Earthquakes, Richter scale's, Seismograph's and Mercalli Scale

http://en.wikipedia.org/wiki/Earthquake
http://en.wikipedia.org/wiki/Seismometer
http://en.wikipedia.org/wiki/Richter_magnitude_scale

http://science.howstuffworks.com/nature/natural-disasters/earthquake.htm
http://news.bbc.co.uk/2/hi/4126809.stm

mercalli:
http://en.wikipedia.org/wiki/Mercalli_intensity_scale