19th century illustration of cracks or fissures in the Earth's crust caused by seismic activity. In its most general sense, the word earthquake is used to describe any seismic event, whether natural or caused by humans, that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by other events such as volcanic activity, landslides, mine blasts, and nuclear tests. An earthquake's point of initial rupture is called its focus or hypocenter. The epicenter is the point at ground level directly above the hypocenter. The Richter magnitude scale assigns a magnitude number to quantify the energy released by an earthquake. The Richter scale, developed in 1935 defines magnitude as the logarithm of the ratio of the amplitude of the seismic waves to an arbitrary, minor amplitude.