～（よ）うものなら vs ～となったら

Question

I encountered the following multiple-choice question on page 71 (question 5 in section "1~6") of my JLPT N2 grammar textbook 「新完全マスター文法　日本語能力試験Ｎ２」:

大きい地震が（　　　）、この家はたちまち壊れてしまうだろう。

ａ 起こらないことには　　　ｂ 起ころうものなら　　　ｃ 起こるとなったら

I chose 「ｃ 起こるとなったら」, but the answer given was 「ｂ 起ころうものなら」. I understand why 「ｂ 起ころうものなら」 makes sense, since this is a clear case of 「もし～たら、大変なことになる」, which is the definition of ～（よ）うものなら given by my textbook. However, I am not sure why 「ｃ 起こるとなったら」 is wrong.

My textbook explains ～となったら as

～としたら・～とすれば・～とすると・～となったら・～となれば・～となると

～と仮定した場合・～ということになった場合、そうする・そうなる。

[...]「～となったら・～となれば・～となると」は、実現する可能性があることにつき、それが実現した場合を考えている。[...]後には話者の判断などを表す文が来る。

Under this definition, 「ｃ 起こるとなったら」 seems to make sense to me:

(c)（？）大きい地震が起こるとなったら、この家はたちまち壊れてしまうだろう。

This house would probably collapse instantly if a big earthquake were to happen.

「大きい地震が起こる」 is a condition that may possibly be fulfilled (実現する可能性があること), and 「この家はたちまち壊れてしまうだろう」 is the speaker's judgment, so the sentence seems to fit the above explanation. Why is the sentence wrong, and what am I missing here?

Answer 1

This なる is similar in function to that in [V dictionary form]-ことになる.

When you say 地震が起こるとなったら, you are not putting yourself in a situation in which the earthquake has already happened. For that, you would simply say 地震が起こったら. Rather, you are supposing a situation in which it has been just determined that the earthquake will happen, or you have just learned it. The house would be still standing.

Answer 2

If the sentence with option c is read or heard without care, few native speaker would feel awkward. If careful, majority would not choose c (Some would, though).

Most probable reason for awkwardness I can think of is that "となったら" is usually followed by something contrary to the foreseeable/expected future. For example:

• 奴らが乗り込んでくるとなったら戦うぜ（戦わないつもりだったが）。
• 台風が来るとなったら旅行は中止するしかない（どんなに行きたくても）。

In your case, the house must be a rat-trap (because it is presumed to collapse たちまち with an earthquake) and likely to be broken down sooner or later. If the sentence were "...この家ですら壊れてしまうだろう," option c would fit.

"ようものなら" is usually followed by something which accelerates/makes more probable the foreseeable/expected future.