The tsunami took hours not days to cross the Indian Ocean. You would think this would be enough time to give someone a call and save a few thousand lives.
This is in a part of the world where a whole village may not have a single telephone, apart from early tsunami warning systems not having been set up for the Indian Ocean. However, an estimated 130,000 plus people died in Indonesia, where there would not have been hours of warning, since the earthquake was just off the coast of Sumatra.
Although it’s thought that most of the 100,000 Tokyo fatalities came from the March 9 raid, it probably also includes numbers from the February 23 attack. There was a further raid on May 26, although there was very little standing east of the Imperial Palace by then.
Still, though, a lot of the deaths were probably from disease due to poor sanitation after the disaster, so a fair fraction of those deaths were probably not on the same day.
It’s probably not in the running, but does anyone know how many were killed by the eruption of Vesuvius?
Not likely in a single day however.
I was going to suggest the mass human sacrifices by the Aztecs but this Wiki article suggests that estimates may be exaggerated.
Aztec sacrifice
The tsunami wave travelled at around 500 mph in open water. There was indeed time to warn people, but there was no infrastructure in place at all; compound that with the fact that nobody knew how far it would go, or that it was travelling both east and west, and that the maximum recorded deep ocean surge was only 60cm above normal levels. Thailand was hit unawares, two hours after the earthquake, as was Sri Lanka about four hours later; when the Maldives were hit, Kenya and Somalia were also warned. Kenya managed to evacuate its coastal areas - to my knowledge, only 1 person died. Somalia, however, had no such communications infrastructure, so despite the wave arriving seven hours after the earthquake, hundreds were also killed.