Albert Barnes on Matthew 28:2

There was a great earthquake. Rather, there had been. It does not mean that this was while they were there, or while they were going, but that there had been so violent a commotion as to remove the stone. The word here rendered earthquake does not of necessity mean that the convulsion extended to the earth, but only that there had been such a concussion as to remove the stone.

And sat upon it. Sat upon it when the keepers saw him. It is not said that he was sitting when he appeared to the women: from Luke it would rather appear that he was standing.

(1) "there was" or "there had been"