Actually, I think that Diego Garcia is an atoll, a low lying island surrounded by coral reefs, and not a volcanic island. Ascension Island is the volcanic mount, hence all the warnings about rip currents to swimmers there.
Topography did save the B.I.O.T. from being another member of the catastrophe, but on a larger scale than local.
From Wikipedia :
The island and base were unaffected by the tsunami caused by the 2004 Indian Ocean earthquake. Service personnel on the island reported only a minor increase in wave activity. The island was protected by its favourable ocean topography. East of the atoll lies the 650-kilometre-long Chagos Trench, an underwater canyon plunging more than 5000 metres. The depth of the Trench and its grade to the shore makes it difficult for tsunami to build before passing the atoll, In addition, undersea coral reefs may have dissipated much of the waves' impact[4]
Apologies if I'm mistaken about the geography and Happy New Year.