A seismically constrained mass discharge rate for the initiation of the May 18, 1980 Mount St. Helens eruption

We calculate the vertical mass discharge rate from Mount St. Helens for the first few minutes of the May 18, 1980 cataclysmic eruption using a new method based on seismic constraints. The observed seismic waves indicate that the seismic source is a series of single forces. We model these forces as thrusts due to a combination of the momentum flux of the erupted products and the pressure of the eruptive jet. The momentum discharge rate is converted to a mass discharge rate based on estimates of the velocity and jet pressure as constrained by a simple fluid dynamical model. Only two parameters are necessary for the calculation: Mach number and sound velocity. The calculated mass ejected in the first 100 s is 1.6×1011 −4.6×1011 kg. Since the total blast deposit is ∼3.2×1011 − 4.1×1011 kg, one possible interpretation is that the directed blast had a significant (>30%) vertical component.