Earthquakes occur as a result of a build up of pressure between colliding sections of the Earth's crust. These sections, known as continental plates, meet at "fault lines". According to classical earthquake theory, small earthquakes should continue to grow into large earthquakes until they spread all along the fault line. However, cities such as San Francisco and Tokyo experience lots of small earthquakes that do not grow into large ones. Geologists believe that this is because regions in the fault line act as barriers to the shock waves and gradually damp down the earthquake. Now a group of US physicists has used equations describing charge density waves in superconductors to create a model of a fault zone. Their model seems to successfully predict the stress and statistical patterns associated with earthquakes (J Rundle et al 1998 Phys. Rev. Lett . 80 5698).