Relationship Between DPRK Nuclear Events and Near-Field Response to Chemical Explosions in the Source Physics Experiment Series

Date: 4/24/2019

Time: 11:00 AM

Room: Cascade I

The Source Physics Experiment (SPE) is developing a greater understanding of explosion phenomenology. In particular we are identifying mechanisms for generating shear which can complicate event discrimination. The program consists of two phases: Phase I in granite and Phase II in dry alluvium. Each phase included a series of chemical explosions of varying yield and depth to provide a range of scaled depth of burial (SDOB).

The test data reveal a near-field shear mechanism in the granite testbed due to slippage along natural joint sets that occurs as the shock wave passes. Specifically, compressive shock causes joint closure during loading with the combined joint surface resistance and in situ stress preventing slippage. Unloading creates an extensional state allowing joint dilation and release of stored shear energy. Evidence is provided by numerous velocity records from the test events indicating unexpectedly large tangential motion. We support this hypothesis using results from explicitly jointed finite element simulations. Phase II data from the unjointed alluvium testbed lack any character indicating a near-field source of shear content, providing a clear contrast between propagation in jointed and in unjointed media.

The composite Phase I data set indicates three regimes. Deeply overburied events do not generate sufficient energy to overcome resistance to slippage due to high confining stress. At “nominal” SDOB there is insufficient confinement stress to lock the joints. Moderately overburied tests provide the condition for shear release. We indicate a notional relationship between SDOB and the likelihood of shear release in SPE. We then link this observation to the recognized DPRK nuclear events that are observed to have large shear magnitudes and are estimated to range from nominally buried to moderately overburied. The comparison suggests an explanation for the confusion identifying the DPRK events as explosions as oppposed to earthquakes.

Presenting Author: David W. Steedman

Authors