Did the US really consider demonstrating the atomic bomb to the Japanese by “blowing the top” off of Mt. Fuji?

Did the US really consider demonstrating the atomic bomb to the Japanese by “blowing the top” off of Mt. Fuji?

In reading this Washington Post article titled Five myths about the atomic bomb, it's mentioned:

The decision to use nuclear weapons is usually presented as either/or: either drop the bomb or land on the beaches. But beyond simply continuing the conventional bombing and naval blockade of Japan, there were two other options recognized at the time.

The first was a demonstration of the atomic bomb prior to or instead of its military use: exploding the bomb on an uninhabited island or in the desert, in front of invited observers from Japan and other countries; or using it to blow the top off Mount Fuji, outside Tokyo. The demonstration option was rejected for practical reasons. There were only two bombs available in August 1945, and the demonstration bomb might turn out to be a dud.

(emphasis is mine)

I can't find any corroborating evidence to this through googling, but this seems crazy to me if only because the effects of bombing Fuji-san could surely be more catastrophic than bombing a conventional target--for example, if the bomb happened to catalyze an eruption, which could also catalyze major earthquakes.

So my questions are:

  1. Did the US really consider demonstrating the atomic bomb to the Japanese by "blowing the top" off of Mt. Fuji?
  2. If so, was any analysis done, by people with appropriate geological expertise, to estimate the risk this could carry to catalyze major eruptions or earthquakes?

"US" and "consider" are rather broad terms. I can't find any evidence that the Manhattan Project targeting committee ever considered anything other than conventional military targets, but there were plenty of other people throwing out ideas of what should be hit.

A rather informal analysis of "blowing the top off a mountain" was done in the form of the Trinity test. The verdict was that a surface detonation of a 20-kiloton bomb would produce a crater about ten meters wide and a meter deep -- a rather unimpressive outcome.

I seriously doubt any formal analysis would have been done. Any demolitions expert could give you the answer in about five seconds: detonating explosives on the surface of a solid object is completely ineffective at causing meaningful damage to that object. In order to blow the top off a mountain with Fat Man or Little Boy, you'd need to drill a hole and lower the bomb into it.

A 20KT warhead is probably very much on the low end of what a volcano can do and how much damage a surface explosion it would do to a mountain is debatable. Mount St. Helens was a 24MT equivalent, from the inside, for example.

Both Hiroshima/Nagasaki drops were airbursts.

A groundburst on Fuji might have turned out to be anything but benign if it churned up a lot of irradiated earth and dropped fallout in the neighborhood. Fallout profiles with bombs depend on many factors, but higher altitude bursts result in "cleaner", less-permanent radiation. However, I also really don't know how much they knew or expected regarding fallout, so it's unclear whether considerations about it would have been a factor.

Last, but not least, you and most press coverage nowadays are looking at the atomic bombs from the point of view of late 20th century aversion (relatively speaking) of civilian casualties.

In WW2 high enemy civilian casualty rates were a feature, not a bug, and the A-bomb was truly novel only in its immediacy and apparent ease of use. Wide revulsion and mutually assured destruction would only come later. Some of the Tokyo raids caused on the same order of civilians deaths in one night.

Not sure when Western countries started avoiding, or at least claiming to avoid, civilian casualties, but even as late as the Vietnam War some consideration was given to bombing dikes which might have caused near 200K deaths.

p.s. as regards the linked article, I have a really, really, hard time taking anyone seriously who cherry picks casualty estimates as they did:

As Stanford historian Barton Bernstein has noted, the U.S. Joint War Plans Committee predicted in mid-June 1945 that the invasion of Japan, set to begin Nov. 1, would result in 193,000 U.S. casualties, including 40,000 deaths.

The invasion of Okinawa, a much more limited affair, had already incurred US casualties as follows, so quoting any 40K dead estimate is raving lunacy or flagrant intellectual dishonesty.

14,009 to 20,195 dead/ 12,520 killed in action/ 38,000 to 55,162 wounded

and at 40K-150K civilians killed as well, a good deal of it due to mainland Japanese coercion, there was considerable ethical justification in wanting to avoid repeating it on the main island.