Which of these 20th century natural disasters caused the most casualties?

The 1931 China floods, or the 1931 'Yangtze–Huai River Floods', were a series of devastating floods that occurred in the Republic of China. It is known as one of the most lethal natural disasters of the 20th century.

China was deeply afflicted by a devastating drought in the early 1930's. The winters were particularly harsh, creating large deposits of snow in mountainous areas. The melting snow flowed downstream and arrived in the middle Yangtze River during periods of heavy rain. The floods eventually inundated an area equivalent in size to England and half of Scotland, or the states of New York, New Jersey and Connecticut combined. The Chinese estimated that 25 million people had been affected by the flood. Historians say that the true number may have been as many as 53 million. Fatality estimates range from at least 423,500 to 4 million.

The 'Great Galveston Hurricane', known regionally as the 'Great Storm of 1900', created almost 12,000 fatalities in the US. Most of these deaths occurred in and near Galveston, Texas.

The 1952 Great Smog of London was a severe air-pollution event that affected the British capital of London. A period of cold weather, combined with an anticyclone and windless conditions was reported to have killed 4,000-12,000 people.

'The 1906 California Earthquake' ranks as one of the worst earthquakes in history. The earthquake struck the California coast with an estimated magnitude of 7.9 and approximately 3,000 people died.