earthquake seismology

earthquake seismology The first recorded accounts of earthquakes date back to 2000 bc. However, the era of earthquake seismology as a modern science began in the late 1800s with the installation of sensitive instruments called seismographs. The key questions of scientific enquiry (what, where, how, and how big?) have been recurring themes in earthquake seismology probably ever since humans first experienced this natural phenomenon and suffered its consequences.

In the attempt to answer these basic questions, three principal tools have been developed and used by seismologists. All three techniques require a seismogram or recording of the seismic waves generated by an earthquake (Fig. 1). These seismic waves are recorded as ground motions by seismographs at the Earth's surface.

When an earthquake is recorded by many seismographs arranged in a spatial network, its origin time, location, focal depth, and sometimes focal mechanism can be calculated. The intent of the first seismographic networks was indeed to determine the locations of, and to eventually evaluate the sources of, earthquakes. Such locations consist of a position defined in some horizontal coordinate system (e.g. latitude and longitude) and depth with respect to a datum, usually the Earth's surface. Earthquake origin times or the time of the rupture initiation of the causative fault can also be computed. The Worldwide Network of Standard Seismograph Stations (WWNSS) in the early 1960s provided the first opportunity to locate and quantify the size of earthquakes in a relatively uniform manner on a global scale (see Worldwide Standardized Seismographic Network).

Locating an earthquake is actually a sophisticated form of triangulation. Present-day location schemes commonly use an iterative least-squares approach (e. g. Geiger's method). Based on the arrival times of seismic waves (usually the initial compressional (P) wave and sometimes later phases such as the shear (S) wave; Fig. 1), the observed travel times to a trial location are compared against theoretical travel times based on a velocity model of the Earth. The differences between these times to all stations are then minimized by converging upon an event location where the sum of the square of the travel time residuals is minimized. The use of computers has made such iterative techniques more efficient.

Prior to the development of the idea of earthquake magnitude, the size of an earthquake could only be measured crudely by the intensity of its effects. The first intensity scale was developed by M. S. de Rossi and F. A. Forel in the 1880s. The most widely used scale today is a modified version of one originally developed by G. Mercalli. This 12-value scale is shown in Table 1. By taking an intensity scale as a basis, maps showing the distribution of reported intensities for a particular earthquake can be developed, and contours encompassing areas of similar intensity can be drawn. These isoseismal lines clearly indicate that generally the larger the earthquake, the larger the felt area and that intensities decrease away from the epicentre, that is, the point on the Earth's surface above the focus of the earthquake. Robert Mallet's detailed study of the 1857 Naples earthquake resulted in the first known isoseismal map.

Table 1. Abridged modified Mercalli intensity scale

1 Equivalent Rossi-Forel (RF) intensities.
I	Not felt except by a few under favourable circumstances (RF I)1
II	Felt only by a few persons at rest, especially on upper floors of
buildings. Delicately suspended objects may swing (RF I-II)
III	Felt quite noticeably indoors, especially on upper floor of
buildings, but many people do not recognize it as an earthquake.
Standing motor cars may rock slightly. Vibration like the passing of
a truck. Duration estimated (RF III)
IV	Felt indoors by many, outdoors by few during the day. Some
awakened at night. Dishes, windows, doors disturbed; walls make
creaking sound. Sensation like heavy truck striking the building.
Standing motor cars rocked noticeably (RF IV-V)
V	Felt by nearly everyone, many awakened. Some dishes, windows,
and other fragile objects broken; cracked plaster in a few places;
unstable objects overturned. Disturbances of trees, poles, and
other tall objects sometimes noticed. Pendulum clocks may stop
(RF V-VI)
VI	Felt by all, many frightened and run outdoors. Some heavy
furniture moved; a few instances of fallen plaster and damaged
chimneys. Damage slight (RF VI-VII)
VII	Everybody runs outdoors. Damage negligible in buildings of good
design and construction; slight to moderate in well-built ordinary
structures; considerable in poorly built or badly designed
structures; some chimneys broken. Noticed by persons driving
cars (RF VIII)
VIII	Damage slight in specially designed structures; considerable in
ordinary substantial buildings with partial collapse; great in poorly
built structures. Panel walls thrown out of frame structures. Fall of
chimneys, factory stacks, columns, monuments, walls. Heavy
furniture overturned. Sand and mud ejected in small amounts.
Changes in well-water levels. Persons driving cars disturbed (RF
VIII + to IX)
IX	Damage considerable in specially designed structures; well-
designed frame structures thrown out of plumb; great in
substantial buildings; with partial collapse. Buildings shifted off
foundations. Ground cracked conspicuously. Underground pipes
broken (RF IX +)
X	Some well-built structures destroyed; most masonry and frame
structures destroyed with foundations; ground badly cracked.
Rails bent. Landslides considerable from river banks and steep
slopes. Shifted sand and mud. Water splashed, slopped over
banks (RF X)
XI	Few, if any, [masonry] structures remain standing. Bridges
destroyed. Broad fissures in ground. Underground pipelines
completely out of service. Earth slumps and land slips in soft
ground. Rails bent greatly
XII	Damage total. Waves seen on ground surface. Lines of sight and
level distorted. Objects thrown into the air

In the early 1930s, Charles Richter, using a particular type of instrument called the Wood–Anderson seismograph, developed the local magnitude (M_L) scale for southern Californian earthquakes. This was a monumental step in earthquake seismology because it allowed for the first time a precise quantification of the size of a seismic event based on instrumental recordings.

Because M_L values were based on the amplitude of the largest wave recorded on a seismogram and were thus simple to calculate, the scale rapidly became a worldwide standard. Since then, several other magnitude scales, such as surface wave (M_s) and body-wave magnitudes (m_b), have come into use. Up to now, however, the M_L scale has been the most commonly used magnitude measure. The moment magnitude scale (M_w) has increasingly become the scale of choice among seismologists because it is based on seismic moment and is the best measure of earthquake size. The seismic moment of an earthquake is a function of the area of the fault that ruptures, the average displacement on the fault, and the shear modulus, a parameter that is related to the rigidity of the rocks in the fault zone. The units of seismic moment are dyne.cm (g.cm² s⁻²). In Table 1 of Earthquake hazards and prediction, some of the world's largest earthquakes ever recorded, their locations, magnitudes, and some of their reported effects are listed.

One of the most significant advances in earthquake seismology has been the development of the technique of focal mechanisms (also referred to as fault-plane solutions) by the Japanese seismologists T. Shida and H. Nakano and Perry Byerly of the University of California at Berkeley in the early 1900s. Focal mechanism studies provided the first insights into the source processes of seismic events.

The point source representation of an earthquake (a shear failure in rock; see earthquake mechanisms and plate tectonics) is that of a double-force couple consisting of two opposing force couples having no net force or torque. The P-wave radiation of such a double-couple results in a pattern of alternating quadrants of compressional and dilatational first motions; S-wave polarizations also show a quadrantal distribution for this type of source. The two orthogonal nodal planes separating these quadrants represent the fault plane and an imaginary plane called the auxiliary plane. When the first motion pattern over the focal sphere is plotted on a stereographic or equal-area projection, as is done for focal mechanisms, the orientations of the nodal planes and the type of faulting can be determined, as can two important axes, the pressure (P) and tension (T) axes of the causative stress field. These two axes approximate the directions of the maximum and minimum principal stresses, respectively. Figure 3 illustrates focal mechanisms for the three principal types of faulting: normal, reverse or thrust, and strike-slip (see Earthquake mechanism and plate tectonics). Combinations of these types, such as oblique faulting, are depicted by variations of these principal types of focal mechanisms.

More sophisticated techniques to analyse the source and rupture process of an earthquake were developed in the 1990s. Parameters such as seismic moment, stress drop (the difference between the initial and final stress acting on the fault resulting in an earthquake), average displacement across the fault, and rupture dimensions can now be calculated, based on an assumed model of the source such as the circular crack model developed by James Brune in 1970. More recently, inversion techniques utilizing seismograms have provided an even more detailed view of the rupture process by deducing the varied distribution of slip on the fault plane.

Using these basic tools, earthquake seismology has evolved into several fields of study focused on various aspects of earthquakes and their effects, including for example: their source processes (see earthquake mechanisms and plate tectonics); seismic wave propagation and Earth structure; seismicity and seismotectonics, the geographic distribution of earthquakes and their relationship to geological structures; and strong motion and seismic hazards (see strong-motion seismology and earthquake hazards and prediction).

Much of what has been learned about the internal structure and composition of the Earth has come from the analysis of seismograms. As seismic waves propagate through the Earth, their velocities vary, they attenuate at different rates as a function of the material they are travelling through, and they are reflected and refracted off compositional and structural boundaries. These effects can be deciphered from seismograms of recorded earthquakes (or explosions) to develop models of the Earth's velocity structure and hence its geological structure. For example, the analysis of a specific reflected phase recorded on seismograms led to the discovery of the Earth's outer core at a depth of 2900 km by R. D. Oldham and Beno Gutenberg at the beginning of the twentieth century.

Not only can the gross structure of the Earth be imaged, but also its finer structure, particularly within the crust. Using mathematical techniques, the travel times of earthquakes are inverted to develop an image of the continental crust. In volcanic areas, inversion has been successful in characterizing underlying magma chambers. Other approaches using artificial sources of seismic waves, such as explosions, and analysis of their reflected and refracted behaviour have been used extensively by the petroleum industry in its search for oil.

Seismicity studies attempt to characterize the spatial distribution of earthquakes, their temporal behaviour, and their sources, particularly as they pertain to geological structures such as faults. Evaluating the seismicity of a region is not only important in terms of understanding what role earthquakes might have in the tectonic deformation of a region and development of geological structures, but is vitally important in terms of addressing the hazard from identified seismic sources such as faults.

Ivan G. Wong

Bibliography

Bolt, B. A. (1993) Earthquakes. W. H. Freeman, New York.
Brumbaugh, D. S. (1999) Earthquakes, science and society. Prentice Hall, New Jersey.