Measuring Earthquakes

Earthquakes are recorded by a seismographic network. Each seismic station in the network measures the movement of the ground at the site. The slip of one block of rock over another in an earthquake releases energy that makes the ground vibrate.
Earthquake size, as measured by the Richter Scale is a well known, but not well understood, concept.
Moment is a physical quantity proportional to the slip on the fault times the area of the fault surface that slips; i
Intensity scales, like the Modified Mercalli Scale and the Rossi-Forel scale, measure the amount of shaking at a particular location.
Intensity is a qualitative measure of the strength of ground shaking at a particular site. The Mercalli Scale is used to determine the intensity. 
For earthquakes that occurred between about 1890 (when modern seismographs came into use) and 1935 when Charles Richter developed the magnitude scale, people went back to the
When an earthquake occurs, one of the first questions is "where was it?" The location may tell us what fault it was on and where damage (if any) most likely occurred. 
The duration of an earthquake is related to its magnitude but not in a perfectly strict sense. There are two ways to think about the duration of an earthquake.
UTC stands for Coordinated Universal Time, and GMT stands for Greenwich Mean Time. The time that earthquakes occur around the world is reported in UTC or GMT, which are essentially the same.
Magnitude calculations are based on a logarithmic scale, so a ten-fold drop in amplitude decreases the magnitude by 1. If an amplitude of 20 millimetres as measured on a seismic signal corresponds to a magnitude 2 earthquake, then: