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Analysis of long-period seismic waves excited by the May 18, 1980, eruption of Mount St. Helens - a terrestrial monopole?

Kanamori, Hiroo and Given, Jeffrey W. (1982) Analysis of long-period seismic waves excited by the May 18, 1980, eruption of Mount St. Helens - a terrestrial monopole? Journal of Geophysical Research B, 87 (B7). pp. 5422-5432. ISSN 0148-0227. doi:10.1029/JB087iB07p05422.

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Long-period (100 to 260 s) Love and Rayleigh waves excited by the eruption of Mount St. Helens on May 18, 1980, and recorded by IDA, SRO, and ASRO stations were analyzed to determine the mechanism of the eruption. The amplitude radiation patterns of both Rayleigh and Love waves are two lobed with a nodal direction in E5°S for Rayleigh waves and in N5°E for Love waves. These radiation patterns preclude any double-couple mechanism. The radiation pattern, the initial phase, the relatively large amplitude ratio of Love to Rayleigh waves and the existence of clear nodes in the radiation patterns of fundamental mode and higher-mode Rayleigh waves suggest that the source is represented by an almost horizontal (less than 15° from the horizontal) single force pointed toward S5°W. The surface wave spectra fall off very rapidly at periods shorter than 75 s suggesting a very slow source process. Although the details of the source time history could not be determined, a smooth bell-shaped time function: f_(o)s(t) = (1/2)f_o(l-cos((t/τ)π)) for 0 ≤ t ≤ 2τ and f_(o)s(t) = 0 for t ≥ 2τ, with τ = 75 s is considered appropriate on the basis of comparison between synthetic and observed seismograms and of the shape of the source spectrum. The peak value of the force f_0 is about 10^(18) dynes. The tailing end of the source time function could not be resolved, and some overshoot may be added. The magnitude and the time history of the force can be explained by a northward landslide followed by a lateral blast observed at the time of the eruption. Two distinct events about 110 s apart can be identified on body wave and short-period surface wave records. The first event may correspond to the earthquake which triggered the landslide and the lateral blast. The second event appears to correspond to a second large earthquake and explosion which took place about 2 minutes after the first earthquake.

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Kanamori, Hiroo0000-0001-8219-9428
Additional Information:1982 by the American Geophysical Union. Received October 15, 1981; revised March 2, 1982; accepted March 12, 1982. We thank all the participants in the coffee break at the Seismological Laboratory. Their enthusiasm and interest prompted us toward completion of this study. In particular, the last part of this paper was written on the basis of discussions during the numerous coffee breaks and on some detailed notes on rocket propulsion and inclined plane handed to us later by Don Anderson, Brad Hager, and Tom Heaton. Sue Kieffer and Kazuaki Nakamura made very helpful suggestions on the mechanism of lateral blast and landslide respectively. We thank Harry Glicken, Stephen Malone and James Moore for useful information on the sequence of the various events. Emile Okal and Sue Kieffer reviewed the manuscript and made helpful comments. The IDA data were made available to us by courtesy of the IDA Project team at the University of California, San Diego, and the SRO and ASRO data were provided by the U.S. Geological Survey. Research supported by the U.S. Geological Survey contract 14-08-0001-19755, National Aeronautics and Space Administration grant NSG-7610, and National Science Foundation grant EAR78-11973. Contribution 3708, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, 91125.
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Caltech Division of Geological and Planetary Sciences3708
Issue or Number:B7
Record Number:CaltechAUTHORS:20141111-105312535
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Official Citation:Kanamori, H., and J. W. Given (1982), Analysis of long-period seismic waves excited by the May 18, 1980, eruption of Mount St. Helens—A terrestrial monopole?, J. Geophys. Res., 87(B7), 5422–5432, doi:10.1029/JB087iB07p05422.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:51571
Deposited On:11 Nov 2014 21:04
Last Modified:10 Nov 2021 19:12

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