Source mechanism of the May 18, 1980, Mount St. Helens eruption from regional surface waves

Abstract

A reference source equalization method using regional surface waves from the Mount St. Helens eruption of May 18, 1980, and the Elk Lake event of February 14, 1981, was used to determine the St. Helens eruption source time history. The time domain deconvolution signals display a complex series of pulses lasting 250 s. An initial pulse was isolated and was found to have a smoothly varying two‐lobed radiation pattern for both Love and Rayleigh waves, suggesting a near‐horizontal point force directed south consistent with previous results. The initial southward force corresponds to the acceleration of the landslide in a northern direction about 5–10 s after the triggering earthquake. Lateral forces were isolated 70–140 s later, consistent with the occurrence of the large lateral blast and the slide changing its direction down the Toutle River Valley. There also appears to be significant source activity between 140 and 250 s after the initial pulse. This later activity exhibits forces in the northwest‐southeast azimuth and may correspond to the final deceleration of the landslide. Several vertical pulses were located between 10–60 and 100–120 s after the eruption began. Much of the vertical force represents the two sets of explosions which occurred during the eruption.