  This paper considers dynamic fracturing of the rockmass surrounding a tunnel
statically loaded by compressional stress as a possible source of seismic
events in underground mines. This begins with two-dimensional dynamic modelling
of failure for six plausible scenarios. In each case, the seismic source
derived from these models has significant negative isotropic and negative
compensated linear vector dipole components as well as a P-axis approximately
aligned with the direction of maximum compressional principal stress. These
features indicate that at wavelengths larger than the diameter of the tunnel
and the extent of damage along it, seismic radiation is controlled by the
elastic convergence of the surrounding rockmass rather than by rock fracturing.
An analytical approximation of the source mechanism is suggested that is based
solely on mechanical and geometric properties: the magnitudes
$\sigma_{\mathrm{max}}$ and $\sigma_{\mathrm{min}}$ of the maximum and minimum
principal stresses orthogonal to the tunnel's axis, the Poisson's ratio $\nu$
of the rockmass, the length $L_{3}$ of dynamic fracturing along the tunnel, the
effective tunnel dimension $\bar{L_{A}}$, and the increase in depth of failure
$\triangle d_{f}^{A}$ in the direction of $\sigma_{\mathrm{min}}$. Furthermore,
it is shown that the scalar seismic moment can be approximated as
$|\mathbf{M}|\approx2[(1-\nu)/(1-2\nu)]|\sigma_{max}|L_{3}\bar{L_{A}}\triangle
d_{f}^{A}$. The suggested approximations are considered in the context of
seismic data from a real underground mine. It is shown that many mechanisms
inverted from observed waveforms are consistent with the suggested model and
that the proposed source mechanism approximation can be used for the forensic
analysis of damaging seismic events and quantitative monitoring of the
evolution of fractured zones around tunnels.
