An
earthquake is the result of a sudden release of energy in the
Earth's crust that creates
seismic waves. The
seismicity,
seismism or
seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time. Earthquakes are measured using observations from
seismometers. The
moment magnitude is the most common scale on which earthquakes larger than approximately 5 are reported for the entire globe. The more numerous earthquakes smaller than magnitude 5 reported by national seismological observatories are measured mostly on the local magnitude scale, also referred to as the
Richter scale. These two scales are numerically similar over their range of validity. Magnitude 3 or lower earthquakes are mostly almost imperceptible and magnitude 7 and over potentially cause serious damage over large areas, depending on their depth. The largest earthquakes in historic times have been of magnitude slightly over 9, although there is no limit to the possible magnitude. The most recent large earthquake of magnitude 9.0 or larger was a
9.0 magnitude earthquake in Japan in 2011, and it was the largest Japanese earthquake since records began. Intensity of shaking is measured on the modified
Mercalli scale. The shallower an earthquake, the more damage to structures it causes, all else being equal.
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![Interior of "Earthquake House"](http://s0.geograph.org.uk/photos/24/20/242053_6cb344b3.jpg "Interior of "Earthquake House"")
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