Seismograms observed at Longmire, Washington, for four eruptions of Mt. St. Helens (May 18, June 13, August 7, and August 8, 1980), can be interpreted as Lamb pulses excited by a nearly vertical single force that represents the counter force of the eruption. These data provide reliable estimates of the impulse of the force K (time integral of the force) from which the total momentum and the kinetic energy, E, of the ejecta associated with the eruption can be estimated. The values of K are estimated to be 1.4 × 1019, 1.4 × 1016, 3.7 × 1015, and 2.8 × 1015 dyne·s for the four eruptions (chronological order), respectively. The corresponding values of E are estimated to range from 0.70 to 2.6 × 1023, 0.70 to 2.6 × 1020, 1.9 to 6.9 × 1019, and 1.4 to 5.3 × 1019 ergs using values of ejecta velocity ranging from 100 to 375 m/s. The ratio of K to the amplitude of the air wave excited by the eruption is 20 to 40 times larger for the main event on May 18 than for the other events suggesting a significant difference in the eruptive mechanism. Our results demonstrate that a digital seismograph in the vicinity of volcanoes provides a simple means for quantification of the explosive power of a volcanic eruption.