The Ancient Earthquake Detector

Ming Fu

Many fascinating technologies were born in the ancient Chinese civilization. One of which is the ancient earthquake detector (dizhenyi).

The original inventor of this earthquake detector was a Han dynasty Imperial scientist and astronomer named Zhang Heng (AD 78-139). The original detector was lost but some of the description of its construction was preserved. This device is crafted entirely out of bronze and consists of an oval shaped pot or urn eight “Chi” or around six feet in diameter with eight dragons with a ball in its mouth mounted around the side, each of which is looking down at a toad with open mouth directly below. Within the urn, a pendulum hangs in the middle and is connected to the dragons’ mouths through lever systems. In the event of a seismic activity even undetectable by human feel, the pendulum would sway delicately and would trigger one or more dragons to release the bronze ball into the toads’ mouth. The detector was described to be engraved and decorated with complex images and patterns.

According to preserved texts and records, the detector was able to pick up earthquakes up to around 400 miles away. Since the original detector did not survive long enough for modern scientists to analyze it, the secret behind its effectiveness remains unknown. The most common explanation is that the detector’s material was tuned to the low resonant frequency that earthquakes produce, so that it only picks up a certain range of frequency and ignores the “noise” of everyday activities. One can also determine the epicenter of an earthquake by analyzing which of the eight balls have fallen. In order for the detector to actually detect earthquakes, the pendulum has to be several feet long. It was also believed that the ancient Chinese took seismic activities as a sign from the heavens, which probably explains the work and effort went into developing such a device.

Not only is the ancient earthquake detector a functional piece of machine that showcases the sensitivity and delicacy of ancient Chinese technology, it is also a example of the understanding of physics at such an early time.