The scale used to measure an earthquake is called the Richter Scale.
芮氏地震規模（Richter magnitude scale），亦稱地震規模（local magnitude，ML）、又譯裡氏、黎克特製震級，是表示地震規模大小的標度。
aftershock = 餘震
tremor = 震動; 微動
An earthquake that measures 5.0 on the Richter scale has a shaking amplitude 10 times larger than one that measures 4.0.
A 5.0 earthquake will release 31.6 times more energy than a 4.
Sources:
http://earthquake.usgs.gov/learn/topics/richter.php
http://en.wikipedia.org/wiki/Richter_magnitude_scale
http://mishunderstanding.wordpress.com/2011/01/23/what-is-the-richter-scale/
http://timeboy.pixnet.net/blog/post/18280579