Lg phase is a regional phase and short period guided wave composed mainly of a sequence of multiple reflected post critical S-waves trapped in the crustal wave guide. Lg phase also can be described as superposition of a higher mode of Love and Rayleigh waves. The mechanisms proposed to explain the excitation of Lg phase by explosions are: shear waves generated by inhomogeneity near the source, scattering effects that transfer P-wave or surface wave (Rg) energy into Lg, S-wave energy generated by spall which is an efficient source of Lg, nongeometrical S phase and P-wave converted S-wave trapped in the crustal wave guide. Lg phase is one of the main phases produced by an underground nuclear explosion even with low yield. This phase has a sharp beginning with high amplitude in the short period seismograms in regional distances. The group velocity of Lg phase is about 3.5 kmls. Lg phase is a proper phase for yield estimation of underground nuclear explosions because of having a symmetric and radial pattern, lack of high attenuation and less scattering of mb(Lg) compared with the magnitude derived from other phases.
In this paper, 16 underground nuclear explosions at Semipalaninsk Test Site (STS, in east Kazakhstan) recorded by SRO, Mashhad, have been investigated. The yields of these explosions were announced by Stevens and Murphy (2001). Maximum peak to peak amplitude for periods of 1 to 1.5 seconds was read in the group velocity window of 3.2 to 3.6 km/s as the amplitude of Lg phase. Then mb(Lg) was determined by using the formula presented by Nuttli (1986a). The relationship between computed mb(Lg) and announced yield (Y in kt TNT) of 16 explosions of STS was derived as mb(Lg) = 0.564 log(Y) + 4.863. This relationship is recommended for yield estimation of STS nuclear explosions from SRO, Mashhad.