The Mt St Helens Explosion...Revisited 40 Years Later.

Abstract:

Release of pressure on high quality steam with the collapse of the north slope of the volcano is regarded as the cause of the Mt St Helens explosion, the slope failing in response to an earthquake. Several issues immediately arise. Slope stability analyses (e.g. Bureau of Land Management computer modeling as well as others) indicates the north slope stable even under insult of an earthquake nor is the failure consistent with earthquake generated landslides which characteristically take several minutes of shaking before they let loose. Near field seismic monitoring yielded the Love waves that are associated with landslides but nothing of a more striking nature. Far field seismic monitoring stations reported a different signature: complete azimuthal uniformity in P-wave arrivals, first motion up and significantly diminished S waves. This event occurred ~11s before the collapse of the north slope which generated the Love waves. This is a signature of an underground explosion and they are important in detecting violations of nuclear test ban treaties. What may be thought peculiar is none of these far field signatures were picked up at near field stations. However, a shadow zone just above the detonation point accompanies explosions at depth and the P wave signatures would not appear there. Explosion excavation literature (which is extensive) points out that an explosion causes material failure by spallation from surface inward to the detonation point in a retrogressive fashion, the reflected shock placing the material under tension. Rock has good compressive strength, little tensile strength. If an explosion at depth were cause of the failure of the north slope, the failure would proceed in a retrograde fashion; from surface inward. Mt St Helens is reported to have experienced “retrogressive failure”. Further, the literature suggests the mountain would fail by slabbing into sections about 500m thick each. There are other observations that urge looking deeper into the explosion at depth scenario for Mt St Helens. However, new material has recently come to fore which argues even more strongly for an explosion at depth responsible of the collapse of Mt St Helens. This material will be discussed and feedback solicited. All material is from referred or professional sources.