Seismology

History[edit]

Scholarly interest in earthquakes can be traced back to antiquity. Early speculations on the natural causes of earthquakes were included in the writings of Thales of Miletus (c. 585 BCE), Anaximenes of Miletus (c. 550 BCE), Aristotle (c. 340 BCE), and Zhang Heng (132 CE).


In 132 CE, Zhang Heng of China's Han dynasty designed the first known seismoscope.^[1][2][3]


In the 17th century, Athanasius Kircher argued that earthquakes were caused by the movement of fire within a system of channels inside the Earth. Martin Lister (1638–1712) and Nicolas Lemery (1645–1715) proposed that earthquakes were caused by chemical explosions within the Earth.^[4]


The Lisbon earthquake of 1755, coinciding with the general flowering of science in Europe, set in motion intensified scientific attempts to understand the behaviour and causation of earthquakes. The earliest responses include work by John Bevis (1757) and John Michell (1761). Michell determined that earthquakes originate within the Earth and were waves of movement caused by "shifting masses of rock miles below the surface".^[5]


In response to a series of earthquakes near Comrie in Scotland in 1839, a committee was formed in the United Kingdom in order to produce better detection methods for earthquakes. The outcome of this was the production of one of the first modern seismometers by James David Forbes, first presented in a report by David Milne-Home in 1842.^[6] This seismometer was an inverted pendulum, which recorded the measurements of seismic activity through the use of a pencil placed on paper above the pendulum. The designs provided did not prove effective, according to Milne's reports.^[6]


From 1857, Robert Mallet laid the foundation of modern instrumental seismology and carried out seismological experiments using explosives. He is also responsible for coining the word "seismology."^[7]


In 1897, Emil Wiechert's theoretical calculations led him to conclude that the Earth's interior consists of a mantle of silicates, surrounding a core of iron.^[8]


In 1906 Richard Dixon Oldham identified the separate arrival of P-waves, S-waves and surface waves on seismograms and found the first clear evidence that the Earth has a central core.^[9]


In 1909, Andrija Mohorovičić, one of the founders of modern seismology,^[10][11][12] discovered and defined the Mohorovičić discontinuity.^[13] Usually referred to as the "Moho discontinuity" or the "Moho," it is the boundary between the Earth's crust and the mantle. It is defined by the distinct change in velocity of seismological waves as they pass through changing densities of rock.^[14]


In 1910, after studying the April 1906 San Francisco earthquake, Harry Fielding Reid put forward the "elastic rebound theory" which remains the foundation for modern tectonic studies. The development of this theory depended on the considerable progress of earlier independent streams of work on the behavior of elastic materials and in mathematics.^[15]


An early scientific study of aftershocks from a destructive earthquake came after the January 1920 Xalapa earthquake. An 80 kg (180 lb) Wiechert seismograph was brought to the Mexican city of Xalapa by rail after the earthquake. The instrument was deployed to record its aftershocks. Data from the seismograph would eventually determine that the mainshock was produced along a shallow crustal fault.^[16]


In 1926, Harold Jeffreys was the first to claim, based on his study of earthquake waves, that below the mantle, the core of the Earth is liquid.^[17]


In 1937, Inge Lehmann determined that within Earth's liquid outer core there is a solid inner core.^[18]


By the 1960s, Earth science had developed to the point where a comprehensive theory of the causation of seismic events and geodetic motions had come together in the now well-established theory of plate tectonics.^[19]