The 4-Ton Steel Ball That Produces Artificial Earthquakes

In the wooded hillside of Hainberg, near Göttingen, Germany, stands an old seismological station. The Wiechert Earthquake Station was built in 1902 by the noted German physicist and geophysicist, Emil Wiechert, to carry out research in the emerging field of geophysics. Wiechert built several seismographs there to record tremors. These instruments have been recording data uninterruptedly since then, becoming the world’s oldest, still functioning seismograph.

Emil Wiechert was interested in learning about the structure of the earth. A few years prior, he had published the first verifiable model of the Earth's interior as a series of shells. He argued that since the density of the Earth's surface rocks was different from the mean density of the Earth, the earth must be made of different layers of rocks of different densities. He concluded, correctly, that the earth has a heavy iron core.

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The Mintrop ball readying for a drop.

Emil Wiechert was joined by many brilliant researchers, among which was a young German geophysicist named Ludger Mintrop.

Ludger Mintrop was an able student of Wiechert, and one of the founders of modern geophysics. In 1908, Mintrop devised a method to produce earthquakes artificially and use the data recorded by seismographs to determine the geological structure underneath the surface. Mintrop built a steel scaffolding, 14 meters tall, from which a 4-ton steel ball was dropped into the bedrock of shell limestone. Portable seismographs were used to register the resulting artificial seismic ground waves at various distances from the drop site. The experiment was a resounding success.

Mintrop successfully created a three-dimension image of the immediate area below the surface of the earth. Through this experiment, Mintrop demonstrated that by producing small, localized, artificial earthquakes, it was possible to identify distinctive boundaries in rock, and between solid and liquid rock layers, and draw conclusions about the nature of geological structures near the surface. Eventually, Mintrop founded the company Seismos GmbH for the purpose of exploring tappable deposits of minerals and rock by using seismic waves, although he replaced heavy iron balls with dynamite. This so-called “seismic exploration” is still used by the crude oil industry to search for deposits of crude oil, natural gas and minerals by inducing artificial shock waves and recording and interpreting the results.

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Recording of the wavelet generated by the drop of the 4,000 kg ironball by the Wiechert-seismograph on August 21, 1908. Photo credit: USGC

The 4-ton steel ball in its steel scaffolding still stands at the earthquake station. It has been modernized with an electric motor and a remote release mechanism. It is still dropped today whenever visitors are around.

There are lots of other interesting stuff at the earthquake station. Visitors can see Wiechert’s famous inverted-pendulum seismograph, which he built in 1902. This instrument is radically different in design from all previous seismographs and consist essentially of a heavy mass that can oscillate freely. The mass is held near the equilibrium position by the pressure of thin springs at the top. When the ground shakes, the frame from which the pendulum is suspended moves too, but the pendulum itself, owning to its heavy mass, stays stationary—an effect called inertia. Records of the movement are made on smoked paper on a rotating drum. Wiechert’s first seismograph recorded horizontal components of the motion, but he later designed instruments to measure vertical components as well. Wiechert seismographs are still used in some of the world’s observatories and continue to supply valuable information.

Also see: The Earthquake House of Comrie

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