Release of Tectonic Strain by Large Underground Nuclear Detonation

The report describes an attempt to observe SH waves from several large underground nuclear explosions at first-zone distances (600 km) from each event. The data base was supplemented by seismograms from the World Wide Standard Station Network and Canadian Network. Good SH arrivals were observed at the shorter ranges (250 km) but not at ranges between 250 km and about 1800 km. In an effort to detect the arrival of any S waves (SV or SH) within this 'shadow zone', a flat-layered earth approximation was assumed (with hence critically refracted S-wave paths), and particle motion diagrams were then constructed. (The flat-layered earth hypothesis suggests small angles of incidence and, therefore, rectilinear particle motion). Such rectilinear motion was observed and in several 'shadow-zone' instances, arrival times were consistent with the model chosen.