Seismology (; derived from Ancient Greek σεισμός (seismós), meaning 'earthquake', and -λογία (-logía), meaning 'study of') constitutes the scientific discipline dedicated to the investigation of earthquakes (or, more broadly, quakes) and the generation and propagation of elastic waves through planetary bodies. This field also encompasses research into the environmental impacts of seismic events, such as tsunamis; other natural seismic sources, including volcanoes, plate tectonics, glaciers, rivers, oceanic microseisms, and atmospheric phenomena; and anthropogenic processes like explosions.
Paleoseismology represents a related discipline that employs geological evidence to deduce information about historical earthquakes. A seismogram is defined as a temporal record of Earth's motion, produced by a seismograph. A seismologist is a scientist engaged in either fundamental or applied seismological research.
History
Ancient and Classical Eras
Scholarly inquiry into earthquakes dates back to antiquity. Early hypotheses concerning the natural origins of earthquakes were documented 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 engineered the earliest known seismoscope.
Genesis of Modern Science
During the 17th century, Athanasius Kircher posited that earthquakes resulted from the movement of subterranean fire within an intricate system of Earth's internal channels. Subsequently, Martin Lister (1638–1712) and Nicolas Lemery (1645–1715) proposed that chemical explosions occurring within the Earth were the causative agents of earthquakes.
The Lisbon earthquake of 1755, occurring concurrently with a period of significant scientific advancement in Europe, spurred intensified scientific endeavors to comprehend the behavior and etiology of earthquakes. Initial contributions included the work of John Bevis (1757) and John Michell (1761). Michell concluded that earthquakes originate within the Earth and represent waves of motion generated by "shifting masses of rock miles below the surface".
Following a series of seismic events near Comrie, Scotland, in 1839, a committee was established in the United Kingdom to develop improved earthquake detection methodologies. This initiative culminated in the creation of one of the first modern seismometers by James David Forbes, which was initially presented in a report by David Milne-Home in 1842. This instrument, an inverted pendulum, recorded seismic activity measurements using a pencil positioned above the pendulum to mark paper. However, Milne's reports indicated that the proposed designs did not prove effective.
Commencing in 1857, Robert Mallet established the foundational principles of modern instrumental seismology and conducted seismological experiments utilizing explosives. He is also credited with coining the term "seismology" and is widely recognized as the "Father of Seismology".
In 1889, Ernst von Rebeur-Paschwitz successfully recorded the first teleseismic earthquake signal, originating from an earthquake in Japan and detected in Potsdam, Germany.
In 1894, Fusakichi Omori demonstrated that the frequency of earthquake aftershocks diminishes following a mainshock, a conclusion derived from his analysis of the 1889 Kumamoto, 1891 Mino–Owari, and 1893 Kagoshima earthquakes.
By 1897, Emil Wiechert's theoretical calculations led him to infer that the Earth's interior comprises a silicate mantle encasing an iron core.
In 1906, Richard Dixon Oldham identified the distinct arrival times of P waves, S waves, and surface waves on seismograms, thereby providing the first clear evidence for the existence of Earth's central core.
In 1909, Andrija Mohorovičić, a pivotal figure in the development of modern seismology, discovered and formally defined the Mohorovičić discontinuity. Commonly referred to as the "Moho discontinuity" or simply the "Moho," this boundary separates the Earth's crust from its mantle. Its definition is based on the pronounced change in the velocity of seismological waves as they traverse regions of varying rock densities.
In 1910, subsequent to his investigation of the April 1906 San Francisco earthquake, Harry Fielding Reid advanced the "elastic rebound theory," which remains a cornerstone of contemporary tectonic studies. The formulation of this theory was significantly dependent on substantial prior advancements in the understanding of elastic material behavior and in the field of mathematics.
The January 1920 Xalapa earthquake prompted one of the earliest scientific investigations into aftershocks following a destructive seismic event. An 80 kg (180 lb) Wiechert seismograph was transported by rail to Xalapa, Mexico, subsequent to the earthquake. This instrument was then deployed to document the ensuing aftershocks. Ultimately, the data acquired from the seismograph revealed that the mainshock originated along a shallow crustal fault.
Harold Jeffreys, in 1926, became the first to propose that Earth's core, situated beneath the mantle, exists in a liquid state, a conclusion derived from his analysis of earthquake waves.
Inge Lehmann's research in 1937 established the presence of a solid inner core within Earth's liquid outer core.
Michael S. Longuet-Higgins, in 1950, clarified the oceanic processes that generate the global background seismic microseism.
By the 1960s, advancements in Earth science culminated in the integration of a comprehensive theory explaining the genesis of seismic events and geodetic motions, forming the now widely accepted theory of plate tectonics.
Types of Seismic Waves
Seismic waves are defined as elastic disturbances that propagate through solid or fluid media. These waves are categorized into three primary types: body waves, which traverse the internal structure of materials; surface waves, which propagate along the boundaries or interfaces between different materials; and normal modes, which represent a form of standing wave.
Body Waves
Body waves comprise two distinct categories: primary waves (P-waves), also known as pressure waves, and secondary waves (S-waves), or shear waves. P-waves are longitudinal waves characterized by compressional and extensional particle motion, which occurs parallel to the direction of wave propagation. As the fastest seismic waves in solid media, P-waves are invariably the initial arrivals recorded on a seismogram. Conversely, S-waves are transverse waves involving shear deformation, with particle motion perpendicular to the wave's propagation direction. S-waves exhibit slower velocities than P-waves, consequently appearing later on seismograms. Due to their inherent lack of shear strength, fluids are incapable of sustaining transverse elastic waves; thus, S-waves propagate exclusively through solid materials.
Surface Waves
Surface waves originate from the interaction of P-waves and S-waves with the Earth's surface. These waves exhibit dispersion, implying that their velocities vary with frequency. The two principal types of surface waves are Rayleigh waves, characterized by both compressional and shear motions, and Love waves, which are exclusively shear. Rayleigh waves arise from the interaction of P-waves and vertically polarized S-waves at the surface and can propagate through any solid medium. Love waves are generated by horizontally polarized S-waves interacting with the surface and require a variation in elastic properties with depth within a solid medium to exist, a condition consistently met in seismological contexts. Surface waves propagate at slower velocities than both P-waves and S-waves, a consequence of their indirect propagation paths involving interaction with the Earth's surface. Their energy attenuates less rapidly than that of body waves (following a 1/distance2 rather than a 1/distance3 relationship) due to their confinement to the surface. Consequently, the ground motion induced by surface waves is typically more pronounced than that caused by body waves, often resulting in surface waves being the most prominent signals on earthquake seismograms. Surface waves are robustly excited by shallow sources, such as superficial earthquakes or near-surface explosions, but are significantly attenuated for deeper seismic origins.
Normal Modes
While both body and surface waves are propagating phenomena, significant earthquakes possess the capacity to induce a global oscillation, causing the Earth to 'ring' akin to a resonant bell. This oscillatory response comprises a superposition of normal modes, characterized by discrete frequencies and periods generally an hour or less. Normal-mode displacements resulting from exceptionally large earthquakes can persist and be detectable for up to a month post-event. Initial observations of normal modes occurred in the 1960s, coinciding with the development of higher-fidelity instrumentation and two of the twentieth century's most powerful seismic events: the 1960 Valdivia earthquake and the 1964 Alaska earthquake. Subsequent to these discoveries, Earth's normal modes have provided crucial constraints for understanding the planet's deep internal structure.
Earthquakes
The 1755 Lisbon earthquake prompted one of the initial scientific investigations into seismic events. Subsequent significant earthquakes, such as the 1857 Basilicata, 1906 San Francisco, 1964 Alaska, 2004 Sumatra-Andaman, and 2011 Great East Japan earthquakes, have driven substantial progress in the field of seismology.
Controlled Seismic Sources
In geophysics, seismic waves generated by explosions or precisely controlled vibrating sources constitute a principal technique for subsurface exploration, complementing various electromagnetic methodologies like induced polarization and magnetotellurics. Controlled-source seismology has been instrumental in delineating geological features such as salt domes, anticlines, other petroleum-bearing rock traps, faults, diverse rock types, and ancient, deeply buried impact craters. A notable illustration is the Chicxulub Crater, an impact structure linked to the dinosaur extinction event, which was initially pinpointed in Central America through the analysis of ejecta within the Cretaceous–Paleogene boundary and subsequently confirmed via seismic mapping derived from oil exploration data.
Seismic Wave Detection
Seismometers are specialized sensors designed to detect and record terrestrial motion resulting from elastic waves. These instruments can be deployed across various environments, including the Earth's surface, shallow subterranean vaults, boreholes, or submerged locations. A comprehensive instrumentation system capable of recording seismic signals is termed a seismograph. Global networks of seismographs continuously monitor ground movements, enabling the ongoing analysis and surveillance of worldwide earthquakes and other seismic phenomena. The prompt localization of earthquakes is crucial for issuing tsunami warnings, given that seismic waves propagate significantly faster than tsunami waves.
Beyond tectonic events, seismometers also capture signals originating from diverse non-earthquake sources. These include nuclear and chemical explosions, localized environmental noise from wind or human activities, persistent signals generated by ocean waves interacting with the seafloor and coastlines (known as the global microseism), and cryospheric occurrences linked to substantial icebergs and glaciers. Seismographs have documented meteor impacts occurring above oceans, with energies reaching 4.2 × 1013 J, an energy output comparable to a ten-kiloton TNT explosion. Furthermore, these instruments have recorded various industrial accidents and acts of terrorism, a specialized area of investigation termed forensic seismology. A significant, enduring impetus for establishing global seismographic monitoring networks has been the detection and analysis of nuclear weapons testing.
Mapping the Earth's Interior
Seismic waves, due to their efficient propagation and interaction with the Earth's internal structure, offer high-resolution, noninvasive techniques for investigating the planet's interior. A pivotal early discovery, initially proposed by Richard Dixon Oldham in 1906 and conclusively demonstrated by Harold Jeffreys in 1926, established the liquid state of the Earth's outer core. Given that S-waves cannot traverse liquids, the presence of a liquid core creates a "shadow zone" on the side of the planet opposite an earthquake, where direct S-waves are absent. Moreover, P-waves exhibit a significantly reduced velocity when passing through the outer core compared to the mantle.
By employing seismic tomography to process data from numerous seismometers, seismologists have achieved a mapping resolution of several hundred kilometers for the Earth's mantle. This advanced technique has facilitated the identification of convection cells and other extensive geological features, including the prominent large low-shear-velocity provinces situated near the core–mantle boundary.
Seismology and Society
Earthquake Prediction
The L'Aquila earthquake on April 5, 2009, a magnitude 6.3 event, triggered a significant public controversy regarding earthquake prediction when Italian authorities charged six seismologists and one government official with manslaughter. A report published in *Nature* indicated that this indictment was widely perceived both domestically and internationally as a prosecution for the failure to predict the earthquake, leading to condemnation from organizations such as the American Association for the Advancement of Science and the American Geophysical Union. Nevertheless, the publication also noted that the residents of L'Aquila did not attribute the indictment to a failure in earthquake prediction, but rather to the scientists' alleged inability to assess and communicate risk effectively. The indictment specifically asserted that, during a special meeting held in L'Aquila the week prior to the earthquake, scientists and officials prioritized reassuring the populace over disseminating sufficient information concerning earthquake risk and preparedness.
Historical records, where available, can be utilized to estimate the timing, location, and magnitude of prospective seismic events. However, several interpretative factors must be considered. The epicenters, foci, and magnitudes of past earthquakes are subject to interpretation; consequently, events described as 5–6 Mw in historical accounts might represent larger earthquakes originating elsewhere but felt moderately in documented populated areas. Furthermore, historical documentation can be sparse or incomplete, potentially failing to provide a comprehensive understanding of an earthquake's geographic extent. Additionally, historical records may only encompass a few centuries of seismic activity, which constitutes a very brief period within a typical seismic cycle.
Engineering Seismology
Engineering seismology involves the study and practical application of seismological principles for engineering objectives. This field primarily focuses on evaluating the seismic hazard of a specific site or region to inform earthquake engineering practices, thereby serving as an interdisciplinary link between earth science and civil engineering. The discipline comprises two main components. The first involves analyzing earthquake history, including historical and instrumental seismicity catalogs, and regional tectonics to determine potential earthquakes, their characteristics, and their frequency of occurrence. The second component entails investigating strong ground motions produced by earthquakes to predict the anticipated shaking from future events with comparable attributes. These strong ground motions can be derived from accelerometer or seismometer observations, or they can be simulated computationally using various methodologies, which are subsequently often employed to formulate ground-motion prediction equations, also known as ground-motion models .
Tools
Seismological instrumentation can generate substantial volumes of data, which are processed by systems such as:
- CUSP (Caltech-USGS Seismic Processing)
- RadExPro seismic software
- SeisComP3
List of Seismologists
Notes
Notes
References
- USGS Earthquake Hazards Program
- A brief history of seismology to 1910 (UCSB ERI)