I think you could improve your writing by planning and structuring it a bit better:
Last week a massvive earthquake hit a small tomw in Japan. It was a highly populated area, and no one had ever expected such a disaster to befall.Here, you start with a specific example, then move to a general statement about two natural disasters, introducing floods. Why not do it the other way around and make the general statement, then give the example:
Earthquake and flood are two of the most terrible disasters in the world. When an earthquake occurred, almost everything was caught unprepared. Buildings and homes were pulled down. Hundreds of people perished while thousands were seriously got injured.
Earthquake and floods are two of the most terrible disasters in the world. Last week a massive earthquake hit a small town in Japan. It was a highly populated area, and no one had ever expected such a disaster to befall.
This seems more logical to me. Then the next part is a descritpion, but of what? Is it a description of the earthquake in Japan, or an earthquake in general? The reason this is not clear is because you say 'an eathquake', which sounds general, and the the past tense, which suggests you are describing the events in japan. Let's say you are describing the Japanese earthquake:
When the earthquake occurred, almost everything was caught unprepared. Buildings and homes were pulled down. Hundreds of people perished while thousands were seriously injured.The change to 'the' is a tiny one, but it liks everything up better, especially after the change of sequence at the beginning.
After this, you start introducing other information about survivors stranded- is this Japan or another event? I live in Japan and I haven't heard of any this in the news. If you're describing another event, then shouldn't you tell the reader that you are changing? You then bring in cholera after the flood- which flood? I think what you're doing is writing a bit fast- you have the ideas in your head, but you must always remember the reader- will they know what you are talking about? I think you are mixing the general statements and your eamples without giving enough information for us to know.
Take the following:
In any natural disasters, the aftermath is sometimes just more serious than the disaster itself, as it always leave a lot of sufferings and hardships on people. Not only were suvivors left stranded and freezing in the open, but also that had no food, no tents and quilts to live on.Now, what I suggest you do is look carefuly at the tenses- why have you changed from the present (talking in general) to the past? Are you now describing a factual event? If so, which one? I could guess that you are referring to the appalling tragedy in pakistan, but that's guessing from the news recently. If you have made that jump, shouldn't you have told your reader? Try rewriting just this bit and we'll look at it again. ;-)