Seismology in Ancient China

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Seismology in Ancient China


Located in a seismically active part of the world, China is beset by earthquakes, earthquakes that have routinely caused thousands of deaths in one of the most densely populated nations on Earth. This has been the case throughout recorded history, and likely much longer. Little wonder, then that scientists in ancient China were the first to develop a seismograph to monitor for earthquakes in their nation. The ability to quickly identify the occurrence and general direction of an earthquake could help the government muster assistance more rapidly. Over time, the original Chinese invention has become an important diagnostic tool, too, providing views of the deep structure of the Earth as well as giving us the ability to monitor for surreptitious nuclear weapons testing.


In the Chinese capital city of Sian, in a room in the Imperial Chancellery for Astronomical and Calendrical Sciences, stood a bronze pot, about 6 feet (1.8 m) in diameter. Around the periphery of the pot were eight dragon heads, each holding a ball in its slightly open mouth. Directly beneath each dragon stood a bronze toad, looking expectantly upward with open mouth. In about a.d. 132, a large earthquake struck a city about 400 miles (644 km) northwest of Sian. It was announced in Sian by the loud ringing of a bronze ball falling from the northwest dragon's mouth into that of a toad.

Skeptical observers questioned the event, suspicious that it was simply a false alarm. Several days later, a messenger arrived, announcing the earthquake and banishing all remaining doubts. This event was recorded by the official historian:

On one occasion one of the dragons let fall a ball from its mouth though no perceptible shock could be felt. All the scholars at the capital were astonished at this strange effect occurring without any evidence of an earthquake to cause it. But several days later a messenger arrived bringing news of an earthquake in Lung-His. Upon this everyone admitted the mysterious power of the instrument. Thenceforward it became the duty of the officials of the Bureau of Astronomical and Calendar to record the directions from which earthquakes came.

Earthquakes were capable of major disruptions in ancient China, just as they are in the modern world. In recorded history, food riots and rebellions followed in the civil unrest that accompanied them. In addition, earthquakes interrupted normal trade, including the shipments of food upon which the cities depended, and which constituted tax payments from many of the provinces. It was also important to send both food and the military to care for the citizenry as well as to quell any thoughts of rebellion. For these reasons, learning of earthquakes as quickly as possible was considered vitally important to Chinese emperors of this time, although how to accomplish this was not known.

Sometime around the year a.d. 130 the brilliant Chinese scientist Chang Heng (78-139) began to see an approach to solving this problem. Although his thoughts have not been recorded, reasonably informed speculation is possible. It is likely that Chang Heng, realizing that the shaking of the earth diminished with distance from the site of an earthquake, also realized that this shaking might continue for very great distances. It would be easy to consider that, at some point, the shaking might still exist, albeit too weakly for humans to feel it. Following this line of thought, it would seem possible to construct a device more sensitive than human perception that might be able to detect earthquakes, even at great distances. The challenge, of course, would be to construct such a device, and to construct it in such a way that it would announce earthquakes in an unmistakable fashion.

The exterior of the device Chang Heng finally developed is described above. The interior, however, is what made it work. Although at least two different models have been proposed, it seems likely that this device consisted of a rod with a weight at the top. This rod was carefully balanced on a base, and the whole structure rested securely on the ground. At the top were eight horizontal rods, each resting gently against the brass balls nestled in the dragons' mouths.

The rod, heavier at the top, was balanced in what physicists call "unstable equilibrium." It is akin to balancing atop a ball; as long as you don't move, you remain balanced. However, as soon as you move a little bit, you continue moving in that direction because of the shape of the ball. The further you move from the point of stability, the more unstable you become. Stable equilibrium is like putting a ball in the center of a bowl; if you move it a little in any direction, the shape of the bowl returns the ball to the bowl's center. In this case, moving further from the center increases the forces that tend to return you to the center.

In this case, an earthquake causes waves to ripple through the earth's crust. These waves would upset the unstable equilibrium of the rod, causing it to fall in the direction from which the waves originated. The rod would then fall against one of the horizontal rods, pushing it into the ball and causing the ball to fall into the bronze toad's mouth with a resulting loud noise. Thus, the device registered both the time and the direction of earthquakes. This is the first known seismograph.


Opinions vary as to whether Heng's "earthquake weathercock" was sufficiently sensitive to have accomplished much. On the one hand, there is little doubt that it detected the occurrence and direction of at least one earthquake at a great distance. On the other hand, some modern seismologists question whether the device itself might not have had too much internal friction to have had much more sensitivity than an attentive person. Unfortunately, no detailed description of the internal workings has survived, and no working models have either. Therefore, it is likely we will never know for certain whether this device was accurate or simply lucky. In either event, it was forgotten by China for well over a millennia, until after modern seismographs were developed in the eighteenth century.

In spite of this, or perhaps because of it, it seems most appropriate to discuss the impact of the modern science of seismology on society, because the earthquake weathercock was the most distant ancestor of today's devices, even though it vanished for so long. Some of these impacts would have been felt by ancient Chinese society, too. Specifically, they are the impact of seismology on earthquake warning and preparedness and the scientific advances stemming from seismology.

Chang Heng built his device almost solely to help detect earthquakes for the purpose of hurrying assistance to the scene as rapidly as possible. The ability to do so significantly helps to mitigate the damage caused by a serious earthquake, a fact to which inhabitants of San Francisco, Kobe, and other cities can amply attest. In fact, the most common refrain from earthquake survivors is that help took too long to arrive, leading to hunger, suffering, and death that could otherwise have been avoided.

In second-century China, of course, the only thing that could be done was to rush aid to a stricken city as quickly as possible. Toward this, Heng's seismograph helped because these preparations took time. By starting to assemble food, troops, and emergency supplies as soon as the ball dropped, they could simply await the arrival of a messenger to tell them which city was affected. Although not perfectly responsive, the government could at least be ready to travel immediately upon hearing this news.

Today, of course, we have many more options, including a limited ability to detect small tremors that often precede larger earthquakes. In addition, modern seismic networks integrate information from hundreds of seismographs spread throughout the world. This lets us instantly identify the exact location, time, and magnitude of events anywhere in the world, and lets governments immediately know that assistance may be required. While a telephone call or e-mail can also notify the government of an earthquake, it is not uncommon for phone lines to be disabled during natural disasters. Thus, the seismograph acts as a reliable backup for today's nearly instantaneous communications.

In addition to the civil disaster and public policy aspects of seismology, it has also given us some handy tools for mapping the Earth's interior. One of the first observations made was that virtually every earthquake on Earth is near the boundaries of tectonic plates. This, along with other conclusive evidence, made the case for plate tectonics overwhelmingly convincing to all but a few die-hard skeptics. Seismology has also helped us map plate boundaries, which is important because these are the areas most subject to both volcanoes and earthquakes.

Finally, seismology has given us a wonderful way to look inside the Earth. Seismology has shown us that the outer core is liquid, for example, and has helped to map chemical and physical variations in rocks of the deep mantel that are otherwise out of our instruments' reach. It has also shown us the topography within the Earth, and much more. It is safe to say that our modern conception of the Earth's structure is shaped almost entirely by our interpretation of seismological information, all stemming from remote descendents of Chang Heng's machine.

Did Heng conceive of all these uses when he ordered the bronze cast for his first device? Probably not. Most likely, he was simply trying to find a slightly better way to help his fellow Chinese. However, the modern descendents of his machine are giving us far more.


Further Reading

Bolt, Bruce. Earthquakes and Geological Discovery. New York: Scientific American Library, 1993.

Temple, Robert. The Genius of China: 3000 Years of Science,Discovery, and Invention. New York: Simon & Schuster, 1986.