Near Field and Regional Modeling of Explosions at the Degelen Test Site

Fundamental to the ability to monitor a Comprehensive Nuclear-Test-Ban Treaty is a good understanding of the nuclear explosion source. Considerable research has been done on the nuclear explosion source over the past 30 years, with mixed success. Although empirical and numerical models of explosion sources do a fairly good job of matching observed seismic signals, a good explanation of the physical basis for the explosion source has been elusive. In particular, numerical models of explosion sources developed using laboratory measurements of rock properties fail to predict observed near field ground motion in hardrock. The basic problem is that the strength of the rock measured in the laboratory is much larger than the apparent strength of the rock as determined from the near field ground motion. Furthermore, additional investigation shows that 1) The strength of the rock is not initially low, but rather decreases dynamically as the shock wave passes; and 2) the strength of the rock is reduced to a level well below that predicted for rubbleized rock under hydrostatic dynamic friction. Maxwell Technologies and the Institute for the Dynamics of the Geospheres (IDG) are in the early stages of a research program directed towards improving the capability to predict the seismic source characteristics of underground explosions in rock. This will be accomplished by development of improved dynamic failure models, constrained by a much better data set than has been available in the past. Near field waveforms are only available from a small number of U.S. nuclear tests, and until recently none have been available from the testing program of the former Soviet Union. IDG now has near field records from at least 10 nuclear explosions at the Degelen test site that will be made available for the project. IDG will also provide near source material properties measurements for all of these events. This unique data set will place strong constraints on the numerical modeling work, as well as providing data from a new and important area to augment our previous data sets which have come primarily from the western United States. In addition, the Russians have data from a seismic line located north of the Degelen test site that was maintained with consistent instrumentation for many years during the Soviet testing program. The 9 seismic stations are spread out at approximately even intervals from the Degelen test site out to a distance of about 150 km. IDG will provide seismic data from these stations for all of the events that have near field records. This provides a rare opportunity to observe and model the seismic wavefield of the explosions as they evolve from the near field of the explosion out to regional distances.