[BillF]

I'm not sure, but wouldn't it be something like inverse-cube law here?

Energy can spread out vertically as well as horizontally. Thus, the energy density or pressure wave at 20 miles would be 1/(2 to the 3rd power) or 1/9 of the values at 10 miles from blast source.

(There might be some edge effects away from the blast due to the massive earth. That might might make the blast effects slightly higher than predicted by inverse-cube, but still way below inverse-square.)

[Elliott Jackalope]

Inverse-square is an equation that applies to 3-d space, so it is the proper equation to apply here. Also realize that this applies to blast force and heat energy, but does not apply to ash dissipation, since there are atmospheric forces at work affecting the distribution of ash.

Let's just say that if the Yellowstone caldera blows, we're not going to be worrying too much about who's going to win the Super Bowl or Titney Rears' latest album...