Earthquakes

Physical Seismicity

As seismicity is a highly variable phenomenon, it is a false presumption that the physical nature of earthquakes can fully be characterized by any single measure. The uniqueness of each seismic event is one factor in the continuing inability of scientists to predict earthquakes shortly before they occur. Seismology is an often-imprecise science that must make delicate measurements of violent and destructive Earth movements.

Magnitude is a measure of the amplitude of seismic waves at a standardized distance from their point of origin, and as such, is a surrogate estimate of seismic energy release. Modern magnitude scales owe much to the work of the Californian seismologist Charles F. Richter, but his scale (known as local magnitude, ML) is now seldom used because it tends to underestimate high magnitudes. In any case, a basic distinction must be drawn between the magnitude of body waves, which travel through the Earth's crust, and that of surface waves, which travel along the surface or interior discontinuities. Earthquakes involve a complex amalgam of six kinds of motion and waves of different frequencies, and hence are usually best characterized by several different magnitude measures. As magnitude scales are logarithmic, there is a vast difference between the energy released by, for example, a magnitude 4 earthquake and a magnitude 8 earthquake. Although high-magnitude seismic events have greater disaster potential than those of low magnitude, there are plenty of examples of major disasters caused by relatively modest tremors. The reasons for this lie in human settlements' vulnerability to earth tremors.

Bracketed duration is the length of time that shaking exceeds a given threshold and represents the duration of strong motion, or how long the earthquake's main shock lasts. Most earthquakes are less than one minute long, although some can be much more protracted, especially if primary shaking gives rise to a complex set of reactions among linked faults. Generally, long earthquakes are more damaging than short ones: for example, 15 seconds of shaking may cause a building to crack, but 65 seconds may propagate the cracks to the point at which the structure collapses.

...