Abstract

Reversal of teleseismic Rayleigh wave polarity has been observed for some underground explosions in eastern Kazakh. Furthermore, the reversed-polarity Rayleigh waves are observed to be time-delayed by several seconds. We analyze two-dimensional, nonlinear numerical simulations of underground explosions to examine the hypothesis that these phase reversals and apparent time delays result from the action of tectonic prestress on the explosion-created nonlinear volume (the shatter zone model of tectonic release). We conclude that tectonic shear stress exceeding about 10 MPa (100 bars) is sufficient to reverse the Rayleigh wave polarity and cause an apparent time delay of several seconds, even in the absence of triggered strain release on faults. Shear stresses of this magnitude are plausible at several hundred meters depth, in that they do not exceed strength bounds for a fractured rock mass, as estimated from Byerlee's law.

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 per cent of the explosion elastic radius. The apparent time delays result from interference between the explosion and tectonic source functions, and can be quantitatively related to the presence of overshoot in the isotropic part of the explosion moment rate tensor. It is therefore conceivable that careful observations of Rayleigh wave phase for the anomalous events (relative to nearby unreversed events) could help constrain the amount of overshoot in the explosion source function, even though the overshoot occurs on a time scale of a few tenths of a second and has a negligible effect on Rayleigh wave spectral amplitudes.

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