Because these natural disasters occur frequently in japan, the japanese have become accustomed to mild floods earthquakes tsunamis droughts

The answer to this is Earthquakes. 
In Japan every year, there are upwards of over 100 000 small earthquakes each year. Only a handful of these become larger earthquakes, and only a handful of these are even noticed.
The Earthquake in March of 2011 was one of the largest and most devastating earthquakes in Japanese history.
Hope this helps :)