Unfortunately, an event like has happened before. In 1883, the Island of Krakatoa split itself in half with a volcanic eruption. The geological tools of the day found that the shockwaves went around the earth 7 times. Even the English channel was affected. Yeah, not too small. Krakatoa was uninhabited, but thousands of people died from four tsunamis and some died from choking on the ash in the air.
A warning is not possible without a rapid response system in place. If you inform the proper authorities, you have to convince them you know what you're talking about. Then, they have to convince someone to listen to them, and it goes on and on. Bureaucracy is there to prevent bad things from happening rapidly. You need a rapid response emergency plan or system to get word form the US Geological Society to remote beaches in Sri Lanka. Let me give you an example. The US ran an antiterrorism drill. The people and agencies involved knew beforehand they would be participating. The drill failed miserably because the 40+ agencies involved couldn't communicate. If US government agencies couldn't communicate efficiently and work together in the same city with preparation AND a "system", how can we get word to Sri Lanka that a wave is travelling toward them at 500 miles an hour without some sort of rapid response system?
I see a lot of tension in this thread, and I understand people being angry. I mean, someone in the world knew that thousands of people were going to die and could do nothing about it. It makes us feel helpless. Now, we have to donate money and push world leaders to set up warning systems.
I just saw Jack Hanna on Anderson Cooper 360. He made a good point. He said that people have built cities and resorts over coastal mangroves. These mangroves are there for a reason. They would absorb the majority of tsunamis. I'm not a "hippie" (but I have nothing against them), but there is a message there. We can inhabit those places, but we do so at a price. Look at what happened to Florida during the hurrican season. Hanna also discussed the "sixth sense" of animals. He explained in much less mystical terms that were every bit as interesting.