A natural disaster is a disaster because it involves a lot of human suffering, not because the event itself is especially big or spectacular. The destruction of an uninhabited island by a volcano is not a natural disaster, because it doesn't really matter to humans. A landslide doesn't matter, however enormous, unless there is a town at the bottom of the hill.
So what does the word ‘natural' add? We use it to demarcate the edges of responsibility. We don't use it very well.