Two earthquakes (M8.6 and 8.2) occurred off the coast of Sumatra on April 11, 2012. Why didn't they generate a tsunami as the 9.0 did in 2004? Watch this animation and learn more at: http://www.iris.edu/hq/retm and http://earthquake.usgs.gov/earthquakes/eqinthenews/2012/usc000905e/