Energy class

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Energy class, also called energy class K or K-class, is a way to measure how much energy an earthquake releases. It is used in the countries of the former Soviet Union, as well as Cuba and Mongolia. K is the logarithm of the seismic energy (in joules) radiated by an earthquake, written as K = log ES.

K values from about 12 to 15 roughly correspond to magnitudes of 4.5 to 6 on other scales. For example, a magnitude Mw 6.0 earthquake would plot around K 13 to 14.5.

The system was developed in Tajikistan in 1954 by Soviet seismologists working in the Garm region, one of the most active seismic areas in the former USSR. The region experiences thousands of quakes each year, so researchers created new methods and equipment. V. I. Bune is credited with early ideas based on seismic energy, with S. L. Solov’ev contributing as well. However, Bune’s initial method was not practical.

A practical revision came from T. G. Rautian in 1958–1960, and by 1961 K-class was used across the USSR. The key change was to estimate ES from the peak amplitudes of the first three seconds of seismic waves—the sum of the largest P-wave and the largest S-wave. This made K-class a local-scale measure for regional earthquakes, roughly up to about M 6.5 (K 15); beyond that, the scale tends to underestimate the true size.

There are several regional variants (KR, KF, KS, KFS, KC, etc.) and different formulas to convert K to other magnitude scales. Some versions are also labeled as M(K) or similar.