High-quality waveform data from regional seismic networks were essential in detection, location, and identification of two North Korean underground nuclear explosions conducted at the P'unggye-ri test site. The generated seismic waves are strongly affected by lateral variations in regional crustal structure surrounding the test site, which leads to difficulties in estimation of source parameters. Thus, these variations become a key matter in the regional seismic verification of North Korean underground nuclear explosions to understand the path effects caused by the crustal structure. The crustal structure that surrounds the test site was therefore investigated in this thesis, by analyzing the regional Rayleigh waves from the 2 North Korean underground nuclear explosion. The estimated yield from the surface-wave magnitude was abnormally higher than that from the body-wave magnitude. The effect of geological properties at the test site on the yield overestimation has been studied in relation with other test sites. In addition, the source characteristics of the explosions were estimated through a source scaling relationship. The crustal structures surrounding the North Korea Test Site were investigated by analyzing Rayleigh waves from the 2" North Korean underground nuclear explosion on May 25", 2009. Regional seismic observation showed lateral variations in crustal structure, which could be more clearly classified into the North Korea-Northeast China region, the Western Margin of the East Sea, and the Japan Basin. Group velocity dispersion curves were measured from the vertical Rayleigh waves of stations in the range of 195 km to 2000.m. The measured dispersion curves were inverted to obtain shear-wave velocity depth models from the surface to 50km, using an initial model from Cipar's study on the crustal structure of North China. A set of mean dispersion curves and a mean velocity model was then measured by averaging dispersion curves and velocity models along the paths of the North Korea-Northeast China region. The averaged shear-wave model revealed that the mean shear-wave velocity of the Moho discontinuity, the depth of which is estimated between 35 km and 42 km, is 4.37 km/s with a standard deviation of 0.15 km/sec. Furthermore, the shear-wave velocity modeled the layer between 3 km and 6 km depth and showed that the velocity decreases from 3.59 km/s to 3.29 km/s. The averaged shear-wave velocity model beneath the Japan Basin showed the mean shear-wave velocity to be 4.26 km/s with a standard deviation of 0.14 km/s at the layer between 16 km and 22 km. The high shear-wave velocity agrees with results from previous studies that stated the crustal structure of the Japan Basin is characteristically oceanic crust. The averaged shear-wave velocity model beneath the Western Margin of the East Sea showed a mean shear-wave velocity of 4.14 km/s with a standard deviation of 0.05 km/s at the layer between 16 km and 22 km. Furthermore, it was observed that the shear-wave velocity from the surface to 11 km decreases in the eastward direction. Group velocity dispersion curves and shear-wave velocity models beneath the Western Margin of the East Sea revealed characteristics of a middle stage between the oceanic crust of the Japan Basin and the continental crust of the North Korea-Northeast China Surface-wave magnitude of the 1st explosion was measured to be 2.81 with a standard deviation of 0.10. The measured surface-wave magnitude of the 2nd explosion was 3.58 with a standard deviation of 0.07. Together with predetermined body-wave magnitudes, the measured surface wave magnitudes were used to estimate the yields of the two explosions. The yields estimated from the surface-wave magnitudes were about 20 times larger than yields from the body-wave magnitudes. Additionally, the two explosions were located on the earthquake side border of discrimination criteria based on differences between body-wave and surface-wave magnitudes. These two results indicated that relatively large surface waves were generated from the North Korean underground nuclear explosions compared with similar body wave magnitudes. The material properties of the emplacement medium at the North Korea Test Site could be the dominant cause for the abnormally large surface waves of the two explosions, according to a review of the nonlinear explosion source model of Denny and Johnson. The empirical scaling relationship of two North Korean underground nuclear explosions was estimated using seismic waveform data recorded at 10 broadband stations in the regional distance range of 194 km-550 km from the test site. The difference in the locations of the two explosions was tiny on the regional distance scale. The tiny difference thus made it possible to establish empirical source scaling relationships for the two explosions by excluding path effects through the spectral ratio of collocated seismograms. Even though the difference in the locations was tiny, the seismic wave forms and spectra of the 2nd explosion showed slight differences from the 1st explosion due to path effects. These effects could be partly smoothed with a stochastic approach that measures network averaged mean spectral ratios. The mean spectral ratio was therefore modeled to obtain the empirical scaling relationship based on the Mueller-Murphy source model. The model analysis calculated the scaling relationship of the yields and depth of burials of the two explosions: the relative depth of burial of the 2d explosion was approximately 1.2 times deeper than the 1st explosion, and the relative yield of the 2nd explosion was approximately 5 times larger than the 1st explosion.

1. Introduction
2. Shear Wave Velocity Structure Surrounding the Korean Peninsula
3. Regional Surfac