Types of Seismic Events and Their Source Descriptions

Abstract

The seismic source characteristics of underground nuclear explosions have now been extensively documented as a result of the testing programs which have been carried out by the various nuclear states over the past 30 years. In particular, the variations in seismic source coupling as functions of the geologic emplacement medium, yield and depth of burial of the explosion have been well-defined experimentally; and first order theoretical source models, such as the Mueller/Murphy model, have been formulated which have been shown to account reasonably well for these observed variations. The most notable feature of the nuclear source seismic coupling in the short-period band is that it is observed to be essentially the same for tamped explosions in virtually all known hardrock and water-saturated media. The only known exceptions to this uniformity are for explosions in clay and water, for which the average mb values for a given yield are higher than those in hardrock by about 0.50 ± 0.25 magnitude units, and for explosions in dry, porous media, such as dry alluvium or tuff, for which the average mb values for a given yield are lower than those in hardrock by about 0.50 ± 0.25 magnitude units. Larger changes in seismic coupling efficiency can occur if the explosion is not fully tamped, as in the cavity decoupling evasion scenario. Thus, it has been experimentally demonstrated that it is possible to reduce the mb value at a given yield by nearly two full magnitude units by detonating the explosion in a suitably large air-filled cavity. This potential variability in explosive source coupling needs to be carefully considered in the assessment of the capabilities of any proposed seismic monitoring systems.