1982 Volume 30 Issue 4 Pages 321-330
On the basis of an extensive data-set of major deep and intermediate-depth earthquakes, the relation between body-wave magnitude mB and seismic moment M0 is discussed in terms of apparent stress. mB is determined from amplitudes and periods of P, PP and S waves recorded by broad-band instruments, and is therefore suitable for measuring the overall size of major earthquakes. The relation between mB and logM0 is approximately linear for the data set of mB greater than 6. This indicates that the apparent stress is, on the average, nearly constant. A gross average of 60 bar is obtained for the earthquakes over a large depth range. For a simple fault model, we have Es=(7·10-5)M0, where Es is the radiated seismic energy. The constant apparent stress implies a decrease of seismic efficiency with an increase of depth. A very similar relation between mB and logM0 is applicable to large shallow earthquakes.