The Collaborative International Dictionary of English v.0.48:

Thereat \There*at"\, adv.
   1. At that place; there.
      [1913 Webster]

            Wide is the gate, and broad is the way, that leadeth
            to destruction, and many there be which go in
            thereat.                              --Matt. vii.
                                                  13.
      [1913 Webster]

   2. At that occurrence or event; on that account.
      [1913 Webster]

            Every error is a stain to the beauty of nature; for
            which cause it blusheth thereat.      --Hooker.
      [1913 Webster] Therebefore