Earthquakes occur where 2 tectonic plates jam together, with one trying to slide over the other. Ultimately, they 'give', sending shockwaves. If the epicenter happens to be underwater, tsunamis result. The Sumatran earthquakes in 2004 and 2005 had very different results, with 2004 resulting in a tsunami that leveled shores thousands of miles away, while the 2005 quake was only felt regionally. Scientists determined that the sediments were the key difference in the 2 epicenters, less than 100 miles apart.