Regional coda magnitudes of underground nuclear explosions

The concept of determining magnitudes, M_τ, of regional events (Δ < 1000 km) by means of coda-duration measurements is re-examined by using short-period vertical-component seismograms from Nevada Test Site explosions. The duration is specified as the time interval between the expected arrival of the S-wave and the time when coda waves fall and stay below 80 μm peak-to-peak ground displacement. The suggested procedure requires that for M_τ = 4.0 the coda duration at a distance of 100 km is 120 s. The adaptivity of the method is examined in terms of the single-station magnitude scatter, and with respect to the potential accuracy of the yield estimation of the explosions. The derived magnitude formula for underground nuclear explosions in granite is of the form: M_τ =0.18+0.001Δ+1.79 log τ+C_s, where C_s is a station correction coefficient for non-WWNSS stations.