The intensity of an earthquake can be expressed in terms of the destruction it causes. Eye witness accounts of earthquake damage at the surface are matched to the Mercalli Intensity Scale which consists of an arbitrary 12 step scale originally designed by Giuseppe Mercalli. The scale relies on subjective information from people who witnessed the earthquake. From their information it is possible to determine the epicenter and the relative earthquake strength.
The intensity of an earthquake at any particular place depends not only on the magnitude of the earthquake, but also on the distance from the epicenter, depth of the focus, and on the surface and sub-surface geological conditions.
The intensity decreases outwards from the focus, forming concentric zones of declining intensity radiating out from the epicenter.
The strength of an earthquake can also be measured and expressed in terms of the magnitude of energy released by the earthquake.
Seismologist Charles Richter devised his system based on the amplitude of the seismic waves recorded by seismographs. When the data from several seismometers are compared a measure of magnitude can be realized within minutes of the earthquake activity.
The Richter scale is logarithmic. An increase in magnitude of one unit corresponds to a tenfold increase in the size of an earthquake. Thus an earthquake of magnitude 6 is ten times larger than one of magnitude 5, and a hundred times larger than one of magnitude 4.