Explanation

Creating a filter: as we blast our way through the original sentence, we should pause at the ambiguous pronoun "it." Furthermore, properly we should not use the word "if," because there is not really a condition in the intended meaning of the sentence. We'll look for another answer choice with a more precise expression.

Applying the filter: choice (B) does not improve on the "if" because the phrase "as an earthquake" seems to state that the blast is or could be an earthquake, so we have a distortion of the intended meaning. In (C), the semicolon is used improperly, leaving a sentence fragment on the latter side. Choice (D) uses the semicolon properly and avoids the problems of (A) and (B), though it's longer. In Choice (E), we lose the whole point that 5.1 on one scale is 7.3 on another. The correct answer is (D).