The planet's deadliest earthquake of the century, by far, was a magnitude 8.0 that struck Tianjin (formerly Tangshan), China, on July 27, 1976. The official casualty figure issued by the Chinese government was 255,000, but unofficial estimates of the death toll were as high as 655,000.
Massive flooding of the Yangtze River in China in 1931 caused more than 3 million deaths from flooding and starvation.
http://www.usgs.gov/public/press/public_affairs/press_releases/pr1133m.html
I was thinking pandemics. The plague wiped out 1/3 of China and 1/2 of Europe. AIDS is slower but just as deadly. Flu killed 500,000 in the USA in 1918.