I think people a long time ago must have known that very few earthquakes occurred in Nara.
この場合、次のような言い方ができますよ。
ーI think people a long time ago must have known that very few earthquakes occurred in Nara.
「昔の人は奈良でほとんど地震が起こらないことを知っていたに違いない」
must have known「知っていたに違いない」
to occur「起こる」
ーThat's why Nara was probably chosen as the capital during the Nara era.
「だから奈良は奈良時代の都として選ばれたんだろう」
capital「都」
ご参考まで！
People in the past might have known that earthquakes don't happen often in Nara.
I think that the ancient people might have known that earthquakes seldomly happen in Nara.
「奈良はほとんど地震が起きないことを昔の人は知っていた」は英語で「People in the past might have known that earthquakes don't happen often in Nara.」と言います。「地震が起きる」は英語で「earthquakes happen」と言います。TOMOKOさんがおっしゃった文章を自然な言い方に変えると「I think that the ancient people might have known that earthquakes seldomly happen in Nara.」となります。
奈良はほとんど地震が起きないことを昔の人は知っていたと思います。
I think that the ancient people might have known that earthquakes seldomly happen in Nara.
カナダ 
