Identification a of shear mechanism in moderately overburied chemical explosive experiments and relation to the DPRK declared nuclear tests

<p>The Source Physics Experiments (SPE) provided new insights into explosion phenomenology. In particular, the data reveal a mechanism for generating shear energy in the near-source region which may explain why certain North Korean declared nuclear tests do not conform to explosion/earthquake discriminants based on relative body wave (m<sub>b</sub>) and shear wave (M<sub>S</sub>) magnitudes.</p><p>The SPE chemical explosive detonations in granite included three scaled depth of burial (SDOB) categories: 1) nominally buried defines the burial depths from which m<sub>b</sub>:M<sub>S</sub> discriminants were derived; 2) deeply overburied, or Green’s function depth; and 3) moderately overburied, or between the two end cases above. This last category is a general descriptor for the North Korean declared nuclear tests which fail the m<sub>b</sub>:M<sub>S</sub> discriminant.</p><p>Near-source three-axis borehole accelerometers indicate that the nominal and deeply buried SPE experiments created the expected spherical shock environment dominated by radial ground motion with insignificant tangential response.</p><p>The moderately overburied SPE experiments indicate a significant contrast. The tangential records in these experiments are quiescent with initial shock arrival and then exhibit a sudden, significant surge immediately following the peak radial component. At distant ranges where the shock wave amplitude has attenuated the environment becomes more consistent with a spherical shock with no significant tangential components.</p><p>We interpret a “shear release” mechanism on an obliquely loaded rock joint:</p><ol><li>During incipient loading the normal shock component forces closure of the joint.</li> <li>In cases of low explosive loading and/or high in situ stress the tangential component is insufficient to cause joint sliding and this load is stored as shearing strain.</li> <li>As the ground shock peak passes the joint unloads and dilates, and the now open joint allows a sudden release of the stored shear strain resulting in sudden joint rupture and slippage.</li> </ol><p>Step 3 above is essential for identifying when this mechanism occurs. For large in situ stress accompanied by low explosive loading (i.e., deep burial, or high SDOB) the joint fails to open and rupture does not occur. For low in situ stress accompanied by high explosive loading (i.e., shallow burial, or nominal SDOB) there is insufficient resistance to tangential slippage and no shear energy is stored for later release.</p><p>The above provides a fully geodynamic definition for why certain explosive events in jointed rock will fall within the correct explosion population of a m<sub>b</sub>:M<sub>S</sub> discriminant while others may not. Moreover, we illustrate that these observations for the SPE results map directly to generally accepted yield and depth combinations for the six declared North Korean nuclear tests.</p>

Acoustic Overpressure Signals from Fully Confined and Vented Chemical Explosions

ABSTRACT We analyzed acoustic overpressure signals generated by overburied underground chemical explosions conducted in hard rock in New Hampshire in 2018. The explosions had comparable yields between 62.8 and 82.6 kg trinitrotoluene equivalent and were buried at depths between 12 and 13 m. Two explosions resulted in crater formation and gas venting, whereas the remaining explosions were fully confined and did not result in ground failure. Acoustic signals from the confined explosions were produced by the ground shock near ground zero. Acoustic signals from cratered explosions represent a combination of a ground shock signal and a time-delayed high-amplitude signal generated by gas venting. The cratering and venting occurred during the free-fall phase observed on the near-source accelerograms. We argue that the main reason for the cratering in this experiment is the low-rock porosity, preventing postexplosion pressure relief in the cavity and promoting long fracture formation during the unloading phase and subsequent containment failure. The ground-shock-induced signals were modeled using the Rayleigh integral of the near-source ground acceleration. The equations of nonlinear acoustics were used to model the observed gas venting signals produced by the gas flow from the explosion cavity to the surface. By comparing the near-source signals produced by venting to theoretical signals from surface blasts we have shown that the venting signals have significantly lower peak pressures and longer signal durations compared to surface blasts of the equivalent impulse. The observed amplitudes of the acoustic signals produced by the venting are significantly higher than the ground-shock related signals expected from overburied explosions. This is important to consider because higher acoustic amplitudes may potentially lead to errors in yield estimation.

Investigating Different Grounds Effects on Shock Wave Propagation Resulting from Near-Ground Explosion

A massive explosion of a liquid-propellant rocket in the course of an accident can lead to a truly catastrophic event, which would threaten the safety of personnel and facilities around the launch site. In order to study the propagation of near-ground shock wave and quantify the enhancement effect on the overpressure, models with different grounds have been established based on an explicit nonlinear dynamic ANSYS/LS-DYNA 970 program. Results show that the existence of the ground will change the propagation law and conform to the reflection law of the shock wave. Rigid ground absorbs no energy and reflects all of it, while concrete ground absorbs and reflects some of the energy, respectively. Ground may influence the pressure-time curve of the shock wave. When the gauge is close to the explosive, the pressure-time curve presents a bimodal feature, while when the gauge reaches a certain distance to the explosive, it presents a single-peak feature. For gauges at different heights, different grounds may have different effects on the peak overpressure. For gauges of height not greater than 4 m, the impact on the shock wave is obvious when the radial to the explosive is small. On the contrary, as for the gauges of height greater than 4 m, the impact on the shock wave is obvious when the radial to the explosive is big. Ground has the enhancement effect on peak overpressure, but different grounds have different ways. For rigid ground, the peak overpressure factor is about 2. However, for the concrete and soil ground, peak overpressure factor is from 1.43 to 2.1.