this is the day the quake happened three years ago
这是个强调句
this is the day 主句
the quake happened 定语从句
three years ago 时间状语定语从句"the quake happened"后面还原句子成分不是"the quake happened on the day"吗？那么原句不应该是"this is the day on which the quake happened three years ago"吗？"你说得没错不过现在这句话只是（省略了）that.还原句子是this is the day （that)the quake happened three years ago