Analysis of seismic body waves excited by the Mount St. Helens eruption of May 18, 1980

Abstract

Seismic body waves which were excited by the May 18, 1980, eruption of Mount St. Helens and recorded by the Global Digital Seismographic Network stations are analyzed to determine the nature and the time sequence of the events associated with the eruption. The polarity of teleseismic p waves (period ∼20s) is identical at six stations, which are distributed over a wide azimuthal range. This observation, together with a very small S to P amplitude ratio (at 20 s), suggests that the source is a nearly vertical single force. A simple model shows that for seismic radiation a volcanic eruption can be represented by a single force applied in the direction opposite to the blast direction. The time history of the vertical force suggests two distinct groups of events, about 2 min apart, each consisting of several subevents with a duration of about 25 s. The magnitude of the force is approximately 2.6×10^(12) N. This vertical force is in contrast with the long-period (∼150 s) southward horizontal single force which has been determined by a previous study and interpreted to be due to the massive landslide. An M_s = 5.2 earthquake initiated the eruption sequence. The direction of the P wave first motion of this event observed at two nearby stations is consistent with the radiation pattern expected for the landslide and suggests that it represents the onset of the landslide. The ground motions observed at station LON (Δ = 67 km) are dominated by Rayleigh waves (i.e., Lamb pulse) and provide tight constraints on the time sequence of the events.

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