In the early times of 585 B.C.E seismology was born through Thales of Miletos developing the theory that earthquakes were due to natural causes, rather than the agitation of the gods, thus beginning the critical analysis of earthquakes. Aristotle contributed to this further in 340 BC with the concept that air had been trapped inside the Earth, and that earthquakes were simply a case of it trying to escape, which proved to be the foundation of seismic knowledge until the seventeenth century. Whilst theories and philosophies were developed over earthquakes it wasn’t until 132 AD that someone attempted to analyse them through means of equipment.
Designing the very first seismoscope, Chinese scholar Zhang Heng utilised an elegant and simple of ball bearing’s falling from an elevated position due to the earthquake’s shaking. Depending on where these balls fell would depict what direction the earthquake was travelling. Despite this ingenious thought it wasn’t until the 1600’s where the concept of understanding earthquakes was yet again put into practice. In 1660 Robert Hooke had laid the foundation for theories revolving around elasticity with his development of stress-strain relationships and laws. Later applied and used to describe the propagation of waves through the Earth. Similar to the thinking of Aristotle, Athanasius Kircher in 1664 suggested that the Earth consisted of a system of channels of fire, the motion of which generated earthquakes via moving against rocks that blocked its path, with volcanoes being a product of fire channels that end at the surface. Diverting from Aristotle’s theory Martin Lister in 1703 and Nicolas Lemcry theorised that the fire channels suggested by Athanasius were explosions that were the product of mixtures of iron, sulphate, salt and water.
November 1st, 1755 marks the date of the Lisbon earthquake, an earthquake that sparked the intrigue of numerous seismologists. One of these individuals was John Bevis who recorded numerous accounts of the earthquake in a mass survey, which proved incredibly useful in John Michell’s analysis of the earthquake in 1761. Michell concluded that the earthquake had a form of source point (now epicentre) from which waves propagated through rocks that acted elastically, in accordance with Hooke’s theorems. Working on this line of logic he attempted to locate this distant source point by comparing the arrival times of the waves across multiple location as well as identifying how sea waves were affected as a means of approximating the depth of the source point.
Utilising Newton’s law of gravitation Henry Cavendish in 1798 was able to estimate the Earth’s mean density, from which he learnt that the density exceeded that of the Earth’s surface rocks leading to the conclusion that the density of the Earth must increase with depth. Which was factored in future measurement of earthquake travel times. Following on from this Thomas Young was the first to identify elastic strain as shear, providing further evidence for earthquakes to propagate through the earth as waves.
With the progression of time earthquakes and their effects became considerable more well documented, with noticeable changes in the elevation of the Chilean coastline being identified by Maria Graham in 1822. This observation of coastal elevation changes would go on to be confirmed by H.M.S Beagle’s captain, Robert Firtzroy, following the 1835 Chilean earthquake.