alive
地震后很多人被活埋了.
alive在这里做主语补足语,也就是补充说明主语people的.
所以提问的这句话相当于：
Many people were buried when they were still alive after the earthquake.
其他例子：
I painted the door green.我把门漆成绿色.
I（主语） painted（谓语） the door（宾语） green（宾语补足语）.
The door was painted green.门被漆成绿色.
The door（主语） was painted（谓语） green（主语补足语）.