A Micromechanical Damage Mechanics Model for the Seismic Coupling of Underground Nuclear Explosions

Abstract:

Seismological discrimination between underground explosions and earthquakes is complicated by the observation that explosions generate significant S wave radiation. Whether these S waves are generated mainly in the non-linear source region, or by slip on nearby faults, or by mode conversion at boundaries along the propagation path remains controversial. In this study we explore the hypothesis that S wave radiation is generated during the ubiquitous generation of fractures observed in the non-linear source region. We model the nucleation, growth, and interaction of these fractures using a fully dynamic micromechanical damage mechanics recently formulated by Bhat et al. (J. Appl. Mech., 2012). Parameters in the model include the size, density, and orientation of initial fractures in the source rock, the coefficient of friction on their faces, and the dynamic critical stress intensity factor for nucleation and propagation. The damage mechanics is built into the ABAQUS dynamic finite element code as a user-defined rheology and is used to calculate the spatial extent and geometry of the fracture damage as well as resultant seismic radiation generated by an explosion with a specified pressure time-function. A fundamental result of these calculations is that any process that breaks the spherical symmetry of the explosion produces S waves. Sources of asymmetry include an anisotropic distribution of initial fracture (e.g. a rift in the granitic texture), an anisotropic regional pre-stress, or even heterogeneity that breaks the symmetry of the radial fractures. We compare our calculations with field experiments (by Weston Geophysical and New England Research) in a granite quarry and with laboratory experiments that use high-speed photography and laser velocimeters to record the evolution of the damage pattern and resultant seismic radiation.