We have compared the seismic magnitude of a wide variety of chemical explosions of known yield, to the magnitude expected for explosions set off in hard rock under conditions most favorable for generating strong seismic signals. Our results are based on numerous chemical explosions that include several different broad groups, mostly taken from practical experience with explosions carried out on territory of the former Soviet Union. To quantify these observations, we define the deficit of an explosion as the expected signal strength if that charge size, or yield, were fired under the most favorable conditions in hard rock, minus the actual strength. We document the size of the deficit using two different measures of signal strength: the energy class K and the seismic magnitude (which may be the teleseismic mb or a regional magnitude).
In general, for ripple-fired chemical explosions carried out in the mining and construction industries, the magnitude deficit is around 1.5 to 2. The type of blasting that comes close to the maximum coupling efficiency (zero deficit) is now rare except for small-yield single-fired explosions that are specially designed to maximize signal strength (such as explosions for seismic refraction surveys). There are a small number of locations where the deficit is small (∼0.5 magnitude units) for quite large chemical yields (several hundred tons). Such explosions, which appear to be uncommon and declining as blasting practices are modernized, may require special attention in the context of verification of the Comprehensive Test Ban Treaty.