I'd like to know what western scientists thought about the causes of earthquakes in the mid to late 18th century (especially pertaining to the one in Lisbon in 1755). I've read that the ancient greeks thought that they were caused by rocking of the continental place, and that they were later believed to be somehow related to magnetism. Was any of this believed at the time I'm interested in?

Are there any sources I could read?

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    $\begingroup$ Kant's Theory of Earthquakes and Volcanic Action is said to be the first modern, naturalistic theory. "Kant supposed that volcanic action was due to the subterranean combination of sulphur and iron, and he rejected the suggestion that earthquakes might be due to the gravitational pull of heavenly bodies.". $\endgroup$ – Nick Jan 28 '19 at 17:48

Aristotle's idea of earthquakes caused by "winds within the Earth" had much currency for centuries, if they were ascribed to natural causes at all. Here is from Agnew's History of Seismology on the prevailing thought in the 17-18th centuries:

"With the decline of Aristotelian thought in early modern Europe, other ideas were put forward, though many of the writers were from northern Europe and so (unlike the Greeks) had little direct experience of earthquakes. They did, however, know about gunpowder: This new technology of chemical explosives suggested that earthquakes might be explosions in the Earth (or in the air); of the various chemical theories put forward, the most popular involved the combustion of pyrites or a reaction of iron with sulfur. Such theories also explained volcanic action; that the most seismic part of Europe, namely Italy, was also volcanic helped to support this association. In the 18th century the development of theories of electricity, and especially their application to lightning, provoked several theories that related earthquakes to electrical discharges."

One of the electrical theories was proposed by Priestley. And as late as 1750, a writer in the Philosophic Transactions of the Royal Society still apologized to "those who are apt to be offended at any attempts to give a natural account of earthquakes".

The Lisbon Earthquake of 1755 was indeed a watershed event. Here is from A Brief History of Seismology to 1910 based on The Founders of Seismology by Davison:

"Empirical observations of the effects of earthquakes were rare, however, until 1750, when England was uncharacteristically rocked by a series of five strong earthquakes. These earthquakes were followed on Sunday, November 1, 1755, by a cataclysmic shock and tsunami that killed an estimated 70,000 people, leveling the city of Lisbon, Portugal, while many of its residents were in church. This event marks the beginning of the modern era of seismology, prompting numerous studies into the effects, locations, and timing of earthquakes.

Prior to the Lisbon earthquake, scholars had looked almost exclusively to Aristotle, Pliny, and other ancient classical sources for explanations of earthquakes. Following the Lisbon earthquake, this attitude was jettisoned for one that stressed ideas based on modern observations. Cataloging of the times and locations of earthquakes and studying the physical effects of earthquakes began in earnest, led by such people as John Michell in England and Elie Bertrand in Switzerland."

But for almost a century this activity was mostly phenomenological, not explanatory. Michell's 1761 explanation was close to the modern one, and was based on Bevis's History and Philosophy of Earthquakes, where he collected accounts of the Lisbon earthquake from first hand sources. Michell wrote:

"Earthquakes were waves set up by the shifting masses of rock miles below the surface.., the motion of the earth in earthquakes is partly tremulous and partly propagated by waves which succeed each another".

He even estimated that the Lisbon earthquake waves had traveled outward at 530 m/sec. Drijhout in 1765 and Young in 1807 also supported the wave theory. However, Michell's work was "very much overlooked" until a century later, according to Mallet, "the first true seismologist" who also coined the word "seismology", see A concise history of mainstream seismology by Ben-Menahem. As Agnew writes, the older theories continued to dominate:

"The type of wave envisaged was like a traveling wrinkle in a carpet; Michell also suggested that the vibrations close to the source were related to waves propagated through the elasticity of the rocks, as sound waves were known to propagate through the elasticity of the air. The elasticity and pressure of gases, more specifically of high-temperature steam, also provided Michell's driving force for the earthquake itself, which he took to be caused by water vaporized by sudden contact with underground fires. (This steam also supported the propagation of waves to great distances.) Michell attempted to locate this "place of origin" by comparing the times of the seiche-inducing wave and the observed sea wave, and also hazarded a guess at the depth. While these ideas were not forgotten, they did not lead to any additional research, and certainly did not replace older theories of earthquakes."

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