Piratecat said:
As has been stated, most people in stone buildings would be safe. Everyone in the open and in a wooden building would be dead. Whole forests leveled, many tens of thousands of people dead. All cattle dead, too, and fields destroyed. There would be mass starvation (or imports) come winter, and the average price of food rises across the continent. So does the pay rates of all kinds of craftsmen, as Thay hires anyone it can get their hands on to come help restore the country.
I'd have to argue with the "whole forests leveled" part. Why would trees gain enough experience from such an act to level up? (Ha ha. Just kidding.) No, really, wood has 10 hit points per inch of thickness, I don't think you're going to topple any tree larger than 3.5 inches in diameter. Saplings will be killed while the bark and leaves are knocked off larger trees, but in the abstract hit point system of D&D, that's just all flavor text and they'll survive.
For that matter, how many trees are there in Thay anyway? I've always pictured it as mostly plains and steppes with trees being sparse. I'd have to wonder if they're even used much for building material in the country. More likley they just use slave labor to quarry Thaymount for stone and use what wood they grow or import for items that can't be made out of other material. I would expect slave quarters to be tents (like yurts) or sod since those would be the available materials if not stone. Those in tents would be toast, but if being in a build protected the occupants from damage, then even a sod hut would probably provide enough protection for the inhabitants to survive. Maybe even waddle and dob house depending on the thickness of the walls and hit points given to them due to quality. I'd say a typical log cabin would survive quite nicely (if they actually could use that much wood if Thay is wood sparse).
Still, it's going to take out most of the commoner slaves and live stock across the whole country. It'll probalby take out most of the armies of Thay if they're not garrisoned properly.
What it does to food production and the country as a whole will probably depend on what time of the year it is. Between after harvest to a little while after planting, they're probably not too bad as all the grain will be inside silos or underground. If it's in summer before harvest, then there's going to be an almost total destruction of the crops. Of course, then theres not really anybody else to feed is there? You're going to be left with the city dwellers of Thay mostly intact. The infrastructure will also be mostly intact as IMHO 35 hit points is not that much when dealing with things like bridges. Especially when sonic damage and once it's over, it's over. A far better choice would have been fire, because although the 35 hit points caused by the fire wouldn't have destroyed such things, it will cause things to start burning which would have resulted in more damage than the initial spell.
So, we're left with a Thay that has been depopulated by slaves, servants, and maybe armies, but still has it's highly trained core of wizards and city folk. The mines would be ok and if the main threat of Thay are the Red Wizards (and any undead armies they can build), then Thay probably has enough threat to defend itself from the outside. What it does mean is that Thay is not much of a threat for many years to come and a place to make money. Thay will be buying slaves and supplies right and left for years to come and spending all their money on rebuilding rather than looking outside of Thay's borders. It would essentially turn Thay from a threat to everybody to an insular country that can't afford to mess too mcuh with other countries affairs because they ahve other things to worry about and spend their resources on. It would not remove them commpletly as individual powerful wizards would have their personal axes to grind and they'd still try and maintain as much of a spy network as possible so they can pull strings here and there.
The other choice is that the Zulkirs would decide the best defence and economics would to be to take what resources they have left, raise and undead army and attack whatever country could threaten them (or is easy pickings) in order to remove any threats and regain their slave population.