The trade-off relationship between yield and depth of North Korea's underground nuclear explosions

North Korea conducted its second nuclear test on May 25, 2009. The location of North Korea's second underground nuclear explosion (UNE) was in the vicinity of P'unggyeri where the first nuclear test was performed. The seismic waves originating from the two UNEs are recorded at regional seismic stations in Korea and China. Mueller and Murphy's description (1971) on the relationship between yield, depth and amplitude of two different events in a fixed medium is applied to North Korea's two UNEs to access the trade-offs between yield and depth of burial. The nearly co-located two UNEs and seismic recordings at the same station give an opportunity to calculate the ratio of displacement amplitude spectra between two events by eliminating path effect. The 95% confidence interval of the mean yield ratio is constrained as a function of depth ratio with all the 92 Pn and Pg spectral ratios. The mean yield ratio varies from 3.45 to 6.36 in the 95% confidence interval with the range of depth ratio from 0.5 to 2.