1.2.3 Earth structure

Early in the twentieth century it became evident from the study of seismic waves that the interior of the Earth has a radially layered structure, like that of an onion (Fig. 1.9). The boundaries between the layers are marked by abrupt changes in seismic velocity or velocity gradient. Each layer is characterized by a specific set of physical proper­ties determined by the composition, pressure and temper­ature in the layer. The four main layers are the crust, mantle and the outer and inner cores. Their properties are described in detail in Section 3.7 and summarized briefly here.

At depths of a few tens of kilometers under continents and less than ten kilometers beneath the oceans seismic velocities increase sharply. This seismic discontinuity, discovered in 1909 by A. Mohorovicic, represents the boundary between the crust and mantle. R. D. Oldham noted in 1906 that the travel-times of seismic compressional waves that traversed the body of the Earth were greater than expected; the delay was attributed to a fluid outer core. Support for this idea came in 1914, when B. Gutenberg described a shadow zone for seismic waves at epicentral distances greater than about 105°. Just as light waves cast a shadow of an opaque object, seismic waves from an earthquake cast a shadow of the core on the opposite side of the world. Compressional waves can in fact pass through the liquid core. They appear, delayed in time, at epicentral distances larger than 143°. In 1936 I. Lehmann observed the weak arrivals of compressional waves in the gap between 105° and 143°. They are inter­preted as evidence for a solid inner core.

Fig. 1.9 Simplified layered structure of the Earth’s interior showing the depths of the most important seismic discontinuities

1.2.3.1 Lithospheric plates

The radially layered model of the Earth's interior assumes spherical symmetry. This is not valid for the crust and upper mantle. These outer layers of the Earth show important lateral variations. The crust and uppermost mantle down to a depth of about 70-100 km under deep ocean basins and 100-150 km under continents are rigid, forming a hard outer shell called the lithosphere. Beneath the lithosphere lies the asthenosphere, a layer in which seismic velocities often decrease, suggesting lower rigidity. It is about 150 km thick, although its upper and lower boundaries are not sharply denned. This weaker layer is thought to be partially molten; it may be able to flow over long periods of time like a viscous liquid or plastic solid, in a way that depends on temperature and composition. The asthenosphere plays an important role in plate tectonics, because it makes possible the relative motions of the overlying lithospheric plates.

The brittle condition of the lithosphere causes it to fracture when strongly stressed. The rupture produces an earthquake, which is the violent release of elastic energy due to sudden displacement on a fault plane. Earthquakes are not distributed evenly over the surface of the globe, but occur predominantly in well-defined narrow seismic zones that are often associated with volcanic activity (Fig. 1.10). These are: (a) the circum-Pacific "ring of fire"; (b) a sinuous belt running from the Azores through North Africa and the Alpine-Dinaride-Himalayan mountain chain as far as S.E. Asia; and (c) the world-circling system of oceanic ridges and rises. The seismic zones subdivide the lithosphere laterally into tectonic plates (Fig. 1.11). A plate may be as broad as 10,000 km (e.g., the Pacific plate) or as small as a few 1000 km (e.g., the Philippines plate). There are twelve major plates (Antarctica, Africa, Eurasia, India, Australia, Arabia, Philippines, North America, South America, Pacific, Nazca, and Cocos) and several minor plates (e.g., Scotia, Caribbean, Juan de Fuca). The posi­tions of the boundaries between the North American and South American plates and between the North American and Eurasian plates are uncertain. The boundary between the Indian and Australian plates is not sharply defined, but may be a broad region of diffuse deformation. A comprehensive model of current plate motions (called NUVEL-1), based on magnetic anomaly patterns and first-motion directions in earthquakes, shows rates of separation at plate boundaries that range from about 20 mm yr"¹ in the North Atlantic to about 160 mm yr"¹ on the East Pacific Rise (Fig. 1.11). The model also gives rates of closure ranging from about 10 mm yr"¹ between Africa and Eurasia to about 80 mm yr"¹ between the Nazca plate and South America.

Fig. 1.10 The geographical distribution of epicenters for 1,000 earthquakes for the years 1961-1967 illustrates the tectonically active regions of the Earth (after Barazangi and Dorman, 1969).

Fig. 1.11 The major and minor lithospheric plates. The arrows indicate relative velocities in mm yr¹ at active plate margins, as deduced from the model NUVEL-1 of current plate motions (data source: DeMets et al., 1990).