Most large earthquakes happen at plate boundaries, though a significant number originate in the middle of continental plates (p.36). The majority of earthquakes go unnoticed by humans. In the build-up to an earthquake, stress accumulates in a volume of rock. The stress comes about because of the plates, whether slipping past one another or one platen subducting under the continental crust. At the point and time where stress exceeds the strength of the rock, fracture takes place. The fracture travels out through the region of stressed rock, and energy is released in all directions as seismic waves. Seismographs to measure earthquake shaking are highly sensitive devices, capable of recording earthquake waves from the far side of the globe. The traces recorded on seismographs can be read by seismologists to determine the location of a distant earthquake (its epicentre) and how energetic it was (its Richter magnitude). Intensity, based on the Mercalli scale, is compiled from eyewitness reports and from estimates of the response of buildings.