Zhang Heng's Seismoscope (132): Detecting Earthquakes in China

Zhang Heng’s seismoscope, presented at the Eastern Han court in AD 132, is the earliest known instrument designed to detect earthquakes. Built by the astronomer, mathematician and poet Zhang Heng (AD 78–139), the bronze device reportedly showed the direction of a distant earthquake before news of it reached the capital, Luoyang.

No original survives, and its inner workings are not recorded. Even so, the description in the Book of the Later Han has made Zhang Heng’s instrument one of the most famous inventions of ancient China.

Zhang Heng: a Han polymath

Zhang Heng was born in AD 78 in Xi’e, in Nanyang commandery (modern Henan). He studied in Chang’an and Luoyang and first became famous as a writer. His “Rhapsody on the Two Capitals”, which took about ten years to complete, describes Chang’an and Luoyang in rich detail, and his short “Rhapsody on Returning to the Fields” is admired as an early nature poem.

He entered court service under Emperor An and twice held the post of Grand Astrologer (taishi ling), in charge of astronomy, the calendar and the reading of omens. This was an important office, since unusual events in the sky and on earth were believed to reflect the moral state of the government.

Zhang Heng’s scientific work covered many fields:

• Astronomy: In his Lingxian (“Spiritual Constitution of the Universe”) he described the heavens as an egg with the earth like its yolk, and counted 124 constellations and about 2,500 stars, 320 of them individually named.
• The armillary sphere: He built a bronze armillary sphere turned by water from a clepsydra, which showed the movement of the stars in step with the sky.
• Eclipses and the Moon: He explained that the Moon shines with light reflected from the Sun and that lunar eclipses occur when the Earth blocks the Sun’s light.
• Mathematics: He worked on the value of pi, using approximations such as the square root of 10 (about 3.162).

The seismoscope of AD 132

China’s frequent earthquakes were seen as warnings from Heaven, so knowing where one had struck mattered to the government. In the first year of the Yangjia era, AD 132, Zhang Heng presented his instrument, called the houfeng didong yi, roughly “instrument for measuring the seasonal winds and the movements of the earth”.

The Book of the Later Han describes it as follows:

• It was a vessel of cast bronze shaped like a wine jar, about eight chi (roughly 1.9 metres) across, with a domed lid.
• Inside stood a central column (duzhu) and eight arms or channels linked to a mechanism.
• On the outside were eight dragons facing the eight directions, each holding a bronze ball in its mouth.
• Beneath each dragon sat a bronze toad with its mouth open upwards.
• When the ground shook, the mechanism caused one dragon to release its ball into the toad below, making a loud sound and showing the direction of the shock.

The history adds that only one dragon would move at a time, so the others did not respond.

The Longxi earthquake

According to Zhang Heng’s biography, the device soon faced a test. One day a ball dropped although no one in Luoyang had felt any tremor, and scholars at court doubted the instrument. A few days later a messenger arrived reporting an earthquake in Longxi, in modern Gansu, several hundred kilometres to the west. The official history says everyone was then convinced, and that officials were afterwards ordered to record the direction of earthquakes.

The Book of the Later Han’s annals record an earthquake in Longxi in AD 138, and this is generally linked to the story, although the exact date of the detection is not certain.

Modern reconstructions

Because the description is short, the mechanism has been debated for more than a century.

• The inverted pendulum model. In the 1950s the Chinese scholar Wang Zhenduo built a model with an upright pendulum that tips over when shaken. It became the familiar image in textbooks and museums, but critics showed it could be set off by small vibrations and was not very sensitive to real earthquake waves.
• The suspended pendulum model. Other researchers argued for a hanging pendulum, closer to the principle of later seismometers. In 2005 a team of Chinese scientists presented a reconstruction based on this idea and reported that it responded to recorded earthquake motions.

Reconstructions also differ in how the falling ball relates to direction. In some models the dragon facing the source drops its ball; in others it is the dragon on the opposite side, since the ground first moves towards or away from the quake. The ancient text does not settle the question.

No reconstruction can be proved correct, and it is uncertain whether any version could reliably indicate direction. What is clear is that Zhang Heng tried to build an instrument to detect earthquakes more than 1,500 years before such devices appeared in Europe.

Why direction mattered

To modern readers the seismoscope can look like a scientific curiosity, but at the Han court it had a clear political purpose. Han thinkers, following the ideas associated with Dong Zhongshu, taught that Heaven responded to misrule with portents such as eclipses, floods and earthquakes. An emperor who learned quickly of a disaster could send relief, order prayers and show that he took Heaven’s warnings seriously.

Communication across the empire depended on mounted messengers, so news from a distant frontier commandery might take days or weeks to arrive. An instrument that announced a quake and pointed towards it allowed the court to prepare before official reports came in. The Eastern Han suffered many earthquakes in the second century AD, and the court annals of the period record them repeatedly, often alongside floods, droughts and locust plagues.

Zhang Heng himself used such events in his memorials to the throne, criticising the influence of palace eunuchs and arguing that the government should reform its conduct. His scientific and political work were therefore closely connected.

Later life and legacy

From AD 136 Zhang Heng served as chancellor of the Hejian kingdom, where he took firm action against lawless local landowners. He was recalled to Luoyang in AD 138 and died in AD 139. His tomb, which still stands at his home district in Nanyang, was restored in modern times.

Zhang Heng is now honoured as one of China’s great scientists. A lunar crater, the asteroid 1802 Zhang Heng and China’s ZH-1 (Zhangheng-1) satellite, launched in 2018 to study electromagnetic signals linked to earthquakes, are named after him. Images of the seismoscope appear on Chinese stamps and in museums worldwide.

The Eastern Han court where he worked had been rebuilt after the restoration of the Han dynasty, and it was also the world of Cai Lun’s paper. China’s later earthquake history includes disasters such as the Shaanxi earthquake of 1556, the deadliest on record.

Timeline

• AD 78: Zhang Heng is born in Nanyang commandery.
• AD 115: He is appointed Grand Astrologer for the first time.
• c. AD 117: He completes his water-powered armillary sphere.
• AD 132: He presents the seismoscope to the court of Emperor Shun.
• AD 136: He becomes chancellor of the Hejian kingdom.
• AD 138: An earthquake strikes Longxi, the quake usually linked to the seismoscope story.
• AD 139: Zhang Heng dies.

Why it matters

Zhang Heng’s seismoscope is the earliest known attempt to detect earthquakes with an instrument. Together with his astronomy and writing, it shows the high level of science and scholarship at the Eastern Han court.