If you scroll down, you see a map of the most severe damage, roughly circular, and centred on the target spot for the bomb.
Except there’s four areas at the bottom right of the map, outside the circle, and rectangular or square: areas 1, 2, 3, 4 and 8. Those areas are shaded to include them in the “city’s most devastated sectors”, same as the areas in the circular section.
What would have caused that damage outside of the main circle, and in non-circular shapes?
Nuclear weapons don’t have unlimited power. Knocking down heavier buildings in one direction is going to use some of the energy and shield, to some degree, the stuff behind it. The variations in the terrain may have also channeled or blocked some of the effects of the blast wave. Local conditions may have also created out of control infernos in some places and not others, and the firestorms that follow nuclear attacks are a factor in how completely things get destroyed.
Also, the structures in those zones may vary. A zone with wooden structures is obviously going to experience more severe destruction than one made out of all concrete.
They’re also drawing the boundaries with lines - if you were to map it out with more precision it might end up looking more like a circle.
There may also be specific reasons that someone else can answer, but I just wanted to give an answer as to what sort of factors may have spread the damage around unequally.
The image from that website, which shades in the same areas as the one from the NPR article, also provides a legend indicating the extent of damage to these areas and what they were. For example, the site labeled ‘15’ in your image was a gas works, which might be more sensitive to an atomic weapon blast. Some of the outlying areas are also indicated to have suffered a lower percentage of damage. The areas labeled ‘1’ and ‘8’ are shown to have suffered 25% and 10% damage, respectively.
I sort of covered this in my previous post, but I forgot to specifically mention that nuclear detonations typically cause fires outside of the area of their blast wave. The flash is so bright (in both visible light, IR, and UV) that even when you’re safe from the blast effects from the bomb, you can still get actual burns on your skin, or severe sunburns if you’re exposed to the flash. You can also experience vision damage or blindness. This effect can also start fires outside of the blast zone. So if you had the right conditions (areas that were particularly vulnerable to fire) plus the obvious hampering of normal firefighting capabilities after a nearby detonation of a bomb, you could get severe destruction from fire without any effects from blast.
There were some extremely damaging fires set by the nuclear blast. Perhaps those outlying very devastated areas were areas that suffered extreme fire damage. Hiroshima suffered a firestorm after the blast.
Building materials might also be a factor, both for effects of the blast and effects of the post-bombing fire.
Those might have been poorer neighborhoods, where construction was cheaper and therefore less sturdy. And neighborhoods often have regular boundaries set by roads or the like.
I doubt that neighborhood wealth and hence construction was much of a deciding factor. Japanese housing at the time was constructed from wood and paper, unlike European housing. This was pivotal in the decision to switch from attempting ‘precision’ bombing using high explosives targeting specific factories or complexes from high altitude to low level firebombing using napalm and incendiaries targeting urban areas, i.e., housing and civilians. The after effect of this can be seen in photos both from cities firebombed and those hit with the atomic bombs where everything has been utterly burned to the ground barring the burned-out husks of the few concrete buildings in housing districts.
The “Japanese Village” was set up at the Dugway Proving Grounds to in 1943 to test the effects of various incendiaries, as was a “German Village.”
On a related topic, I just watched a documentary on Operation Meetinghouse, a bombing raid conducted on Tokyo on March 9-10, 1945. 334 B-29s took off with 279 of them dropping 1,665 tons of bombs. An estimated 100,000 people died, more than either Hiroshima or Nagasaki.
It’s a U.S. Army Air Intelligence Bomb Damage Assessment. The numbers denote military facilities. The shaded areas outside the central blast radius are military facilities that were also damaged or destroyed by the blast - they have the shape of the facilties, not the shape of the damage. Note that the damage those facilities suffered range from 10% damage (for a Rayon mill) to 100% (for a gas works, which presumably suffered secondary explosions and fires).
The shaded area inside the blast radius circle is showing the most heavily devastated area. The shaded areas outside the circle are just outlining nearby military facilities that suffered at least some damage. NPR’s caption is incomplete and a bit misleading, but then again the infographic format on the photo itself is a bit confusing.
