Seismology

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Earthquakes
Types Mainshock Foreshock Aftershock Blind thrust Doublet Interplate Intraplate Megathrust Remotely triggered Slow Submarine Supershear Tsunami Earthquake swarm
Causes Fault movement Volcanism Induced seismicity
Characteristics Epicenter Epicentral distance Hypocenter Shadow zone Seismic waves P wave S wave
Measurement Seismometer Seismic magnitude scales Seismic intensity scales
Prediction Coordinating Committee for Earthquake Prediction Forecasting
Other topics Shear wave splitting Adams–Williamson equation Flinn–Engdahl regions Earthquake engineering Seismite Seismology
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Seismology(/saɪzˈmɒlədʒi,saɪs-/; fromAncient Greekσεισμός (seismós) meaning "earthquake" and -λογία (-logía) meaning "study of") is the scientific study ofearthquakes(or generally,quakes) and the generation and propagation ofelastic wavesthrough theEarthor otherplanetary bodies. It also includes studies ofearthquake environmental effectssuch astsunamisas well as diverseseismic sourcessuch as volcanic, tectonic, glacial,fluvial, oceanicmicroseism, atmospheric, and artificial processes such as explosions andhuman activities. A related field that usesgeologyto infer information regarding past earthquakes ispaleoseismology. A recording ofEarthmotion as a function of time, created by aseismographis called aseismogram. Aseismologistis a scientist works in basic or applied seismology.

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

In 132 CE, Zhang Heng of China'sHan dynastydesigned the first knownseismoscope.^[1][2][3]

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

TheLisbon earthquake of 1755, coinciding with the general flowering of science inEurope, set in motion intensified scientific attempts to understand the behaviour and causation of earthquakes. The earliest responses include work byJohn Bevis(1757) andJohn 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 nearComrieinScotlandin 1839, a committee was formed in theUnited Kingdomin order to produce better detection methods for earthquakes. The outcome of this was the production of one of the first modernseismometersbyJames David Forbes, first presented in a report byDavid Milne-Homein 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 Malletlaid 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 theEarth's interiorconsists of a mantle of silicates, surrounding a core of iron.^[8]

In 1906Richard Dixon Oldhamidentified the separate arrival ofP-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 theMohorovičić discontinuity.^[13]Usually referred to as the "Moho discontinuity" or the "Moho," it is the boundary between theEarth'scrustand themantle. 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 April1906 San Francisco earthquake,Harry Fielding Reidput 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 ofaftershocksfrom a destructive earthquake came after the January1920 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 Jeffreyswas 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 Lehmanndetermined that within Earth's liquidouter corethere is a solidinner 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 ofplate tectonics.^[19]

Seismic waves areelastic wavesthat propagate in solid or fluid materials. They can be divided intobody wavesthat travel through the interior of the materials;surface wavesthat travel along surfaces or interfaces between materials; andnormal modes, a form of standing wave.

There are two types of body waves, pressure waves or primary waves (P-waves) andshearor secondary waves (S-waves). P-waves arelongitudinal wavesthat involvecompressionandexpansionin the direction that the wave is moving and are always the first waves to appear on a seismogram as they are the fastest moving waves through solids.S-wavesaretransverse wavesthat move perpendicular to the direction of propagation. S-waves are slower than P-waves. Therefore, they appear later than P-waves on a seismogram. Fluids cannot support transverse elastic waves because of their low shear strength, so S-waves only travel in solids.^[20]

Surface waves are the result of P- and S-waves interacting with the surface of the Earth. These waves aredispersive, meaning that different frequencies have different velocities. The two main surface wave types areRayleigh waves, which have both compressional and shear motions, andLove waves, which are purely shear. Rayleigh waves result from the interaction of P-waves and vertically polarized S-waves with the surface and can exist in any solid medium. Love waves are formed by horizontally polarized S-waves interacting with the surface, and can only exist if there is a change in the elastic properties with depth in a solid medium, which is always the case in seismological applications. Surface waves travel more slowly than P-waves and S-waves because they are the result of these waves traveling along indirect paths to interact with Earth's surface. Because they travel along the surface of the Earth, their energy decays less rapidly than body waves (1/distance²vs. 1/distance³), and thus the shaking caused by surface waves is generally stronger than that of body waves, and the primary surface waves are often thus the largest signals on earthquakeseismograms. Surface waves are strongly excited when their source is close to the surface, as in a shallow earthquake or a near-surface explosion, and are much weaker for deep earthquake sources.^[20]

Both body and surface waves are traveling waves; however, large earthquakes can also make the entire Earth "ring" like a resonant bell. This ringing is a mixture ofnormal modeswith discrete frequencies and periods of approximately an hour or shorter. Normal mode motion caused by a very large earthquake can be observed for up to a month after the event.^[20]The first observations of normal modes were made in the 1960s as the advent of higher fidelity instruments coincided with two of the largest earthquakes of the 20th century the1960 Valdivia earthquakeand the1964 Alaska earthquake. Since then, the normal modes of the Earth have given us some of the strongest constraints on the deep structure of the Earth.

One of the first attempts at the scientific study of earthquakes followed the 1755 Lisbon earthquake. Other notable earthquakes that spurred major advancements in the science of seismology include the1857 Basilicata earthquake, the 1906 San Francisco earthquake, the1964 Alaska earthquake, the 2004Sumatra-Andaman earthquake, and the 2011Great East Japan earthquake.

Seismic waves produced byexplosionsor vibrating controlled sources are one of the primary methods ofunderground exploration in geophysics(in addition to many differentelectromagneticmethods such asinduced polarizationandmagnetotellurics). Controlled-source seismology has been used to mapsalt domes, anticlines and other geologic traps inpetroleum-bearingrocks,faults, rock types, and long-buried giantmeteorcraters. For example, theChicxulub Crater, which was caused by an impact that has beenimplicated in the extinctionof thedinosaurs, was localized to Central America by analyzing ejecta in theCretaceous–Paleogene boundary, and then physically proven to exist using seismic maps fromoil exploration.^[21]

Seismometersare sensors that detect and record the motion of the Earth arising from elastic waves. Seismometers may be deployed at the Earth's surface, in shallow vaults, in boreholes, orunderwater. A complete instrument package that records seismic signals is called aseismograph. Networks of seismographs continuously record ground motions around the world to facilitate the monitoring and analysis of global earthquakes and other sources of seismic activity. Rapid location of earthquakes makestsunamiwarnings possible because seismic waves travel considerably faster than tsunami waves. Seismometers also record signals from non-earthquake sources ranging from explosions (nuclear and chemical), to local noise from wind^[22]or anthropogenic activities, to incessant signals generated at the ocean floor and coasts induced by ocean waves (the globalmicroseism), tocryosphericevents associated with largeicebergsand glaciers. Above-ocean meteor strikes with energies as high as 4.2 × 10¹³J(equivalent to that released by an explosion of ten kilotons of TNT) have been recorded by seismographs, as have a number of industrial accidents and terrorist bombs and events (a field of study referred to asforensic seismology). A major long-term motivation for the global seismographic monitoring has been for the detection and study ofnuclear testing.

Because seismic waves commonly propagate efficiently as they interact with the internal structure of the Earth, they provide high-resolution noninvasive methods for studying the planet's interior. One of the earliest important discoveries (suggested byRichard Dixon Oldhamin 1906 and definitively shown by Harold Jeffreys in 1926) was that theouter coreof the earth is liquid. Since S-waves do not pass through liquids, the liquid core causes a "shadow" on the side of the planet opposite the earthquake where no direct S-waves are observed. In addition, P-waves travel much slower through the outer core than the mantle.

Processing readings from many seismometers usingseismic tomography, seismologists have mapped the mantle of the earth to a resolution of several hundred kilometers. This has enabled scientists to identifyconvection cellsand other large-scale features such as thelarge low-shear-velocity provincesnear thecore–mantle boundary.^[23]

Forecasting a probable timing, location, magnitude and other important features of a forthcoming seismic event is calledearthquake prediction. Various attempts have been made by seismologists and others to create effective systems for precise earthquake predictions, including theVAN method. Most seismologists do not believe that a system to provide timely warnings for individual earthquakes has yet been developed, and many believe that such a system would be unlikely to give useful warning of impending seismic events. However, more general forecasts routinely predictseismic hazard. Such forecasts estimate the probability of an earthquake of a particular size affecting a particular location within a particular time-span, and they are routinely used inearthquake engineering.

Public controversy over earthquake prediction erupted after Italian authoritiesindictedsix seismologists and one government official formanslaughterin connection witha magnitude 6.3 earthquake in L'Aquila, Italy on April 5, 2009.^[24]A report inNaturestated that the indictment was widely seen in Italy and abroad as being for failing to predict the earthquake and drew condemnation from theAmerican Association for the Advancement of Scienceand theAmerican Geophysical Union.^[25]However, the magazine also indicated that the population of Aquila do not consider the failure to predict the earthquake to be the reason for the indictment, but rather the alleged failure of the scientists to evaluate and communicate risk.^[26]The indictment claims that, at a special meeting inL'Aquilathe week before the earthquake occurred, scientists and officials were more interested in pacifying the population than providing adequate information about earthquake risk and preparedness.^[27]

In locations where a historical record exists it may be used to estimate the timing, location and magnitude of future seismic events. There are several interpretative factors to consider. The epicentres or foci and magnitudes of historical earthquakes are subject to interpretation meaning it is possible that 5–6 Mw earthquakes described in the historical record could be larger events occurring elsewhere that were felt moderately in the populated areas that produced written records. Documentation in the historic period may be sparse or incomplete, and not give a full picture of the geographic scope of an earthquake, or the historical record may only have earthquake records spanning a few centuries, a very short time frame in aseismic cycle.^[28][29]

Engineering seismology is the study and application of seismology for engineering purposes.^[30]It generally applied to the branch of seismology that deals with the assessment of the seismic hazard of a site or region for the purposes of earthquake engineering. It is, therefore, a link betweenearth scienceandcivil engineering.^[31]There are two principal components of engineering seismology. Firstly, studying earthquake history (e.g. historical^[31]and instrumental catalogs^[32]of seismicity) andtectonics^[33]to assess the earthquakes that could occur in a region and their characteristics and frequency of occurrence. Secondly, studying strong ground motions generated by earthquakes to assess the expected shaking from future earthquakes with similar characteristics. These strong ground motions could either be observations fromaccelerometersor seismometers or those simulated by computers using various techniques,^[34]which are then often used to develop ground motion prediction equations^[35](or ground-motion models)[1].

Seismological instruments can generate large amounts of data. Systems for processing such data include:

• CUSP (Caltech-USGS Seismic Processing)^[36]
• RadExPro seismic software
• SeisComP3^[37]

• European-Mediterranean Seismological Center, real-time earthquake information website.
• Seismological Society of America.
• Incorporated Research Institutions for Seismology.
• USGS Earthquake Hazards Program.
• A brief history of seismology to 1910(UCSB ERI)