Trouble with this sentence: 未曽有の被害をもたらした７月の西日本豪雨災害

Question

未曽有の被害をもたらした７月の西日本豪雨災害。その災害から私たちを守ってくれるはずの避難所で、命の危険を感じて被災した自宅に引き返した人たちがいます。「食物アレルギー」がある人や、アレルギーのある家族がいる人たちです。避難所にアレルギーに対応した食品がなかったため、避難所にとどまることができなかったのです。災害大国とも呼ばれる日本で、なぜ教訓が生かされないのか？被災地の１つ、広島県の自治体を通して見えてきた課題を取材しました。(source)

未曽有の被害をもたらした７月の西日本豪雨災害 means that ７月の西日本豪雨 is a disaster that has caused unprecedented damages but I don't understand if we only talk about heavy rains or all types of disasters(for the damage comparison), if it's unprecedented in comparison to other heavy rains/disasters in western Japan, in Japan, or in the world?

Also, I don't know what is the (omitted)object of the verb 引き返す(the verb appears as transitive on jisho)?

Answer 1

未曽有の is a fixed no-adjective that literally means "unparalleled in history" or "unprecedented", but it's merely a Buddhism-origin fixed phrase and not a technical term. (character-wise, 未 = not-yet, 曽 = in the past, 有 = exist.) You should not take it too seriously and wonder what is compared with it.

If you are interested in the actual fact, 2018 Western Japan Flood is the deadliest one caused by rain in the last 36 years in Japan (after Nagasaki Flood in 1982, which not many people remember), but of course there are several deadlier floods happened in this period in the world. This is the worst natural disaster that happened in Japan after Tohoku Earthquake in 2011, which everyone still remembers. Taking these facts into consideration, we can say this 未曾有 is used in the sense of "worst one caused by rain in the last few decades in Japan", but this does not mean 未曾有 itself has such a specific definition.

引き返す meaning "to head back" is an intransitive verb. While the transitivity of a compound verb tends to be determined by the second verb, there are always exceptions, and jisho.org made a mistake. (They make tons of mistakes regarding word class...)

Answer 2

未曽有の被害をもたらした７月の西日本豪雨災害
Fair question. The sentence is ambiguous. There is no way to know what it is comparing to.
But it can't be about all kinds of disasters, because there have been much worse disasters in the past.
So the sentence is most certainly referring only to heavy rains in Japan.

According to the dictionary ひきかえす has several meanings, but one of them is "to return".
ひきかえす【引（き）返す】進んできた道をもとへ戻る。ひっかえす。
https://dictionary.goo.ne.jp/jn/183678/meaning/m0u/引返す/

So it can intransitive.

自宅に引き返した人 means "people who returned to their homes".

Answer 3

西日本豪雨災害 is the name of a disaster that happened this July in Western Japan. The Japan Meteorological Agency decides the name if a natural disaster caused serious damage. The naming rule is 'Year + Month + Area + Type Of Disaster'. You can omit the year and month if it happened quite recently. When the JMA mentions a disaster as 'unprecedented', it is so compared to other disasters that have been observed in Japan.