Earthquake Savior: The History of Seismographs

Written by Snow

A seismograph is an instrument that detects and records seismic waves. It consists of a seismometer (or seismometer), a collector, and a power supply system. It is equipped with sensors that convert ground movement into electrical signals, which can be output on paper or a screen. Seismographs were originally designed to locate naturally occurring earthquakes, but with the development of related technologies, seismographs are also used in many other areas, such as oil exploration, crustal surveys, and monitoring volcanic activity.

The most familiar seismograph to us may be the "Houfeng Seismograph" described in textbooks. It was designed and manufactured by Zhang Heng in 132 AD during the Eastern Han Dynasty and was first recorded in the "Book of the Later Han Dynasty·Biography of Zhang Heng". In fact, the description of the Houfeng Seismograph in this document is only 196 words. The models (pictures) we see are all restored by later generations based on historical records, and the restoration plan is still controversial. But this ancient invention is considered to be the earliest device for humans to monitor earthquakes.

Early History of the Modern Seismograph

People discovered early on that pendulums can detect earthquakes (a restoration scheme for the Houfeng seismograph also used a pendulum), because objects suspended in the same position can clearly show the movement of the ground. The seismograph in the modern sense was born in the 19th century. In 1855, Italian physicist Luigi Palmieri designed a complex mechanical device, which connected a U-shaped tube filled with mercury to a conductive device, a clock, and a recording device. The movement of mercury during an earthquake recorded the time, relative intensity, and duration of the earthquake. However, Palmieri's invention is not the prototype of the modern seismograph, but the functions it realizes are what the modern seismograph needs, not just detecting earthquakes.

【Figure 1】Schematic diagram of Luigi Palmieri’s seismograph

The first modern seismograph currently recognized by the academic community was invented in 1875. It was a two-dimensional seismograph (north-south and east-west components) invented by Italian physicist Filippo Cecchi. He also used a pendulum, which could record the movement of the pendulum over time to simulate an earthquake. Although this device was not sensitive, it opened the era of analog seismographs.

An important development came from several scholars working in Japan at the time. After the Yokohama earthquake in 1880, British engineer John Milne (who also carefully studied Zhang Heng's seismograph), Scottish physicist James Alfred Ewing and Scottish engineer Thomas Gray organized the Japan Seismological Society, which developed a series of seismograph devices. Milne invented the famous horizontal pendulum seismograph and successfully recorded several earthquakes in Japan. Its feature is that it uses light to pass through a slit onto photosensitive paper, and the movement of the earthquake is "recorded" by light. Ewing designed a damped seismograph, and the damping design is one of the key components of the seismograph, because the instrument must record earthquake movement without swinging freely; Gray invented a method to record vertical movement, and thus invented the three-way pendulum seismograph. At this point, the modern seismograph was basically completed. After returning to the UK, they used these instruments to establish a small global seismic network.

【Figure 2】Schematic diagram of Milne seismograph

【Figure 3】Milne horizontal pendulum seismograph on display at the National Museum of Nature and Science in Tokyo, Japan

Important early works include the inverted pendulum damped seismograph invented by German physicist Emil Wiechert in 1893, and the electromagnetic seismograph invented by Russian physicist Boris Borisovich Golitsyn in 1906, which introduced a galvanometer for the first time, greatly improved the sensitivity, and realized a complete three-dimensional observation system. In the following decades, the relevant technologies of analog seismographs were rapidly updated, and modern seismology was developed as a result. Their basic principles originated from the work of Milne and others, so some people call Milne the father of modern seismology.

【Figure 4】Weichert seismograph (left) and Galitzine seismograph (right)

【Figure 5】Schematic diagram of vertical seismograph

Modern seismograph

In the 1950s, due to the negative impact of nuclear tests, seismological research was not sufficient to monitor small underground nuclear tests, so the World Standardized Seismic Network (WWSSN) was born in 1960, and seismographs were put into large-scale use. At the same time, semiconductor and computer technology developed rapidly, and digital seismographs that could amplify signals (electronic feedback technology) and process data through computers were developed.

【Figure 6】STS-1 seismometer

In 1976, Swiss scientists first developed the force balance feedback seismometer (STS-1 seismometer), which greatly broadened the frequency band, improved the dynamic range, and realized computer storage and display functions. In particular, the force balance feedback system is the core part of most seismometers today. In fact, modern seismometers are mainly composed of seismometers (wideband, large dynamic feedback) and high-resolution digital recorders.

The principle of force balance feedback seismometer: the feedback force is balanced with the inertial force, and the feedback current is proportional to the acceleration of ground vibration. The overall response and output of the instrument are mainly determined by the parameters of the feedback network, rather than the factors of the mechanical system and transducer.

【Figure 7】STS-2 seismometer is a standard device for calibrating other seismometers

【Figure 8】my country's BBVS three-component integrated force balance seismometer

There are other forms of seismometers, especially fiber-optic seismometers that have appeared in recent years. Fiber-optic seismic sensing was originally developed because the noise signal detected was similar to seismic waves. Seismic waves can cause micron-level changes in optical cables, and the points disturbed by seismic waves can be measured from the received data. A signal transmitter is installed at one end of the optical cable to emit laser pulses, and the backscattered signal of impurities in the optical fiber is used to extract the vibration information of the surface. Distributed fiber-optic sensing technology can instantly turn several kilometers of ordinary optical fiber into thousands of seismometers, thereby conveniently obtaining high-density monitoring of the underground. At present, my country has applied this technology in the West-East Gas Transmission System Project.

【Figure 9】Submarine fiber optic network can be used for earthquake monitoring

In addition to Earth, there are also dedicated seismometers on probes to the moon and Mars, providing humans with rich information about the extraterrestrial world.

Today, seismographs can detect vibrations as small as one ten-millionth of a centimeter in very quiet locations, making great progress in monitoring earthquakes. With better monitoring equipment and more comprehensive earthquake science, we can better understand the changes in the earth and better respond to disasters. Of course, don't forget to learn emergency knowledge to be prepared.

References

https://en.wikipedia.org/wiki/Seismometer#Other forms

https.//www.britannica.com/science/seismmograph/Basic-principles-of-the-modern- seismograph

https://mp.weixin.qq.com/s/LDEiTRROPKYHEIXTDPb8w

https://mp.weixin.qq.com/s/O3qfQXDMaSH6GJ-VkWNkzq

Wang Xizhen, Teng Yuntian. New technology and development of seismic sensors. Progress in Geophysics. 2010.25(2):478~485 Feng Rui, Interesting Seismology (25): Zhang Heng rewrote history. Progress in Earthquake Science, 2021,51(1):

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.