Earthquake bomb

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. Because of 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 the 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.

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