Earthquake and Earth deformation



Geophysicists are cognizant, if not in fear, of the chances of one or more great earthquakes occurring in the United States during the next several to 100 years. The San Andreas fault is, of course, the major threat. The worst earthquake along it occurred in 1857 in southern California, and it is just a matter of time before another follows the expected 100–200 year cycle. Warning would be a nice thing, but seismologists are not particularly successful in this, possibly because there is insufficient instrumental coverage of the earth's surface to study earthquakes. There is the international seismic network, but instruments have only local coverage and usually are set for optimizing the range of deformation, foreshocks, and other crustal changes that could provide warning. It would be a first step to determine what changes actually do give reliable forecasts of major earthquakes, and a range of seismographs, strain meters, and satellite detectors could be used to good advantage. According to the Panel on Crustal Movement Measurements (National Research Council Assembly of Mathematical and Physical Sciences' Committee on Geodesy and Seismology), ‘…The frequency with which geodetic networks are re-observed is often of the order of years or tens of years, while most… seismographs are designed to register ground motion excited by an earthquake, and their response is most favorable in a range of periods from, say, one tenth of a second to a few minutes.’ (Geodetic Monitoring of Tectonic Deformation—Toward a Strategy, 119 pp., National Academy of Sciences Press, Washington, D.C., 1981.)