earthquake mechanisms and plate tectonics

earthquake mechanisms and plate tectonics Since the dawn of man and his first experiences with earthquakes, the question of their origin has been asked. However, the responses were by and large mystical until about the past millennium. The modern concept of earthquakes, their causes and mechanisms, had its beginning in the 1880s. G. K. Gilbert, an American geologist, was one of the first to suggest that earthquakes were the result of displacement along geological faults. In his observations of the Wasatch Mountains in north-central Utah in the western United States, Gilbert suggested that the mountains were the result of upwards incremental displacements (earthquakes) along a range-bounding fault that moved due to the constant accumulation of strain. In the century prior to this, it was believed that earthquakes were the result of magmatic processes or some form of geologically generated explosions, and that faulting simply accompanied earthquakes rather than being their cause.

In 1910, H. F. Reid suggested that earthquakes were the result of a phenomenon called ‘elastic rebound’, based on his observations of the great 1906, magnitude (M) 8, San Francisco (California) earthquake. This theory states that an earthquake is generated by rupture or sudden displacement along a fault when it has been strained beyond its elastic strength. In the process of strain accumulation, the opposing sides of the fault are stressed until failure occurs, that is, sudden displacement takes place, and then they rebound back to an unstrained position. The result of each cycle of strain accumulation along a fault generates an earthquake. The elastic rebound theory has become the accepted model for the generation of most, but not all, earthquakes. Some types of volcanic earthquakes and deep earthquakes may have different mechanisms. Also, displacement along faults does not necessarily always result in earthquakes. Slow displacement, or creep, is an aseismic (i.e. non-seismic) process that has been observed along faults worldwide.

Most earthquakes are essentially the product of tectonic stresses which are generated at the boundaries of the Earth's tectonic plates. Such tectonic earthquakes, as we shall refer to them, can range in size from magnitudes less than zero, resulting from fault slippage of a few centimetres, to the largest events, M greater than 9, where fault displacements are on the order of many metres. The size of an earthquake is not only a function of the amount of displacement but also the area of the fault plane that ruptures (see earthquake seismology). Hence the larger the rupture area, the larger the earthquake. An M 7 earthquake ruptures a fault area of about 1000 km² or about 50 km long and 20 km wide.

The majority of the seismic energy released in the world is from earthquakes occurring along the plate boundaries, particularly around the Pacific Rim or the so-called ‘Ring of Fire’ (Fig. 1). Specifically, the Earth's greatest earthquakes are the result of incremental plate movement within subduction zones. The largest known earthquake was the 1960, M 9.5, earthquake that occurred along the South American subduction zone off the coast of Chile (see earthquake hazards and prediction and earthquake seismology). Earthquakes that occur along or in the vicinity of the plate boundaries are called interplate earthquakes, in contrast to intraplate earthquakes that occur in the interiors of the tectonic plates (see intraplate seismicity). Intraplate earthquakes seldom exceed M 8 in size.

Earthquakes can result from very rapid displacement on all types of faults. Faults can be classified into three general types based on their sense of displacement: normal, reverse or thrust, and strike-slip. The great subduction zone earthquakes are generated along the great thrust faults that constitute the boundaries between downgoing and overriding tectonic plates. Major strike-slip faults, such as the San Andreas fault, 1300 km long, which separates the Pacific and North American plates in California, and the North Anatolian fault in Turkey, 1100 km long, have also ruptured repeatedly in earthquakes as large as M 8. In extensional tectonic regimes where the Earth's crust is being extended or stretched, normal faulting has generated large earthquakes but not quite as large as those along subduction zones. In the interior of the western United States, normal-faulting earthquakes have created the horst and graben landscape of the Basin and Range province and the Rio Grande rift.

Most of the world's earthquakes occurring outside the subduction zones have their origins in the Earth's upper crust. At these relatively shallow depths, of generally less than 15–20 km, the temperatures are usually low enough (less than about 350 ° ± 100 °C) for sudden displacement to occur along faults cutting brittle rocks. This form of displacement is often called ‘stick-slip’; that is, strain builds up in the rocks next to the fault but there is no slip for some time because the rocks on either side of the fault are stuck together by features such as asperities. Eventually the resistance to sliding is overcome by shear stresses acting along the fault—giving a very rapid increment of slip. At greater temperatures (except for uppermost mantle rock, where the limiting temperature is thought to be about 650 ± 100 °C), faults will behave plastically and deform by aseismic creep instead of brittle failure. The deepest earthquakes in the world occur at depths of 600–700 km within subducting tectonic plates. Because the temperatures at these depths are extremely high, the mechanism by which these earthquakes are generated is not well understood.

Earthquakes often occur in sequences. The principal and largest event in a sequence is called the main shock. Foreshocks precede the main shock, and aftershocks can occur for tens of years after the main shock. Many tens of thousands of aftershocks occurred for several years after the 1964, M 9.2, Prince William Sound, Alaska earthquake, with several events as large as M 6. Aftershocks are generally the result of the release of residual stress in areas not ruptured in the main shock. Aftershocks need not, however, occur on the same fault as the main shock. For example, the 1992, M 7.3, Landers, California earthquake was followed 3 hours later by the M 6.2, Bear Lake earthquake, the source of which was an unnamed fault located 30–40 km to the west of the Landers zone.

During the process of fault slip and, hence, earthquake generation, tectonic strain energy is expended by the crushing of rock within the fault zone, production of heat, and a release of a small percentage of energy as seismic waves. The point on the fault plane where the earthquake rupture is initiated is called the focus or hypocentre. The point on the Earth's surface directly above the hypocentre is the epicentre. The focal depth of an earthquake is the depth of the hypocentre below the Earth's surface.

Seismic waves can be classified into three basic types: compressional or primary (P) waves, shear or secondary (S) waves, both which are body waves, and surface waves. P- and S- waves are called body waves because they can travel through the interior of a body such as the Earth. The P-wave, the fastest wave (i.e. it has the highest velocity) results in particles in the medium in which it is travelling, moving in a longitudinal direction (the same direction as the movement of the wave). S-waves generate transverse particle motion and they generally have velocities that are about a half to two-thirds that of the P-wave velocity.

In contrast to body waves, surface waves are confined to the outer layers of the Earth. There are two types of surface waves: Love and Rayleigh waves. The Love waves has a particle motion, which, like the shear wave, is transverse to the direction of propagation but with no vertical motion. Rayleigh waves have an elliptical and retrograde particle motion confined to the vertical plane in the direction of propagation. Deep earthquakes generally do not generate surface waves.

At the frequencies of most engineered structures, most of the seismic energy radiated as seismic waves is contained in the S-wave. It is for this reason, and also the fact that S-waves approaching the Earth's surface result in horizontal ground movement, that most of the ground shaking damage from earthquakes is due to S-waves (see strong-motion seismology).

Ivan G. Wong

Bibliography

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Yeats, R.,, Sieh, K.,, and and Allen, C. (1997) The geology of earthquakes. Oxford University Press, New York.