Analysis of amplitude and travel‐time anomalies for short‐period P ‐waves from NTS explosions

SUMMARY Short‐period P ‐wave amplitudes from explosions at the Nevada Test Site (NTS) have strong, slowly varying azimuthal patterns, which may result from receiver effects, tectonic release interference, attenuation differences or near‐source heterogeneity. In order to determine which factor is responsible for the non‐isotropic radiation patterns, m b and travel‐time anomalies have been measured for 57 explosions at the Pahute Mesa and Yucca Flat sub‐sites, as well as the Central Nevada explosion FAULTLESS, using teleseismic WWSSN and Canadian Seismograph Network stations. Magnitude anomalies from 12 earthquakes in the western United States have also been determined to characterize the near‐receiver effects. Stations north to northeast of NTS record amplitudes that are a factor of 2.5 lower than those to the southeast and west for Pahute Mesa events and a factor of 1.7 lower for Yucca Flat events. Stations to the northeast of NTS also record travel times that are 1.4 s faster than those to the northwest and southeast for Pahute Mesa tests, and 1 s faster for Yucca Flat tests. The m b and travel‐time anomalies are positively correlated for the NTS explosions, suggesting the predominance of elastic focusing and defocusing, rather than attenuation or tectonic release effects. Applying the regional event m b values as receiver corrections leaves a small common component in the NTS m b patterns, which must be produced by velocity heterogeneity deep enough (>200 km) to affect both Pahute Mesa and Yucca Flat events. However, the principal long‐wavelength variation in the receiver‐corrected magnitudes is unique to Pahute Mesa. The travel‐time patterns for the two NTS sub‐sites are quite similar and have a systematic counter‐clockwise rotation from the FAULTLESS travel‐time pattern, indicating that a deep (>400 km) high‐velocity anomaly is responsible for the strong common component of the travel‐time variations. The spatially varying components of the m b and travel‐time patterns between NTS sub‐sites are isolated by subtracting the station average m b and travel‐time anomalies for Yucca Flat from those for Pahute Mesa. The intersite differences have smooth azimuthal variations, with a positive correlation between differenced m b and travel‐time anomalies. This suggests that focusing and defocusing by near‐source velocity heterogeneity, most of which is probably concentrated in a high‐velocity body in the crust and upper mantle (<200 km deep) northeast of Pahute Mesa, causes the strong intersite amplitude variations.