Analysis of Teleseismic Signals from Underground Nuclear Explosions Originating in Four Geological Environments

Summary A comparison of Yellowknife records of the short‐period teleseismic P codas of underground nuclear explosions with synthesized records enables us to differentiate between the primary signal and anomalous signal‐generated noise. By primary signal we are referring to the P coda generated by a symmetric point source in a layered crust; both source and receiver crusts satisfy the requirements of the Haskell‐Thomson matrix theory. For epicentral distances for which multipathing in the upper mantle does not occur, the large‐amplitude phases in the first several cycles are dominated by the primary signal. Complexities in the next portion, which consists of smaller amplitude phases, are probably due to signal‐generated noise resulting from the presence of lateral heterogeneities near the source such as faults and perturbations in layer thickness and composition. Finally complexities in the latter portion of the P coda may, in some cases, be due to explosion‐generated noise resulting from the presence of topographical irregularities at greater distances from the source. Of the four geological environments studied, the events from Amchitka Island appear to have the smallest complexity factor and the event from Novaya Zemlya, the largest. The complexity of the events from the E. Kazakh site and the Nevada Test Site lie in between these two values.