Moment-Tensor Inversions of Single Cast Mining Cast Blasts
Figures 1-2 - Figure 1 plots an example isotropic source spectrum from the inversion. The green dashed line is the corresponding standard deviation of the estimates. The spectrum is characterized by a strong peak in the frequency range of 6-10 Hz. The peak is superimposed on a spectral plateau that extends to a few tens of hertz. This kind of spectral behavior is not expected from a typical contained explosion source. We estimated the amplitudes of the spectral plateau by calculating the mean of the modulus between 20 and 40 Hz, which is indicated by the thick horizontal blue line in the left figure along with its plus/minus one standard deviation (the dotted lines). The amplitudes of the plateau and the spectral peak amplitudes are plotted in the figure below. Also plotted is the scaling relationship between yield and source moment from Denny and Johnson's study of a set of nuclear and chemical explosion data*. The amplitudes of the plateau are fairly consistent with Denny and Johnson's results whereas the peaks of the spectra are an order of magnitude or more larger.
* Denny, M. D. and L. R. Johnson (1991). The explosion seismic source function: models and scaling laws reviewed, in Explosion Source Phenomenology, S. R. Taylor, H. J. Patton and P. G. Richards (Editors), AGU, Washington, D.C. 1-24, Fig. 8.
Figures 3-4 - Time-dependent source moment tensors of the eight shots were estimated by the frequency-domain linear inversion method. As an example, the figure above shows the result for one of the shots. The moment-tensor is dominated by its isotropic component. On the average, 80% of the moment tensor is composed of its isotropic component. Nevertheless, apparent asymmetry is observed among the diagonal components with larger vertical component M33. The time histories of the diagonal components are similar to each other, indicating similar source processes that involve volume change. Although there are variations from shot to shot in terms of the degree of time history complexities as is illustrated in the plot of M33 components on the right, the general behavior of the moment tensor time histories indicate more complex source processes than that of a simple isotropic source, which would have a single pulse suggested by classical explosion source models.
Site Characterization |
Moment-Tensor Inversion |