This time, I was startled by the magnitude of the earthquake.
I was surprised by the big earthquake which we haven't had for quite a while.
ご質問ありがとうございます。
① "This time,"＝「今回は」
"I was startled by the magnitude of the earthquake."＝「私は地震の大きさに驚かされた。」
② "I was surprised by the big earthquake"＝「私は大きい地震に驚いた」
"which we haven't had for quite a while."＝「（大きい地震は）最近無かった。」
ご参考に。
アメリカ合衆国 
