NOTE: I've combined all my various writings about tsunamis from December 2004 and January 2005, including this entry, into a single tsunami article, which might make a good introductory reference.
I am wondering why the Tsunami did not hit Australia which on the map seems to be at least as much on the path as Africa.
- As I mentioned in an earlier post on my site, tsunamis are not necessarily always circles of waves emanating from a point source; they can be directional. In this case, the main paths of the tsunamis were directly to the west and east of the earthquake, not so much north and south. So the hardest-hit areas were directly west and east, regardless of distance. Northwestern Sumatra and Thailand, Sri Lanka and India, and even Somalia (thousands of kilometres and hours away) got big waves. Bangladesh (which is not only nearby, but very low-lying, and so would seem particularly vulnerable), southern Sumatra, much of Burma, Antarctica, and other areas had much smaller waves and less destruction.
- Australia is not only southeast of the earthquake, it is somewhat shielded from it by most of the islands of Indonesia, including Sumatra itself.
The combination of the directional tsunamis and the significant shielding meant that—unlike other partially shielded areas (southwestern Sri Lanka and the southwest coast of India, for instance), where the tsunamis were powerful enough to diffract around the coastlines and still cause damage—even the exposed northwest coast of Australia saw only relatively small effects: larger swells and surf, but no tsunami-style run-up and destruction.