Seismic Scale

a scale used to evaluate the intensity of oscillations on the earth’s surface during earthquakes. There are many seismic scales in which the intensity of oscillations is evaluated by the degree of damage caused to buildings, the scale and shape of residual deformations in the ground, and other indicators of the external effect of earthquakes. The USSR uses a scale of 12 degrees (GOST 6249–52), in which the readings of the pendulum of an SBM seismometer are used to supplement the above-mentioned indicators in determining the size of an earthquake; the MSK-64 scale (seeEARTHQUAKES), which defines techniques of determining intensity more precisely, is also used.

Work has been under way since 1973 to develop a new seismic scale, in which the intensity of earthquakes would be evaluated not only by the results of visual observations but also by readings from instruments, such as seismographs and accelerographs. Such instruments register the displacement, velocity, and acceleration—the basic elements of the oscillation process—acquired by particles of the ground at the moment of an earthquake. Thus, a value of 9 corresponds to a velocity x of oscillations of ground particles on the order of 24.1–48.0 mm/ sec and an acceleration ẍ of 241–480 cm/sec²; the values of ẋ and ẍ are correspondingly lower for lower values.

In addition to an estimation of the intensity of oscillations on the earth’s surface, a classification of earthquakes by magnitude is used. The magnitude is an arbitrary quantity proportional to the logarithm of the energy emitted by the earthquake focus. For example, the intensity of the 1948 Ashkhabad earthquake is estimated at a level of 10, but the magnitude was 7.0; the 1966 Tashkent earthquake was of intensity 8 and had a magnitude of 5.3. The relation between magnitude m, intensity I_∫, and depth of focus h for an earthquake is expressed as l_∫ = bM - v log h + C, where the coefficients b, v, and C are determined empirically and vary somewhat from region to region.

Other seismic scales are used in certain countries; for example, a scale of seven degrees is used in Japan. Seismic scales are used in studying the external effect of earthquakes, in plotting isoseismals on maps, and in seismic zoning and microzoning.