Earthquake Zones

Radioactive decay in the Earth's dynamic interior drives a process of mantle convection that causes the rigid plates of the solid outer crust to migrate. Subduction zones occur where plates are driven under other crustal material and consumed back into the mantle. Mountain zones, island arcs, and foretroughs are associated with such areas, and earthquakes can be common. For example, in the foretrough in front of the Himalayan Mountains (in India and Bangladesh), many high-magnitude earthquakes have occurred. Seismicity is associated with consumption of the crust, orogeny (mountain building), and downwarping (gentle bending of the crust) in front of the mountain chains.

Earthquakes occur more rarely at the trailing margins of plates, and tend to be less strong where crust is being created. However, there are various zones in which violent transcurrent, or strike-slip, movement occurs as the plates slide past one another. Some of these are areas in which the faults have a morphological expression at the Earth's surface; for example, at the San Andreas fault complex in California and the Boconó Fault in Venezuela. Most faults are buried by sediment and rock, so earthquakes commonly have no direct connection with the shape of the landscape at the surface.

Seismicity results from stress in the Earth's crust, which is a cyclical process of accumulation and release. In some areas, known as seismic gaps, the accumulation is long-standing, and major earthquakes are expected as a result of very large buildup of strain. Ten major seismic gaps occur at the plate boundaries along the Pacific Ocean rim, and three are in the Caribbean. Even more than the Mediterranean basin and Indian Ocean, the Pacific is susceptible to earthquake-induced tsunamis, which are also known as seismic sea waves. They can cause considerable mortality in coastal regions, primarily by drowning.

Not all earthquakes are caused by the tectonic processes that result from crustal plate migration. Nuclear explosions, meteorite impacts, and rock falls have all caused tremors, albeit small ones relative to the vast energy release resulting from tectonic seismicity. Volcano-seismic activity occurs when magma rises to the surface and stresses the surrounding crust. This form of seismicity tends to be localized and to consist of harmonic tremors, the rhythmic fracture of rocks. It may be associated with bradyseisms, local, volcanically induced alterations in the level of the Earth's surface. Volcanoseismic [Page 143]activity is associated with the world's major volcano belts: in Japan, the Andes, and the Mediterranean, for example. The combination of moderate earthquakes and bradyseismic activity that occurs when magma rises can, however, cause casualties and destruction, as occurred at Pozzuoli, southern Italy in the early 1980s, where 70,000 people had to be evacuated from severely damaged homes.

The disaster potential of earthquakes results from the interaction of the global pattern of seismicity and the geography of population and human settlement. The great seismic belt stretches from Portugal across the Mediterranean and Asia as far as the Japanese island arc. About 90 percent of recorded deaths in earthquakes have occurred on the great continental land mass in the band between 25 degrees north and 45 degrees north: more than 70 percent have occurred in the 30–40 degrees north band. This is an area of ancient settlement and traditionally consists of aseismic construction techniques (construction that is not built to resist earthquakes), rather than antiseismic. However, modern construction techniques and the increased use of reinforced concrete are gradually reducing the risk due to building collapse during earthquakes. Nevertheless, excavations of the Palace of Knossos on Crete show evidence of damage by as many as six major seismic events in early classical times. In 62 C.E., 17 years before it was inundated by volcanic materials from Mount Vesuvius, the Roman town of Pompeii suffered considerable damage in an earthquake. In 1556, at Shensi, China, 830,000 people were killed in a single earthquake.

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