Adam Clarke on Matthew 28:2

A great earthquake - Σεισμος, a shaking or commotion of any kind: probably the word means no more than the confusion caused among the guards by the angel's appearance. All this had taken place before the women reached the sepulchre.

The angel of the Lord descended from heaven - Matthew is very particular in this, to show that the word angel is not to be taken in the sense of an ordinary messenger, who might have come from Joseph of Arimathea, or from any other; but in the sense of an extraordinary messenger, who descended from God, out of heaven, for this very purpose. It is likely that the angel had descended, rolled away the stone, and was sitting on it, before the women reached the tomb.