The ocean canyons are very deep in that area of the Indian Ocean and this prohibits develop of tsunamis.
URL = www.globalsecurity.org...
Initial indications are that Diego Garcia was not affected by the Andaman Tsunami of 26 December 2004. It is located south of the tip of India, well with in range of what the tsunami, with a max elevation of 22 and an average elevation of only 4 feet. Civilians monitoring shortwave radio reported on rec.radio.shortwave that a female operator, in answer to a query from an aircraft after giving weather information, reported no ill effects from the earthquake.
Officials said the Diego Garcia Navy Support Facility, [...] suffered no damage related to the earthquake and ensuing tsunamis. [...] Diego Garcia, the southernmost island in the Chagos Archipelago, sits about 1,000 miles south of India and roughly 2,000 miles from the earthquake’s epicenter. Even though an earthquake like Sunday’s will radiate destructive waves in all directions, the damage caused by the water differs greatly depending on the undersea topography.
Favorable ocean topography minimized the tsunami’s impact on the atoll. Diego Garcia is part of the Chagos Archipelago, situated on the southernmost part of the Chagos-Laccadive Ridge. To the east lies the Chagos Trench, a 400 mile long, underwater canyon that ranges in depth from less than 1,00 meters below the surface to depths that plunge to over 5,000 meters. It is one of the deepest regions of the Indian Ocean. Diego Garcia is located to the west of Chagos Trench, which runs north and south. The depth of the Chagos Trench and grade to the shores does not allow for tsunamis to build before passing the atoll. The result of the earthquake was seen as a tidal surge estimated at six feet.
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If you look at this image you can see the enormous expanse of deep water that is non-conducive to allowing tsunami growth.
The darker the red the deeper the sea floor. Diego Garcia base is near the middle of this image (72E, 7S) while the earthquake was at 95E, 3N. Note that I dropped the significant digits in the lat/long numbers for this example. Unmathematically speaking, the quake was about 33 degrees, or 1,980 miles from the base. You could certainly get more exact with spatial programs but I'm supposed to be working...
