The six declared nuclear tests of the Democratic Peoples' Republic of Korea defy current earthquake/explosion discrimination methods that rely on relative energy contained in body waves vs. that contained in surface waves. These events indicate excess shear energy relative to the legacy tests upon which the discriminants are based. We use near-source velocity data from a series of buried chemical explosive tests in a jointed medium to describe that a certain range of yield-scaled depth-of-burial (SDOB) of the explosive source can create the conditions to generate this excess shear energy. The relation between level of explosive loading and in situ stress conditions can determine whether, upon the unloading phase of the explosive pulse, the joints dilate, resulting in a sudden release of stored shear energy.

1. INTRODUCTION

A challenge for seismologists in the nuclear monitoring community is discriminating between explosions and earthquakes based on the received seismic signals. One accepted method for event discrimination is to compare body wave magnitude (mb) to surface wave magnitude (MS) since explosively-caused motion is dominated by compressional waves while that caused by earthquakes is dominated by shear, which appears in the recording stations in the form of surface motion such as Rayleigh waves. This is illustrated in Figure 1 which indicates two distinct populations, with earthquake events indicating MS dominance and explosions trending to mb dominance.

However, mb and MS estimates for the first two declared nuclear tests conducted by the Democratic Peoples' Republic of Korea (DPRK), labeled KIM1 and KIM2 in Figure 1, place these events within the earthquake population. This misalignment indicates that certain explosive events can produce additional shear energy that is not produced in the general population, suggesting a deficiency in this discriminant for monitoring. Confidence in future application of this discrimination approach requires better understanding of explosion phenomenology.

The Source Physics Experiment Program (SPE) combined numerical modeling and field tests to study possible mechanisms of excess shear observed in explosion events (Snelson, personal communication). To this end SPE Phase I included a series of chemical explosive tests conducted in the Climax stock granite at the Nevada National Security Site (NNSS), an analog location to the DPRK test site at Pyongye-ri (Coblentz and Pabian, 2015). A particular subject of study is the effect of yield-scaled depth-of-burial (SDOB), calculated as the device burial depth divided by the cube root of its yield, of the KIM events. Yield scaling is based on the considerations of similitude as discussed in Sedov, 1933.

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