Global tectonic implications of anomalous seismicPtraveltimes from the nuclear explosion longshot

A recently developed method of three-dimensional seismic-ray tracing is used to reinterpret P-wave traveltime residuals of the Longshot nuclear explosion on Amchitka, Aleutian Islands, in terms of plate tectonic structures near the source and near the teleseismic stations. The observed pattern of P residuals from Longshot can be explained by a descending lithospheric plate, 80 km thick, that reaches a depth of 250 km beneath the Aleutian arc and has P-wave velocities 7–10% higher than the surrounding mantle. The anomalous high velocity at 100- to 200-km depth indicates that the descending plate at that depth is colder than the surrounding normal mantle by several hundred degrees. The P traveltime anomaly associated with the dipping plate is eliminated from the total P residuals to obtain new worldwide station residuals. The station residuals are then grouped according to a proposed tectonic code that distinguishes between active zones of plate convergence, divergence, and transcurrent shear, as well as between volcanically active regions and stable oceanic and continental regions. The tectonically grouped station residuals show a strong correlation with various tectonic features. On the average, P arrivals in continental shields are earlier by 1 sec than in younger (but stable) continental and oceanic provinces, and earlier by 2 sec than in active volcanic regions. The station residuals indicate that lateral velocity anomalies within the upper 200 to 250 km of the earth's mantle are commonly associated with tectonic features and that lateral velocity contrasts may in some cases exceed 10% of the average velocity. A new P-residual map for the United States and adjacent Canada is presented.