If there is an earthquake, this building would probably collapse.
If an earthquake happens, this building probably won't be able to withstand it.
ご質問ありがとうございます。
① "If there is an earthquake, this building would probably collapse."=「もし地震があったら、この建物は多分倒れるだろうね。」
② "If an earthquake happens, this building probably won't be able to withstand it."=「もし地震が起きたら、多分、この建物は耐えられないだろうね。」
ご参考までに。