Earthquakes

Distribution of Seismicity

The global distribution of seismicity is conditioned by the pattern of tectonic plates, which is a function of patterns of convection in the Earth's mantle and variations in the planet's spin rate. Most earthquakes are concentrated at the plate margins and in particular around the subduction zones, where crustal material is subsumed by overriding plates. Transcurrent plate margins also generate significant seismicity, as occurs along the west coast of North America, where the Pacific Plate is moving north against the continental plates to its east. Collisional plate margins generate seismicity through their impact on orogeny, or mountain building. In such regions, seismicity is usually associated with landslides. Finally, the regional geometry of stress creates some “hot spots” of intraplate seismicity, which are not associated with the plate margins. Particular problems are experienced with “seismic gaps”—sections of plate margins, or other seismogenic areas, where strain has built up and not been released in a major earthquake for decades or centuries.

Owing to the world distribution of the human population, the pattern of human effects of earthquakes does not coincide exactly with the global pattern of seismicity. Major earthquakes occur in unpopulated areas (e.g., in Alaska and Siberia) with limited consequences for human settlement. Much less severe earthquakes can have terrible human consequences if they occur in areas of dense settlement.

Earthquake Mechanisms

It is a common misconception that earthquakes can be characterized adequately by a single measure. This is usually given as magnitude or intensity, but in reality more information is needed. Magnitude is a surrogate measure of the energy liberated in the shaking, known as strong motion. Modern scales owe much to the pioneering work of the Californian seismologist Charles F. Richter. However, although his name is widely used in the mass media, the Richter scale has been abandoned by seismologists as it tends to be inaccurate at high magnitudes. Instead, a combination of measures is used that characterizes the magnitude of different seismic waves, particularly body waves, which travel through crustal material, and surfaces waves, which travel along surfaces and discontinuities in rock formations. The six kinds of seismic waves (i.e., P waves, S waves, body waves, surface waves, Rayleigh waves, and Love waves) involve different forms of motion. For example, the first waves to arrive (P waves) at a point on the Earth's surface are compression-extension movements of a longitudinal kind, and these are followed by the undulatory motion of S waves.

In terms of surface effects and damage, two other measures are vitally important. The first is hypocentral depth, the distance below the surface at which earthquake shaking begins. In most geophysical models of earthquake generation, intensive microfissuring coalesces to create sudden variations in stress within a block of rigid crustal [Page 811]material. As stress and strain are liberated, shaking spreads out dynamically from the small area in which the earthquake hypocenter, or focus, is born. Hypocenters that are deeper than 50 to 80 km (kilometers) seldom generate damaging earthquakes, and so the lethal seismic events tend to be those that have shallow foci, perhaps 5 to 20 km below the surface. In the Tonga Trench, in the South Pacific, major earthquakes occur as deep as 700 km, but by the time the seismic waves reach the surface they have dispersed their energy and are harmless. Conversely, the earthquake that occurred in Bam, Southern Iran, in December 2003 had a hypocenter only 10 km below the surface, and it killed 26,271 people.

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