Issue link: https://wardsworld.wardsci.com/i/1382081
Earthquake (continued) Characteristics Earthquakes vary immensely in size, from tiny events that can be detected only with the most sensitive seismographs, to great earthquakes that can cause extensive damage over widespread areas. Although thousands of earthquakes occur every day, and have for billions of years, a truly great earth- quake occurs somewhere in the world only once every year. When a great earthquake occurs near a highly populated region, tremendous destruction can occur within a few sec- onds. In 1976, 600,000 people were killed in Tangshan, China, by a single earthquake. The city of Lisbon, one of the principal capitals of that day, was utterly destroyed, with high loss of life, in 1755. Cities as Tokyo and San Francisco have been leveled by earthquakes. In these cases, much of the damage was not due to the shaking of the earthquake itself, but was caused by fires originating in the gas and electrical lines that interweave modern cities, and by damage to fire-fighting capability which rendered the cities helpless to fight the conflagrations. Cause The locations of earthquakes that occurred between 1900 and 2013 are shown on the map in Fig. 2. The map shows that earthquakes are not distributed randomly over the globe but tend to occur in narrow, continuous belts of activity. Approxi- mately 90% of all earthquakes occur in these belts, which define the boundaries of the Earth's plates. The plates are in continuous motion with respect to one another at rates on the order of centimeters per year; this plate motion is responsible for most geological activity. Plate motion occurs because the outer cold, hard skin of the Earth, the lithosphere, overlies a hotter, soft layer known as the asthenosphere. Heat from decay of radioactive minerals in the Earth's interior sets the asthenosphere into thermal convection. This convection has broken the lithosphere into plates which move about in response to the convective motion in a manner shown schematically in Fig. 3. The plates move apart at oceanic ridges. Magma wells up in the void created by this motion and solidifies to form new sea floor. This process, in which new sea floor is continually created at oceanic ridges, is called sea-floor spreading. Since new lithosphere is continually being created at the oceanic ridges by sea-floor spreading, a like amount of lithosphere must be destroyed somewhere. This occurs at the oceanic trenches, where plates converge and the oceanic lithosphere is thrust back down into the asthenosphere and re- melted. The melting of the lithosphere in this way supplies the magma for the volcanic arcs which occur behind the trenches. Where two continents collide, however, the greater buoyancy of the less dense continental material prevents the lithosphere from being underthrust, and the lithosphere buckles under the force of the collision, forming great mountain ranges such as the Alps and Himalayas. Where the relative motion of the plates is parallel to their common boundary, slip occurs along great faults which form that boundary, such as the San Andreas fault in California. According to the theory of plate tectonics, the motion of the plates is very similar to the movement of ice floes in arctic waters. Where floes diverge, leads form and water wells up, freezing to the floes and producing new floe ice. The formation of pressure ridges where floes converge is analogous to the development of mountain ranges where plates converge. + ward ' s science Fig. 2 Seismicity of the Earth 1900–2013. (Credit: USGS)