YOU SAID:

unbenounced to his mother the earthqake flabbergasted his dad

INTO JAPANESE

彼の母親には知らされていなかったが、地震は彼の父親を驚かせた

BACK INTO ENGLISH

Unbeknownst to his mother, the earthquake stunned his father.

INTO JAPANESE

彼の母親は知らなかったが、地震は彼の父親を驚かせた.

BACK INTO ENGLISH

Unknown to his mother, the earthquake stunned his father.

INTO JAPANESE

母親は知らなかったが、地震は父親を驚かせた。

BACK INTO ENGLISH

Unknown to his mother, the earthquake stunned his father.