Air pressure waves from Mt. St. Helens eruptions

Barograms were assembled for the May 18, 1980 eruption and compared to airblast wave propagations from large explosions. Wave amplitudes at 50 to 300 km distances were what might be expected from a nuclear explosion of between 1 megaton and 10 megaton yield. Pressure time signatures could not be received for the first compression phase, because of the slow paper recording speed. The 900 s negative phase duration was too long for comparison with the negative phase of an explosion. Nevertheless, positive and negative amplitudes were about equal, as often observed at long distances from explosions. Calculations made for a simple finite amplitude propagation model show rough bounds on the source compression rate, to give the observed inaudible waves at least to 54 km distance, yet cause audibly rapid compression at Seattle, near 150 km, and beyond.