I'm worried about the fact that their will probably be a massive/big earthquake someday.
I worry that there will probably be a massive/big earthquake someday.
ご質問ありがとうございます。
「いつか」＝someday
「巨大」＝huge, massive, big, gigantic. catastrophic
「地震」＝earthquake
「〜かもしれません」＝probably, most likely, supposed to be
「心配する」＝worry
地震みたいな自然災害の話ではcatastrophicは特にいい言葉だと思います。
例文：I worry that there might be a catastrophic earthquake someday.
ご参考になれば幸いです。
I'm quite worried we're going to have a massive earthquake one of these days.
この場合、次のような言い方ができますよ。
ーI'm quite worried we're going to have a massive earthquake one of these days.
「いつか巨大地震が起こるかもしれないとかなり心配しています」
a massive earthquake で「巨大地震」
one of these days で「いつか・近いうちに」
ご参考まで！
アメリカ合衆国 
カナダ 
