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。