Nonlinear Modeling of Tectonic Release from Underground Explosions
S-CUBED LA JOLLA CA
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Reversal of teleseismic Rayleigh wave polarity has been observed for some underground explosions in eastern Kazakh, and in some cases the polarity is reversed at all azimuths of observation. We analyze two-dimensional, nonlinear numerical simulations of underground explosions to examine the hypothesis that these phase reversals result from the action of tectonic prestress on the explosion-created nonlinear volume. We conclude that the effect of tectonic prestress on surface wave excitation is potentially large. When a shear prestress of 7.5 MPa 75 bars is introduced, with horizontal principal stresses more compressive than the vertical principal stress, the explosion Rayleigh wave amplitude is reduced by a factor of 4 i.e., Ms is reduced by 0.6. The large surface wave reduction is accompanied by no significant perturbation of body wave magnitude mb. The nonlinear model predictions imply that if tectonic release is modeled elastically as the relaxation of the deviatoric part of the prestress into a spherical cavity, the appropriate cavity radius is approximately 80 percent of the explosion elastic radius.
- Geology, Geochemistry and Mineralogy
- Numerical Mathematics