Recording experiment on Rainier Mesa in conjunction with a reflection survey

摘要

The chemical explosion of the NPE was recorded on the surface of Rainier Mesa along the same line which had previously been the site of a high resolution reflection survey. Six three-component accelerometer stations where distributed along the 550 meter line, which was offset about 600 meters from the epicenter of the explosion. The bandwidth of the acceleration data extends to 100 Hz. Even though the separations of the stations was only about 100 meters, the waveforms and the amplitudes exhibited considerable variability, especially for the transverse component of motion. The maximum accelerations ranged between 0.27 g and 1.46 g, with the maximums of the average traces being 0.57 g on the radial component, 0.28 on the transverse component, and 0.50 g on the vertical component. Using the results of the reflection survey to help constrain the velocity model, the acceleration data were inverted to obtain a preliminary estimate of the seismic moment tensor of the NPE. This result is a strong diagnostic for the NPE being an explosion, showing a somewhat asymmetric extensional source with very small shear components. When interpreted in terms of a spectral model and scaling relationships, the isotropic moment tensor indicates a yield of 1.4 kt, an elastic radius of 116 meters and a cavity radius of 15.5 meters. This interpretation includes a source time function which contains appreciable overshoot, and, if shown to be reliable, this feature of the explosion could have a significant effect upon the analyses of other types of seismic data.