Bottomlands WI: The bomb goes off

In the Harry Turtledove book Down in the Bottomlands, the one with the dry Mediterranean basin, a nuclear bomb intended to blow open the Strait of Gibraltar is defused and disaster averted, but WI it went off, causing an earthquake that let the Atlantic into the Mediterranean basin?
 
I only read the book once like 10 years ago. Mainly I remember that the Bottomlands was really dry. But how big was the nuke? Was it some huge 50 megaton hydrogen bomb smuggled in on a mule? Seems unlikely. Or was it some unexplained tech differences that let very high yield nukes be much smaller? Or was it some BS "nuclear Voodoo" type of writing? I am going to guess it's the last one.
 
Not being a nuclear physicist I can't begin to guess if the bomb could do it or not, but lets assume it could.

Well, Tartesh sees a lot of its people devastated by the floods and will certainly be hurt, but since few of their people and none of their major cities were in the inhospitable depression, IIRC, they're not devastated population-wise. Hiowever, 80% of their power (to quote wiki) was from bottomlands hydro plants, sothey'll have a major power crisis. They are also sure to want bloody revenge against the nuclear attack. "Starbombs" may fly.

Krepalgan gets an *Atlantic coast. Morgaf maybe succeeds in getting the two nations at war while their own hands are kept clean.

Otherwise, too little information to go into too much detail.
 
Climate probably will change as moist air from the new Mediteranian brings rain and such to Europe. Interesting and a strange short story in my opinion.
 
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