a long time coming vs coming for a long time.

No, they do not have the same meaning. They both have errors.

The first sentence is incorrect for two reasons: (1) the last "coming" should be deleted; (2) tsunamis do not take a long time to come. They come fast and cause disasters.

The second sentence is grammatically correct, but the catastrophic tsunami does not go well with the idiom "a long time coming (待ち遠しい)." This idiom is for something happy or desirable.

Andy and Emily have been dating for ten years. Their marriage is a long time coming.

Jim had been working 7 days a week. His promotion was a long time coming.

ご質問の文で、おっしゃりたいことを察して書くと、こんな感じです。
A catastrophic tsunami has been long anticipated to occur.

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kevin@musicolingo.com