Issue link: https://wardsworld.wardsci.com/i/1382081
Stick-slip friction and elastic rebound As the plates move past each other, little of the motion at their boundaries occurs by continuous slippage; most of the motion occurs in a series of rapid jerks. Each jerk is an earthquake. It happens because, under the pressure and temperature condi- tions of the shallow part of the Earth's lithosphere, the frictional sliding of rock exhibits a property known as stick-slip, in which frictional sliding occurs in a series of jerky movements, inter- spersed with periods of no motion—or sticking. In the geologic time frame, then, the lithospheric plates chatter at their bound- aries, and at any one place the time between chatters may be hundreds of years. The periods between major earthquakes is thus one dur- ing which strain slowly builds up near the plate boundary in response to the continuous movement of the plates. The strain is ultimately released by an earthquake when the frictional strength of the plate boundary is exceeded. This pattern of strain buildup and release was discovered by H. F. Reid in his study of the 1906 San Francisco earthquake. During that earthquake, a 250-mi-long (400-km) portion of the San Andreas fault, from Cape Mendocino to the town of Gilroy, south of San Francisco, slipped an average of 12 ft (3.6 m). Subsequently, the triangulation network in the San Francisco Bay area was resur- veyed; it was found that the west side of the fault had moved northward with respect to the east side, but that these motions died out at distances of 20 mi (32 km) or more from the fault. Reid had noticed, however, that measurements made about 40 years prior to the 1906 earthquake had shown that points far to the west of the fault were moving northward at a slow rate. From these clues, he deduced his theory of elastic rebound, illustrated schematically in Fig. 4. The figure is a map view, the vertical line representing the fault separating two moving plates. The unstrained rocks in Fig. 4a are distorted by the slow movement of the plates in Fig. 4b. Slippage in an earthquake, returning the rocks to an unstrained state, occurs as in Fig. 4c. Classification Most great earthquakes occur on the boundaries between lithospheric plates and arise directly from the motions between the plates. Although these may be called plate boundary earthquakes, there are many earthquakes, some- times of substantial size, that cannot be related so simply to the movements of the plates. Near many plate boundaries, earthquakes are not restricted to the plate boundary itself, but occur over a broad zone—often several hundred miles wide—adjacent to the plate boundary. These earthquakes, which may be called plate boundary— related earthquakes, do not reflect the plate motions directly, but are secondarily caused by the stresses set up at the plate boundary. In Japan, for example, the plate boundaries are in the deep ocean trenches offshore of the Japanese islands, and that is where the great plate boundary earthquakes occur. Earthquake (continued) + ward ' s science Fig. 3 Movement of the lithosphere over the more fluid asthenosphere. The lithosphere spreads away from the oceanic ridges. It descends again into the asthenosphere at the trenches. (Credit: USGS) Fig. 4 Schematic of elastic rebound theory. (a) Unstrained rocks (b) are distorted by relative movement between the two plates, causing strains within the fault zone that finally become so great that (c) the rocks break and rebound to a new unstrained position.