This remains a gray area because Hardy delicately draws a veil over it and does not describe the rape/ seduction. One reason, of course, is that in a nineteenth century novel it would not be possible to provide a graphic description. But I believe it is also because Hardy wants to preserve the modesty of his "pure" heroine. She is ill-used by the world and lovingly, achingly evoked as desirable and vulnerable--much like Wessex. If one reads Tess as a character who symbolizes the rural landscape that is destroyed (raped) by progress than the question becomes a wider one with a more complex response. The rape/ seduction is then a metaphor.
Tess was not raped. In the following chapter she said, "If I had gone for love o' you, if I had ever sincerely loved you, if I loved you still, I should not so loathe and hate myself for my weakness as I do now!... My eyes were dazed by you for a little, and that was all." She stayed with him for a few weeks afterwards and was "dazed" by him. She was not raped because she didn't try to stop him. Thomas Hardy suggests by that statement it may have happened more than once.
The act of Alec can never be considered as seduction. Tess was actually being raped by Alec.