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Earthquakes and Plate Tectonics
From: Cambridge University Press
| By:
Agustin Udias |
EDITOR'S INTRODUCTION |
Earthquakes and tremors are almost a part of daily routine in California and southern Europe.Why isn't it the same in London and New York? Part of the explanation lies in the idea of plate tectonics. In this brief extract from his book, Principles of Seismology, Agustmn Udmas outlines the connection between earthquakes and our understanding of the structure of the planet. |
 he term seismicity was probably used for the first time by Montessus de Bailore in 1906 to describe the distribution of earthquakes and their characteristics within a particular region. The most important aspects of seismicity are given by the geographic distribution of earthquakes' foci, their magnitude, their occurrence over time, their mechanisms, and the damage produced by them. Studies of seismicity are, then, based on seismic catalogs that include parameters such as the dates and times of occurrence, hypocenter coordinates, magnitudes and focal mechanisms of earthquakes and their correlation to regional geological and geophysical characteristics. |
 Historical descriptions of earthquakes can be traced back to written records of old civilizations such as those of China and, for Europe, of Greece and Rome. Among the first universal catalogs of earthquakes were those published by J. Zahn in 1680 and by J. J. Moreira de Mendonca in 1758. Modern catalogs started about 1850 with the work of Perrey, Mallet, and Milne. Today global and regional catalogs are being compiled by various agencies, such as the ISC (International Seismological Centre, Newbury) and NEIC (National Earthquake Information Service, Denver). Among the first studies of seismicity were those of Montessus de Ballore between 1850 and 1923, Sieberg between 1923 and 1933, Gutenberg and Richter (Seismicity of the Earth and Associated Phenomena, 1954) and Karnik (Seismicity of the European Area, 2 vols, 1969, 1971). Studies of seismicity are fundamental for understanding the seismotectonic and geodynamic conditions of a region and for the assessment of its seismic risk. |
From antiquity it has been known that some regions are more prone to the occurrence of earthquakes than are others. Thus we can separate seismically active regions from those that are more stable. Once the determination of epicenters had become sufficiently accurate, it was observed that earthquakes occur in narrow zones that surround relatively stable regions (Figure 1). These bands or alignments of earthquakes coincide in some cases with the margins of continents, or with oceanic ridges under the oceans. However, not all continental margins correspond to seismic zones, so that they can be divided into active and passive margins, which classification is related to plate tectonics. For example, the western margin of the Americas is active whereas the eastern margin is passive, the northern margin of Eurasia is passive whereas its southern and eastern margins are active, and all margins of Africa are passive. |
The depths of earthquakes vary from the surface to about 700 km. According to their depth, earthquakes are classified as shallow (less than 30 km deep), that is, inside the crust, intermediate (30--300 km deep), and deep (more than 300 km deep).  Intermediate and deep earthquakes occur in active zones where, according to plate tectonics, lithospheric plates are introduced into the mantle in zones of subduction. As a general rule, for oceanic ridges there are only shallow earthquakes whereas for island arc regions there are shallow, intermediate, and deep earthquakes (Figures 1 and 2). Although shallow earthquakes are classified as those less than 30 km deep, most of them take place at depths less than 20 km, that is, in the upper rigid part of the crust that is called the seismogenic layer. Earthquakes at greater depths correspond to zones where this rigid and relatively cold material of the upper crust is introduced into the upper mantle by lithospheric subduction. However, very deep earthquakes may be produced by processes related to phase changes in focal material instead of brittle fracture. |
According to plate tectonics, the global distribution of epicenters is related to boundaries between lithospheric plates (Figure 1). Earthquakes at plate boundaries are called interplate earthquakes. Less commonly, earthquakes also take place in plate interiors and these are called intraplate earthquakes. The most active region in the world corresponds to the margins of the Pacific Ocean. Earthquakes with large magnitudes take place along this zone in the Americas from the Aleutian Islands to southern Chile and from the Kamchatka peninsula in Asia to New Zealand. Besides shallow earthquakes, throughout most of this long region, intermediate and deep shocks take place along the margin of Central and South America and on the other side of the Pacific along the systems of island arcs (Aleutians, the Kuriles, Japan, the Philippines, Fiji, etc.) (Figure 2). |
Another large seismically active region is known as the Mediterranean-Alpine-Himalayas region and extends from West to East from the Azores to the eastern coast of Asia. This region is related to the boundary between the plates of Eurasia to the North and Africa, Arabia, and India--Australia to the South (Figure 1). Its seismicity involves shallow, intermediate, and deep earthquakes. A third seismic region is formed by earthquakes located on ocean ridges that form the boundaries of oceanic plates, such as the Mid-Atlantic Ridge, East Pacific Rise, etc. (Figure 1). In these regions earthquakes of shallow depths are concentrated in relatively narrow bands following the trend of the oceanic ridges. On comparing Figures 1 and 2, we can see the location of shallow and deep earthquakes and the presence of zones of concentrated seismicity and others with earthquakes spread over wide areas. In general, boundaries between oceanic plates and between oceanic and continental plates have simpler distributions of seismicity than do boundaries between continental plates. |
 An example of a complex distribution of seismicity in a boundary between two continental plates is the Mediterranean region (Figure 3) (A. Udías and E. Buforn, 'Seismotectonics of the Mediterranean Region', Adv. Geophys. 36 (1994), pp. 121--209). Inside this region the most active zones in order of activity correspond to Greece, Turkey, Italy and the Alps, the Carpathians and Dynarics, North Africa, southern Spain and the Pyrenees, and the Rhine Graben. Along this region, seismic activity is formed by a continuous occurrence of earthquakes of small and moderate magnitudes (M < 6) with the occurrence of large earthquakes (M > 6) separated by larger intervals of time. Destructive earthquakes have happened more or less often in all active regions. Most earthquakes are of shallow depths with four zones of intermediate and deep earthquakes, namely the Hellenic arc, the Carpathians, Sicily--Calabria, and southern Spain and Morocco. The deepest earthquakes have taken place in southern Spain (640 km) and Sicily-Calabria (450 km). In the other two regions the maximum depth is about 200 km. As can be seen from Figure 3, earthquakes are spread over wide areas, indicating the presence of several small plates between the two large plates of Eurasia and Africa. |
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