shake loose gas bubbles in molten rock some distance from the epicentre. As the gas bubbles rise to the surface they may then amplify the pressure waves into full aftershocks.