The recent magnitude 8.6 earthquake (11/4/2012) and its aftershock of magnitude 8.2 struck Indonesian island of Sumatra. The mainshock last for five minutes and aftershock last for four minutes. Though tsunami alert was announced but no tsunami of remarkable height was not appeared any coasts of the world specially Sumatran coast.
Now the question is the magnitude 8.6 earthquake was not enough for tsunami? This earthquake off the west coast of northern Sumatra, Indonesia, occurred as a result of strike-slip faulting within the oceanic lithosphere of the Indo-Australia plate. The quake was located approximately 100 km to the southwest of the major subduction zone that defines the plate boundary between the Indo-Australia and Sunda plates offshore Sumatra. At this location, the Indo-Australia plate moves north-northeast with respect to the Sunda plate at a velocity of approximately 52 mm/yr.
This quake seems to be a large earthquake within the Indian Plate and the plate has broken in a sort of lateral way. It's a sort of tearing earthquake, and this is much less likely to cause a tsunami because it's not displacing large volumes of water.