The earthquake bomb, or seismic bomb, was a concept that was invented by the British aeronautical engineer Barnes Wallis early in World War II and subsequently developed and used during the war against strategic targets in Europe. They differ somewhat in concept from traditional aircraft-borne bombs, which usually explode at or near the surface, and destroy their target directly by explosive force. By contrast, an earthquake bomb is dropped from very high altitude to gain very high speed, and upon impact penetrates and explodes deep underground, causing massive caverns or craters – known as camouflets – as well as much more severe shockwaves. In this way, they can affect targets that are too massive to be affected by other types of conventional bomb, as well as difficult targets such as bridges and viaducts.
Earthquake bombs were used towards the end of World War II for massively reinforced installations (e.g. submarine pens with concrete walls several meters thick), underground caverns, buried tunnels, and bridges.
An explosion in air does not transfer much energy into a solid, as their differing acoustic impedance makes an impedance mismatch that reflects most of the energy. Due to the lack of accuracy of bombing in the face of anti-aircraft defences, air forces used area bombardment, dropping large numbers of bombs so that it would be likely that the target was hit. Although a direct hit from a light bomb would destroy an unprotected target, it was comparatively easy to armour ground targets with many yards of concrete, and thus render critical installations such as bunkers essentially bombproof. If a bomb could be designed to explode in water, soil, or other less compressible materials, the explosive force would be transmitted more efficiently to the target object.
Wallis's idea was to drop a large, heavy bomb with a hard armoured tip at supersonic speed (as fast as an artillery shell) so that it penetrated the ground like a ten-ton bullet being fired straight down. It was then set to explode underground, ideally to the side of, or underneath a hardened target; the resulting shock wave would produce the equivalent of a 3.6 magnitude earthquake, destroying any nearby structures such as dams, railways, viaducts, etc. Any concrete reinforcement of the target would likely serve to enclose the force better.
Wallis also argued that, if the bomb penetrated deep enough, the explosion would not breach the surface of the ground and would thus produce an underground cavern (a camouflet) which would remove the structure's underground support, thus causing it to collapse. The process was graphically described as a "trapdoor effect" or "hangman's drop".
Wallis foresaw that disrupting German industry would remove its ability to fight, and also understood that precision bombing was virtually impossible in the late 1930s. The technology for precision aiming was developed during World War II, and Barnes Wallis' ideas were then shown to be superbly successful, considering the standards at the time.
Wallis' first concept was for a ten-ton bomb that would explode some 130 feet (40 m) underground. To achieve this, the bomb would have had to be dropped from 40,000 feet (12 km). The RAF had no aircraft at the time capable of carrying a ten-ton bomb load aloft, let alone lifting it to such a height. Wallis designed a six-engine aeroplane for the task, called the "Victory Bomber", but he was not taken seriously by the military hierarchy of the day.
Wallis then took a different line in developing a means to destroy Germany's industrial structure with attacks on its supply of hydroelectric power. After he had developed the bouncing bomb and shown its possibilities, however, RAF Bomber Command were prepared to listen to his other ideas, even though they often thought them strange. The officer classes of the RAF at that time were often trained not in science or engineering, but in the classics, Roman and Greek history and language. They provided enough support to let him continue his research.
Later in the war, Barnes Wallis made bombs based on the "earthquake bomb concept", such as the 6-ton Tallboy and then the 10-ton Grand Slam, although these were never dropped from more than about 25,000 feet (7.6 km). Even from this low height, the earthquake bomb had the ability to disrupt German industry while causing minimum civilian casualties. It was used to disable the V2 factory, bury the V3 guns, sink the battleship Tirpitz and damage the U-boats' protective pens at St. Nazaire, as well as to attack many other targets which had been impossible to damage before. One of the most spectacular attacks was shortly after D-Day, when the Tallboy was used to prevent German tank reinforcements from moving by train. Rather than blow up the tracks — which would be repaired in a day or so — the bombs were targeted on a tunnel near Saumur which carried the line under a mountain. Twenty-five Lancasters dropped the first Tallboys on the mountain, penetrating straight through the rock, and one of them exploded in the tunnel below. As a result, the entire rail line remained unusable until the end of the war.
After World War II, the United States developed the 43,000-pound (20,000 kg) T12 demolition bomb that was designed to create an earthquake effect. Given the availability of nuclear weapons with surface detonating laydown delivery, however, there was little or no development of conventional deep penetrating bombs until the 1991 Gulf War. During the Gulf War, the need for a conventional deep penetrator became clear. In three weeks, a cooperative effort directed by the Armament Systems Division at Eglin Air Force Base in Florida developed the 5,000-pound (2,300 kg) GBU-28 that was used successfully by F-111Fs against a deep underground complex not far from Baghdad just before the end of the war.
The United States has developed a 30,000-pound (14,000 kg) Massive Ordnance Penetrator, designed to attack very deeply buried targets without the use of nuclear weapons with the inherent huge levels of radioactive pollution and their attendant risk of retaliation in kind.
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