The most common explanation, and one that I find repulsive, is that such natural disasters are punishment from God for something or the other. But as Kettle writes:
Certainly the giant waves generated by the quake made no attempt to differentiate between the religions of those whom it made its victims. Hindus were swept away in India, Muslims were carried off in Indonesia, Buddhists in Thailand. Visiting Christians and Jews received no special treatment either. This poses no problem for the scientific belief system. Here, it says, was a mindless natural event, which destroyed Muslim and Hindu alike.
A non-scientific belief system, especially one that is based on any kind of notion of a divine order, has some explaining to do, however. What God sanctions an earthquake? What God protects against it? Why does the quake strike these places and these peoples and not others? What kind of order is it that decrees that a person who went to sleep by the edge of the ocean on Christmas night should wake up the next morning engulfed by the waves, struggling for life?
Also read: Douglas Adams on atheism.