Earthquake prediction and control

No natural event is as destructive over so large an area in so short a time as an earthquake. Throughout the centuries earthquakes have been responsible not only for millions of deaths but also for tremendous damage to property and the natural landscape. If major earthquakes could be predicted, it would be possible to evacuate population centres and take other measures that could minimize the loss of life and perhaps reduce damage to property as well. For this reason earthquake prediction has become a major concern of seismologists in the United States, Russia, Japan, and China.

World seismicity patterns show that earthquakes tend to occur along active plate boundaries where there is subduction (Japan) or strike-slip motion (California) and along strike-slip faults (as in China, where they are the result of the northward migration of India into Asia). Investigators agree that much more has to be learned about the physical properties of rocks in fault zones before they are able to make use of changes in these properties to predict earthquakes, though the use of Global Positioning Systems (GPS) at satellite ground stations over the years is providing quantitative data on a millimetre scale concerning the relative movement of crustal blocks across seismic faults. Recent research has suggested that rocks may become strained shortly before an earthquake and affect such observable properties of the Earth’s crust as seismic wave velocity and radon concentration. Leveling surveys and tiltmeter measurements have revealed that deformation in the fault zone just prior to an earthquake may cause changes in ground level and, in certain cases, variations in groundwater level. Also, some investigators have reported changes in the electric resistivity and remanent magnetization of rocks as precursory phenomena.

Since the San Francisco earthquake of 1906, seismic activity along the nearby San Andreas Fault has been closely monitored. It has been observed that numerous semicontinuous microearthquakes have occurred along some sections of the fault. These small quakes seem to release built-up strain and thus prevent large earthquakes. By contrast, intervening sections of the fault are apparently locked and thus manifest no microshocks. Consequently, seismic strain accumulating in these locked sections is expected to be released one day in a major quake.

Seismological research includes the study of earthquakes caused by human activities, such as impounding water behind high dams, injecting fluids into deep wells, excavating mines, and detonating underground nuclear explosions. In all of these cases except for deep mining, seismologists have found that the induction mechanism most likely involves the release of elastic strain, just as with earthquakes of tectonic origin. Studies of artificially induced quakes suggest that one possible method of controlling natural earthquakes is to inject fluids into fault zones so as to release strain energy.

Seismologists have done much to explain the characteristics of ground motions recorded in earthquakes. Such information is required to predict ground motions in future earthquakes, thereby enabling engineers to design earthquake-resistant structures. The largest percentage of the deaths and property damage that result from an earthquake is attributable to the collapse of buildings, bridges, and other man-made structures during the violent shaking of the ground. An effective way of reducing the destructiveness of earthquakes, therefore, is to build structures capable of withstanding intense ground motions.

Other areas of application

The fields of engineering, environmental, and urban geology are broadly concerned with applying the findings of geologic studies to construction engineering and to problems of land use. The location of a bridge, for example, involves geologic considerations in selecting sites for the supporting piers. The strength of geologic materials such as rock or compacted clay that occur at the sites of the piers should be adequate to support the load placed on them. Engineering geology is concerned with the engineering properties of geologic materials, including their strength, permeability, and compactability, and with the influence of these properties on the selection of locations for buildings, roads and railroads, bridges, dams, and other major civil features.

Urban geology involves the application of engineering geology and other fields of geology to environmental problems in urban areas. Environmental geology is generally concerned with those aspects of geology that touch on the human environment. Environmental and urban geology deal in large measure with those aspects of geology that directly influence land use. These include the stability of sites for buildings and other civil features, sources of water supply (hydrogeology), contamination of waters by sewage and chemical pollutants, selection of sites for burial of refuse so as to minimize pollution by seepage, and locating the source of geologic building materials, including sand, gravel, and crushed rock. Since the late 1990s the importance of environmental geology has increased considerably in most developed countries as societies became aware of the environmental impact of humankind.