Judging by his response to South Shore it will probably be a strongly worded letter.

A black (or was it dark?) shaman!
From under Orgrimmar... yeah

Thrall...but he wouldn't do something like that to Jaina.

This would actually be a great way to help both Thrall and Jaina's characters grow if it was Thrall that destroyed Theramore. There are two ways I could see that going down:

1) We already know Garrosh is going to become corrupted. So say Garrosh tricks Thrall. He begs Thrall to come help him, there is some convincing reason he gives that Theramore needs to be tsunami'd right now, or the world will be in grave danger. Thrall is forced to choose between saving the greater good or saving a small human town. He chooses the former. Thrall and Jaina both must come to terms with this choice.

2) There actually IS a reason to tsunami Theramore. I don't know what it could be-- but it would have to be big, with far-reaching consequences, and the rest plays out as above.

A third way that still involves Thrall but less directly is say there is a tsunami, or something, approaching Theramore. Jaina pleads with Thrall to come help, she can't do it on her own, and the draenei and dwarf shaman are not powerful enough. Thrall is torn between some Big Important Event with World Consequences (as above) or saving Theramore from this natural disaster. Again, coming to terms, blah blah blah.

It would be so good for both characters and the story as a whole if it was Thrall who did this to Jaina, but the writers would have to come up with a good reason for Thrall to do this (tricked into thinking it was for the greater good, or it actually being such) because Thrall is not a dick, as much as some players think he is. ;P

I just cannot imagine Thrall doing it for two reasons:

1.) His friendship with Jaina. Unless we're talking Orgim being destroyed and every orc facing extinction, it just is not in Thrall's character to do something so destructive.

2.) I would assume Thrall is on the shelf as a character for most of the MoP expansion. I mean, assuming a year has gone by since the end of Cataclysm, he's probably busy being the father he never knew to his child, plus trying to help Azeroth heal. It just seems too early for him to come back into play.

So, let's assume it's not Thrall. Who else is there who might have this level of power? Or we could go another direction - how do we know this isn't a strike by Azshara? Granted, that does not make a lot of sense right now, but I don't have anything better at the moment.

Come to think of it, wouldn't a wave that size also impact Ratchet and Azshara?

I just assumed she fell off a dock or her boat sunk or something normal like that.

I'm thinking the same, it seems like a Tsunami of that magnitude would have far-reaching effects.

But then, look at Booty Bay. It was hit by a wave, and it should not even exist anymore, but nearly every building is intact with sea debris all that's left behind.