Essay: Seismology – definition and history

Seismology is the scientific study of earthquakes and the propagation of the elastic wave through the Earth or through other planet-like bodies. The earliest seismoscope was invented in by Zhang Heng of China’s Han dynasty in 132 AD[2]. This instrument is reported to have detected a 400-mile distant earthquake. However, it does not tell any other information about the earthquake.
It is not until the early 1800s, a research team led by K.A. Von Hoff and A. Perrey began to look for possible correlations between earthquake occurrence and astronomical cycles. The research style, known as “Humboldtean”, focused on the accumulation of large volumes of data. For earthquake studies, this meant the first systematic catalogues of shocks [1]. This period also started the earliest investigation of the relationships between earthquakes and other geological processes.
The establishment of the behaviour of elastic materials by S.D. Poisson started a different approach to earthquake studies. Poisson found that in such materials wave motions were propagated at two speeds. These results were applied to earthquake studies by W. Hopkins (1847), and by R.Mallet (1848 onwards). Mallet discovered that earthquake waves radiate from a central focus and argued that the observatories should be established to monitor earthquakes [1]. Moreover, he believed that quantitative mechanical principles could determine the magnitude and location of an earthquake.
Around the same time, James Forbes designed the most significant instrument in seismology: seismometer. It consisted of a vertical metal rod having a mass moveable upon it. By adjusting the pendulum, a pencil placed on the prolongation of the metal rod could write a record on a stationary, paper-lined, spherical dome. Forbs were also the first to describe mathematically the behaviour of a seismic instrument in an “earthquake” [2]. Soon after, higher-quality pendulums were developed by John Milne, Sir James Ewing and Thomas Gray in Japan. Milne developed the horizontal pendulum, showing different seismic waves with distinct arrival times in 1897. However, these early instruments were undamped, which did not continuously record time. E. Wiechert introduced the first seismometer with viscous damping in 1898, which was capable of producing a useful record of the entire duration of an earthquake [3].
By 1900, Richard Oldham reported the identification of body waves and surface waves, later in 1906, he detected the presence of Earth’s core from the absence of direct P and S arrivals. In 1909, Andrija Mohorovicic discovered the crust-mantle boundary due to the velocity discontinuity. The student of Wiechert, K. Zoppritz, determined a set of widely-used travel times and worked out the equations for transmission of elastic waves at an interface [1]. Seismic surveying using explosions and other artificial sources was developed during the 1920s and 1930s for prospecting purposes in hydrocarbon exploration sites in Mexico and the United States.
Since then, seismology has been used in investigating Earth’s interior structure. In 1926, H. Jeffreys was the first to claim that below the mantle, the core of Earth is liquid [4]. 11 years after, Danish seismologist Inge Lehmann discovered the Earth’s solid inner core. In 1935, Charles Richter developed his logarithmic scale of earthquake magnitude, which allows a huge ranged of earthquakes sizes to be conveniently measured. By 1940, the seismic-wave velocity structure of mantle and core had been worked out, but many travel-time investigations were specific to particular earthquakes. H. Jeffreys and K.E. Bullen used global data to iteratively construct improved epicenters and travel times. This study showed that the Earth was nearly spherical with only a few major internal discontinuities. Later, they published the final versions of their travel-time tables for many seismic phases, which are accurate enough to still be in use today.
In 1961, the Worldwide Standardized Seismograph Network (WWSSN) was established. The availability of records from these seismographs led to rapid improvements in many aspects of seismology. The year 1960 brought the first detection of Earth’s free oscillations from the great Chilean earthquake. Several groups applied Fourier analysis to their records. At 1960 International Union of Geodesy and Geophysics (IUGG) meeting, the peaks were found to match the computed periods of oscillation. Analyses of normal mode data also emphasized the significance of geophysical inverse theory, providing tools in evaluating Earth’s structure from indirect observations.
Between 1969 and 1972, the Apollo mission brought seismometers on the Moon. As the result, the first lunar quakes were recorded. A seismometer placed on Mars by the Viking 2 probe in 1976 was hampered by wind noise, and only one possible Mars quake was identified.
Nowadays, seismology is widely used to detect both near-surface geology and Earth’s interior and construct 3D models of the Earth.
2019-2-17-1550415032