Lamb pulse observed in nature


  • Hiroo Kanamori,

  • Jeffrey W. Given


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.