Seismology

Seismology is the scientific discipline concerned with the study of earthquakes and the propagation of elastic waves through planetary bodies. It encompasses the examination of naturally occurring seismic events, such as tectonic earthquakes, volcanic tremors and oceanic microseisms, as well as induced seismicity generated by human activities including explosions and industrial processes. The field also explores environmental effects associated with seismic events, notably tsunamis, landslides and ground deformation. Seismology relies heavily on instrumental records, and the measurement of ground motion using seismographs produces seismograms that allow scientists to infer the characteristics of both the seismic source and the structure of the Earth’s interior. Specialists in this domain are known as seismologists, and the closely related field of palaeoseismology draws upon geological evidence to reconstruct ancient seismic activity.

Historical Development

Interest in earthquakes has roots extending back to antiquity, with early natural philosophers attempting to explain the causes of ground shaking. Greek thinkers such as Thales of Miletus, Anaximenes and Aristotle proposed theories linking earthquakes to subterranean processes. In China, during the Han dynasty, Zhang Heng designed the first known seismoscope in 132 CE, marking an early attempt to create an instrument capable of detecting distant tremors.
During the seventeenth century, European scholars continued to speculate on the internal workings of the Earth. Athanasius Kircher suggested that earthquakes were caused by the movement of fire through channels beneath the surface, while Martin Lister and Nicolas Lemery proposed chemical reactions as a cause of ground motion. A turning point in scientific understanding came after the 1755 Lisbon earthquake, which stimulated systematic studies of seismic phenomena. John Bevis and John Michell produced some of the earliest analytical works; Michell recognised that earthquakes originated deep within the Earth and propagated as waves caused by shifting rock masses.
Further progress was made during the nineteenth century. A series of earthquakes in Comrie, Scotland, in 1839 led to the establishment of a committee dedicated to improving seismic detection. Their efforts supported the development of early seismometers, including one of the first modern designs attributed to James David Forbes and reported by David Milne-Home in 1842. Although this inverted-pendulum instrument had limitations, it represented an important step in instrumental seismology.
From 1857, Robert Mallet advanced the field through controlled experiments using explosives to investigate wave propagation. He also introduced the term seismology, and his contributions earned him recognition as the Father of Seismology. In 1889, Ernst von Rebeur-Paschwitz recorded the first teleseismic signal, detecting an earthquake in Japan from an observatory in Potsdam, which demonstrated the global reach of seismic waves.
The early twentieth century brought further major discoveries. Emil Wiechert calculated that the Earth comprised a silicate mantle surrounding an iron-rich core. Richard Dixon Oldham identified distinct P waves, S waves and surface waves on seismograms and provided the first clear evidence for the existence of a central core. In 1909, Andrija Mohorovičić discovered the discontinuity separating the Earth’s crust from the mantle, now known as the Mohorovičić or Moho discontinuity, defined by a sudden increase in seismic wave velocity.
Following the 1906 San Francisco earthquake, Harry Fielding Reid formulated the elastic rebound theory, explaining that earthquakes occur when accumulated strain along faults is suddenly released. This remains central to modern tectonic studies. In 1920, after the Xalapa earthquake in Mexico, the deployment of a Wiechert seismograph allowed researchers to record aftershocks and determine that the event originated from a shallow crustal fault.
By the mid-twentieth century, instrumental improvements and theoretical advances had deepened understanding of Earth’s interior. In 1926, Harold Jeffreys argued that the Earth’s outer core is liquid, and in 1937 Inge Lehmann identified a solid inner core within it. In 1950, Michael S. Longuet-Higgins advanced knowledge of oceanic microseisms by describing the wave interactions responsible for background seismic noise. The emergence of plate tectonic theory during the 1960s integrated these insights into a comprehensive explanation of the origin and global distribution of earthquakes.

Types of Seismic Waves

Seismic waves are elastic disturbances that propagate through solids or fluids and constitute the primary information source for inferring Earth’s internal structure. They are broadly categorised into body waves, surface waves and normal modes.
Body waves travel through the Earth’s interior and include two principal types.• P waves, or primary waves, are longitudinal waves involving alternating compression and rarefaction, with particle motion parallel to the direction of propagation. They travel fastest and therefore appear first on a seismogram.• S waves, or secondary waves, are transverse waves involving shear motion perpendicular to the direction of travel. They are slower than P waves and arrive later. Because fluids lack shear strength, S waves propagate only through solid materials.
Surface waves travel along the Earth’s surface or along interfaces within the Earth and generally cause the most intense ground shaking due to their relatively slow decay with distance. They are dispersive, meaning their velocity depends on frequency. Two principal forms occur:• Rayleigh waves, involving a combination of compressional and vertical shear motion, can propagate in any solid medium.• Love waves, involving horizontal shear motion, require a change in elastic properties with depth—conditions commonly met in the Earth’s crust. Surface waves are strongly excited by shallow seismic sources and often produce the largest amplitudes on seismograms.
Normal modes represent standing-wave oscillations of the entire Earth, excited by exceptionally large earthquakes. These oscillations can persist for weeks and provide valuable constraints on the density, elasticity and structure of deep Earth layers. Their detection became possible in the 1960s thanks to the availability of high-fidelity global seismographic networks.

Earthquakes and Their Scientific Importance

Large earthquakes have historically prompted major advancements in seismology. The 1755 Lisbon earthquake inspired the earliest systematic studies, while the 1857 Basilicata earthquake in Italy and the 1906 San Francisco earthquake contributed to the development of modern theoretical models. The 1964 Alaska earthquake and the 2004 Sumatra–Andaman event generated extensive global seismic records, enabling improved understanding of rupture dynamics, tsunamigenesis and global wave propagation. The 2011 Tōhoku earthquake in Japan further accelerated the refinement of hazard assessment and early warning systems.
Seismology also informs the study of fault mechanics, plate boundaries and crustal deformation. By analysing the timing, frequency and characteristics of seismic events, scientists interpret how tectonic stresses accumulate and are released. This knowledge supports seismic hazard mapping, urban planning, and the design of earthquake-resistant structures.

Controlled Seismic Sources and Applications

Beyond natural earthquakes, controlled seismic sources such as explosions, vibroseis trucks and air guns are widely used in exploration geophysics. These artificial sources generate waves that reflect and refract at subsurface boundaries, allowing geophysicists to construct detailed images of geological structures. Applications include the identification of salt domes, anticlines and other traps that may contain hydrocarbons, as well as mapping fault zones and impact craters. A notable example is the confirmation of the Chicxulub crater in the Yucatán region, linked to the mass extinction at the end of the Cretaceous period, which was verified through seismic imaging of buried structures.
Seismological techniques are also employed in monitoring volcanic regions, assessing glacier dynamics, investigating river-induced vibrations and studying atmospheric interactions that produce microseisms. These diverse applications demonstrate the field’s broad relevance across Earth sciences and its importance to both academic research and applied industries.