The following are excerpts from the MAGIC Far East Summaries of intercepted Japanese operational messages sent to Tokyo from Hiroshima and Nagasaki reporting on the atomic bombings. To emphasize the timing of these reports, the times are in bold type (the "I" indicates the Tokyo time zone).
Note in particular (yellow highlights) that already by 10 AM on the day before the Nagasaki bombing
the military had already known that even simple bomb shelters protected. Although
they were speaking only of blast and flash burns, unaware that people with high
radiation doses would begin dying in a few days, in fact even simple shelters gave enough shielding
to protect against radiation, too, if the entrance didn't face the explosion. (A meter of earth
absorbs about 99% of the radiation passing through it. A meter and a half, 99.9%. So if the line from
a sheltered person to the explosion core passed through over a meter of earth,
they'd not get a deadly dose even just 500 meters from the hypocenter
.)
The military's initial attempt to keep what had happened at Hiroshima secret, and shortcomings of the air raid warning systems, made the deaths at Nagasaki especially tragic, because a goodly fraction of the populace could have reached the excellent air raid shelters dug into the mountain sides, and even most in slit-trench shelters with earth covering would have survived. The deeply affecting photographs taken at Nagasaki by Yosuke Yamahata include this image of a young girl emerging unscathed from a simple underground shelter (part of a San Francisco Exploratorium exhibit) .
From SRS-507, MAGIC Far East Summary 8 Aug 45:
1. Hiroshima – Japanese Reports On Atomic Bomb Attack:
a. At 2155I on 7 Aug the 12th Flying Div (Hq Ozuki,
80 m.
SW of Hiroshima) sent to
the Air General Army at Tokyo the following report, entitled “Eyewitness Account (and Estimates Heard) of
the Sixth Air Army and of this Division in regard to the Bombing of Hiroshima (#2 Report)”:
“(1) Report on the bomb used (an estimate) and resulting conditions: a violent, large, special-type bomb, giving the appearance of magnesium, was dropped over the center of the city of Hiroshima this morning by a formation of three or four planes (it is also said that there was only one plane; some say that the bomb was attached to a parachute). It is estimated that, after being dropped from a plane, the bomb exploded at a certain altitude above the ground (500 to 1000 meters). There was a blinding flash and a violent blast. (Over the center of the city the flash and the blast were almost simultaneous, but in the vicinity of the airfield* the blast came two or three seconds later.) Then a mass of white smoke billowed up into the air.
*The airfield is just south of the city.
“(2) the flash was instantaneous, burning objects in the immediate vicinity, burning the exposed parts of people’s bodies as far as three kilometers away, and setting fire to their thin clothing.
“(3) The blast leveled completely or partially as many as 60,000 houses within a radius of three kilometers, and smashed glass blocks, etc.
“(4) Losses: the majority of the houses within the city were completely or partially leveled. The conflagration spread all over, and many important areas were destroyed by fire. The majority of government buildings were either leveled or destroyed by fire. Many people were injured by burns from the flash and by objects shattered by the blast, particularly by glass fragments, and, as far as was observed, [word missing] one-third of the residents were either seriously or slightly injured.
“(5) Countermeasures:
“(a) Personnel, aircraft, etc., have been moved underground. (Partially underground [word missing] walls, if they are strong are all right.)
“(b) We must keep a strict watch even for a small number of planes.
“(c) Planes must be dispersed and their tanks emptied.
“(d) The height of the planes should be lowered (even if it be only the height of the wheels) and the ailerons fixed in place. Walls of valleys should be used to protect planes on the ground.”
b. Available parts of a later report from the 12th Flying Div, sent out at 1000I on the 8th, include the following:
(1) “Because conflagration broke out suddenly and the spread of the fire was rapid, we think that 70 or 80 percent of the people in the city were casualties.”
(2) “As a result of the horrible catastrophes brought about by the recent air raid, there appears to be a gradual increase in the circulation of wild and fantastic rumors. Moreover, there has been an increasing number of cases in which the fighting spirit of victims or eyewitnesses has been broken. However in this prefecture* the mobile police officials are doing everything in their power to [words missing] the spread of such frenzied rumors, and are using every means at their disposal to provide against this in advance. In order to calm the people’s fears, they are particularly stressing the fact that, even in the recent air raid, those who took refuge immediately in the safe underground shelters escaped injury completely.”
*Apparently a reference to Yamaguchi prefecture (where the message originated), which lies to the west of Hiroshima prefecture.
c. A Japanese Navy report, transmitted from Kure at 1124I on the 8th but apparently prepared somewhat earlier, includes the following statements:
(1) “The concussion was beyond imagination, demolishing practically every house in the city.”
(2) “Present estimate of damage: about 80% of the city was wiped out (destroyed or burned). Only a portion of the western section escaped the disaster. Casualties have been estimated at 100,000 persons.”
(3) “Relief squads have been dispatched to the area to assist the Army and rescue operations. About 1,000 Army troops and 10,000 [word missing] medical supplies were moved in by dawn on the 7th.”
From SRS-508, MAGIC Far East Summary 10 Aug 45:
1. Further Japanese Reports on the Atomic Bomb:
a. A “third report”, sent out by the 12th Flying Div (Hq Ozuki, 80 m. SW of Hiroshima) at 1950I on 8 Aug, adds the following information on results of the 6 Aug attack on Hiroshima:
(1) “The altitude of the bomb Was about 700 meters at the time of the explosion.”
(2) “Within a radius of 500 meters, there were many of whose bodies showed internal [word missing]. There were practically no casualties among those who were in [word missing] which had wooden log [word missing]. No damage was sustained by those wartime [word missing] of a semi-underground type which were within a radius of two kilometers. Residential buildings within a radius of three kilometers were [word missing]. There were some casualties from burns four and one-half kilometers from the center of the explosion; casualties were rare beyond that limit.”
(3) “These fires are more difficult to extinguish than fires caused by oil. In general, fires broke out ten minutes after the explosion and are believed to have been set off by sparks within collapsed houses.”
(4) “Research programs: researches into the atom, Uranium 235, are being* conducted at the Tokyo Imperial University under Army supervision and at the Kyoto Imperial University under Navy supervision. [Words missing] have not gone beyond the limits of theory. At [place missing], about five months ago, research was made into the [practical] application of Uranium 235, but no announcements have been made since then. However, it is believed that research into the [word missing] of the atom has since been completed and [words missing] research into this is considered to be of considerable value. It is believed that it is essential that this be completed immediately.”
*Possibly, “will be”.
c. A 7 Aug message to all Japanese naval commands, probably from the Navy General Staff, states: “Although we [are making investigations] concerning [expected] atomic bomb attacks by the enemy, you are to make every effort to strengthen [word missing] as follows and to [minimize] damage:
“(1) As regards the aircraft [word missing] you will have fighters intercept them. For that purpose, we shall consider the question of aviation fuel [word missing] in the light of actual results.
“(2) You are to endeavor to [word missing] against the parachute of the [atomic] bomb and to [shoot it down].
“(3) You are to step up the [word missing] of aircraft fitted for [word missing] attacks.”
d. On 9 Aug the following “intelligence report” was sent to all naval commands:
“The bombers which carry the atomic bombs are B-29s attached to the 313th [Bomber Wing] based at Tenian, and have heretofore appeared on the communication circuit between 30 minutes to one hour before bombing.”
The message adds that, “if those planes appear on the circuit”, certain specified signals will be broadcast from the Tokyo area.
From SRS-509, MAGIC Far East Summary 11 Aug 45:
1. Hiroshima – Japanese Reports On Atomic Bomb Attack:
a. A Japanese Army intelligence message, sent out from Tokyo at 2245I on 9 Aug, states:
“Forty per cent of the people of Hiroshima are dead or wounded. However, the spirit of the townspeople is steadily reviving.”
Note: At 1200I on the 8th, the 12th Flying Div (Hq Ozuki, 80 m. SW of Hiroshima) and reported that “we think that 70 or 80 per cent of the people in the city were casualties”; and a Navy report, also transmitted on the morning of the 8th, stated that “casualties have been estimated at 100,000 persons” (FES 9 Aug 45). The population of Hiroshima in 1940 was 343,988.
b. Another Tokyo intelligence message of 9 Aug, recommending protective measures against atomic bomb attacks, includes the following:
“It is necessary that the shelter trenches be covered. Underground rooms are especially suitable. There were comparatively few shelters destroyed [in Hiroshima]. Those in shelters at points two kilometers from the center were not [seriously injured].”
From SRS-510, MAGIC Far East Summary 13 Aug 45:
1. Hiroshima – Japanese Reports On Atomic Bomb Attack:
An 8 Aug message from the Eleventh Area Army (N Honshu) states:
“Imperial GHQ is investigating the new-type enemy bomb and is devising countermeasures against it. Since this bomb is dropped by parachute, each unit will at once issue orders to have AA guns fire on bombs dropped by parachute.”
From SRS-512, MAGIC Far East Summary 15 Aug 45:
1. Hiroshima – Further Report On Atomic Bomb Attack:
An 13 Aug the Japanese Navy Bureau of Military Affairs sent to the Japanese naval attaches in Switzerland and Portugal the following account of the atomic bomb attack on Hiroshima:
“(1) Two or three B-29s came in at a high altitude and dropped near the center of the city three of what appeared to be bombs attached to parachutes. Since all three were of identical structure, it is supposed that two of them were concerned with the detonating apparatus. The bomb exploded at an altitude of 500 meters.
“(2) The whole city was illuminated by a yellowish light and people on the ground became aware of a great heat in the atmosphere. At the same time residential buildings collapsed and subsequently caught on fire. Pine trees were split and some strong roofs were set on fire simultaneously with the explosion, but since most of the fires occurred several dozen minutes after the explosion, we believe the fires were also due to carelessness in the demolished buildings.
“(3) Effects of the bomb blast: Within 500 meters, human intestines were laid bare and eyeballs pulled out, and even at 2,000 meters there were deaths from the bomb concussion, but people in long reinforced trenches were safe. Within two kilometers, bodies were stark naked, there were deaths from burns, and wooden constructions were totally destroyed. Wooden constructions within four kilometers were half-destroyed.
“(4) Effects of the thermic rays: Almost everyone within four kilometers suffered burns, but even at seven kilometers slight burns were received. The majority of these occurred only on the side which was exposed to the flash of the explosion [word missing] in the parts exposed to the high radiation temperature. The effect on vegetation was comparable to that on human beings. Even at seven kilometers, parts of the branches and leaves have withered.
“(5) There is practically no defense measure.”
From SRS-515, MAGIC Far East Summary 18 Aug 45:
5. Nagasaki – Japanese Report On Atomic Bomb Attack:
There are available the last four parts of an 11-part message, sent on 12 Aug by the Sixteenth Area Army (Hq Fukuoka, NW Kyushu), which apparently discusses the results of the atomic bomb attack on Nagasaki.* Those parts of the message read as follows:
“The special-type bomb which drifted in the air above exploded 500 m above the ground. At the same time that the explosion occurred, there was a flash, and overpowering heat wave and a great blast. The heat wave ignited [word missing] on the ground near the spot of the explosion and burned men and horses.
“The effects of the blast extended beyond a radius of 12 kilometers. The houses in the [word missing] area [more than]two kilometers from the center of the explosion were practically all flattened, roof tiles, window glass and [word missing] were [words missing].
“After the concussion, fires burst out and almost everything burned up. House was in 4 to 6 kilometers of where the bomb fell were badly [word missing]. In the area within a radius of 8 to 10 kilometers, 30 to 40 per cent of the roof tiles were blown off civilian homes, and [words missing] within houses were scattered all about. In the area within a radius of 10 to 12 kilometers, all glass was broken.”
*This message is believed to refer to Nagasaki and not Hiroshima because (i) the effects of the blast described in the message extended over distances considerably greater than those given in any of the available Japanese reports on Hiroshima, and (ii) the message was originated by the Sixteenth Area Army which has a command including the Nagasaki area but not Hiroshima area.
6. Hiroshima – Further Report On Atomic Bomb Attack:
A Japanese Army report, sent from Hiroshima on 13 Aug, reads as follows:
“(1) The substance of this bomb is not an ordinary explosive charge or incendiary material. We conclude that it is an atomic bomb. It is dropped from a single plane or several planes, not necessarily with a parachute.
“(2) The explosion occurred about 300 meters South of the Gokoku Shrine in the center of Hiroshima, at a height of approximately 350 meters.
“(3) The force of the blast, at a point on the ground at the center of the explosion, is estimated at 8 kilograms per square centimeter (enough to knock a man down). However, more accurate and careful examination will be necessary.
“(4) We are quite sure that the burns are caused by the flash, which includes infra-red to gamma rays. Moreover, we suspect that beta rays may also be involved. Apparently the duration of the flash is more than momentary.
“(5) Some of the fires were caused by highly inflammable materials (such as straw, thin boards, paper, black cloth, etc.) which caught fire from the flash.
“(6) People were killed principally by being crushed or burned to death in collapsing houses. Eighty per cent of the wounded were burned and the outlook for their recovery is not favorable.
“(7) This type of bomb explodes [word missing] an ordinary bomb and produces an intense blue-white flash. The flash usually burns exposed skin. Since the emission of light lasts for some time, the immediate loss of [words missing] is low. If the exposed parts are covered, one can avoid burns.”
Note: At this point there is a fragmentary sentence which appears to state that, since the blast travels at a speed much slower than that of the flash, it is possible for persons at a distance from the center of the explosion to avoid injury, “if steps are taken immediately upon perceiving the flash”.
“(9) The principal countermeasures appear to be the following:
“(a) When it is known that enemy planes are approaching (even during the preliminary alert), people should leave their homes and take shelter in covered air raid shelters.
“(b) Those who cannot enter an air raid shelter will take cover and lie low and, after perceiving the flash, will immediately escape to open ground.
“(c) People should dress so as to expose as little of the body as possible and where heavy clothing in so far as possible [words missing] white underwear.
“(d) Glass windows cause many injuries, so they are to be removed at once. Japanese houses are to be rebuilt as half-underground shelters.
“(e) Fire-fighting apparatus will be placed in an outdoor shelter where there is little danger from falling houses.
“(f) Aircraft [words missing] covered [words missing] or will be housed underground.
“(g) Shelters used by AA artillery and other AA units should, as far as possible, be made high and camouflaged.
“(h) In general, it is necessary to [words missing] fuel and ammunition [words missing].
“(i) Army hospitals must prepare to [words missing] a large number of burned patients, and all medical supplies must be carefully stored underground.
“(j) Other points are as in ordinary air raid preparations.”