Earthquakes are a reality of our world, and they have been for centuries. While at present it would be exceptionally difficult to find someone who is unaware of this phenomenon and its causes, this was not always the case. Seismology, the study of seismic sources, the waves they produce, and the properties of the media through which these waves travel, is just over 100 years old in its modern form. Through the historical journey of seismology, let’s look at how people came to develop a better understanding of earthquakes.
In early cultures, the most frequent explanations given for earthquakes were very similar to those of any natural disaster: earthquakes were caused by divine powers. Up until the 18th century, this idea was prevalent in scientific discussions in European culture. In Chinese and Greek cultures, seismic events were explained with the help of nature. A variety of causes were suggested by Greek philosophers, the most prominent being the study of Aristotle, who made a connection between earthquakes and winds blowing in underground caverns. This view remained the principal theory during the medieval periods.
Due to the decline of Aristotelian ideology in early modern Europe, new ideas were proposed, mostly by northern Europeans who did not have many experiences related to earthquakes. Instead, they were knowledgeable about gunpowder. They connected the two and suggested that earthquakes might be explosions in the Earth. During the 18th century, the development of electricity produced theories that linked earthquakes with electrical discharges.
Undeniably, the turning point for seismological thinking was the Lisbon earthquake of 1775, which was extremely destructive. It provided evidence of motion at large distances. After this disaster, it was proposed by writers such as John Bevis and John Michell that the distant motion was created by a wave spreading from a distinct location. This wave was likened to a moving wrinkle in a carpet. While this idea was of importance, it couldn’t replace older theories and did not make way for any significant research.
Due to numerous earthquakes happening near Comrie in Scotland in 1839, a committee was established in the United Kingdom for the purpose of developing detection methods for earthquakes. As a result, one of the first modern seismometers was produced by James David Forbes, initially presented in a report by David Milne-Home in 1842. Though the design for the seismometer did not prove effective, it was undoubtedly a step forward in the field.
In 1857, geophysicist, civil engineer, and inventor Robert Mallet laid the foundation of modern instrumental seismology and carried out multiple experiments using explosives. Considered responsible for inventing the word “seismology”, he is often recognized as the father of the scientific study of earthquakes and seismic waves.
Forty years after Mallet’s discovery, Emil Wiechert concluded that the Earth’s interior consisted of a mantle of silicates, salts containing anions of silicon and oxygen, surrounding a core of iron. Following Wiechert, in 1906, Richard Dixon Oldham found the first direct evidence that the Earth has a central core.
Andrija Mohorovičić, one of the founders of modern seismology, discovered in 1909 the Mohorovičić discontinuity, generally referred to as the “Moho discontinuity” or plainly “Moho”. It referred to the boundary between the mantle and the Earth’s crust. Seismological waves' unique shift in velocity as they traverse varying rock densities serves as their defining characteristic.
In the time following, Harry Fielding Reid proposed the "elastic rebound theory" in 1910 after researching the April 1906 San Francisco earthquake; it is still the cornerstone of contemporary tectonic investigations. The substantial advancement of earlier, separate streams of research on the behavior of elastic materials and in mathematics was a prerequisite for the formulation of this theory.
Harold Jeffreys was the first to assert that the Earth's core is liquid beneath its mantle in 1926, based on his research into earthquake waves. Succeeding Jeffreys, Inge Lehmann discovered that the Earth has a solid inner core inside of its liquid outer core in 1937. By the 1960s, Earth science had advanced to the point that plate tectonics, a now well-established theory that explains the causes of seismic events and geodetic motions, had been developed. At present, seismology is the greatest tool to understand the phenomena that are earthquakes.
works cited:
Oldroyd, David (2007). "The Study of Earthquakes in the Hundred Years Following Lisbon Earthquake of 1755"
Society, The Royal (2005-01-22). "Robert Mallet and the 'Great Neapolitan earthquake' of 1857”
Barckhausen, Udo; Rudloff, Alexander (14 February 2012). "Earthquake on a stamp: Emil Wiechert honored"
Rudnick, R. L.; Gao, S. (2003-01-01), Holland, Heinrich D.; Turekian, Karl K. (eds.), "3.01 – Composition of the Continental Crust"
"Reid's Elastic Rebound Theory". 1906 Earthquake . United States Geological Survey.
Hjortenberg, Eric (December 2009). "Inge Lehmann's work materials and seismological epistolary archive"
Jeffreys, Harold (1926-06-01). "On the Amplitudes of Bodily Seismic Waues"
Agnew, Duncan Carr. History of Seismology. 2002, igppweb.ucsd.edu/~agnew/Pubs/agnew.a66.pdf. Accessed 16 Feb. 2023.