Why No Tsunami This Time?
The original earthquake took place along the edge of a major ocean trench, and was a result of the Indus plate slipping under the Sunda (or Burma - I can't remember which) plate. This kind of fault is sometimes called a plunging fault. What happens when a portion of one plate slips under another is a tremendous release of energy, and usually some movement along either or both plate boundaries. The upper plate is usually shoved up even higher, and the lower plate is shoved down (See Figure 1). Whenever the land moves up and down, and there is contact with water, the energy of the land's movement is transferred to the water, and you have one or more tsunami waves.
This latest earthquake, based upon where it's located and its depth, was more likely a simple horizontal strike, or one where the two plates slid on top of one another. It also appears that there was little up/down motion, which is what causes tsunamis.