12. Observation of Earthquakes and Explosions at the Bottom of the Western Pacific: Structure of Oceanic Lithosphere Revealed by Longshot Experiment

  1. George H. Sutton,
  2. Murli H. Manghnani,
  3. Ralph Moberly and
  4. Ethel U. Mcafee
  1. Toshi Asada1 and
  2. Hideki Shimamura2

Published Online: 17 MAR 2013

DOI: 10.1029/GM019p0135

The Geophysics of the Pacific Ocean Basin and Its Margin

The Geophysics of the Pacific Ocean Basin and Its Margin

How to Cite

Asada, T. and Shimamura, H. (1976) Observation of Earthquakes and Explosions at the Bottom of the Western Pacific: Structure of Oceanic Lithosphere Revealed by Longshot Experiment, in The Geophysics of the Pacific Ocean Basin and Its Margin (eds G. H. Sutton, M. H. Manghnani, R. Moberly and E. U. Mcafee), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM019p0135

Author Information

  1. 1

    Geophysical Institute, University of Tokyo, 113, Japan

  2. 2

    Geophysical Institute, Hokkaido University, Sapporo 060, Japan

Publication History

  1. Published Online: 17 MAR 2013
  2. Published Print: 1 JAN 1976

ISBN Information

Print ISBN: 9780875900193

Online ISBN: 9781118663592

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Keywords:

  • Geophysics—Pacific area—Congress;
  • Woollard, George Prior, 1908

Summary

Results of observations using ocean bottom seismographs (OBS's) for the past five years are reviewed. About seventy operations have been made to date with these instruments. Studies of natural earthquakes near trenches have clearly shown that the frequency content of seismic waves and rates of occurrence of earthquakes are in marked contrast for earthquakes occurring landward as compared with those occurring oceanward of trench axes. Q values in the western Pacific basin exceed 6000, whereas Q values on landward sea floors are generally ten or more times smaller. As for explosion seismology, the OBS has proved to be powerful for detecting refraction signals from small charges; for example, it has detected refraction arrivals from 2 kg charges at distances of more than 100 km in Pacific basins. Two longshot explosion seismology studies were made (August 1973 and September 1974) in the western Pacific basin on which the profile lengths were about 1300 km. Since the ambient noise in deep oceanic basins has been shown to be greater in the optimum frequency range (below 10 Hz) for detecting waves from distant shots, 1.5-ton explosions were found to be inadequate for obtaining clear records, whereas 5- or 7-ton shots (used for the second experiment) proved adequate for the longshot experiments. The experiments have illustrated the stratified oceanic lithosphere. The upper layer, immediately below Moho, is 8.2 km / sec and is underlain by a rather thin, high-velocity layer of 8.6 km/ sec. The thickness of theoceanic lithosphere at the site of the second experiment is estimated to be about 80 km. The lithosphere is underlain by a very thin, low-velocity layer of about 8.4 km/ sec. Since the thickness of the low-velocity layer is less than 50 km, the layer may not be recognizable by surface wave studies. Beneath that layer is another 8.6 km/ sec layer with a positive velocity gradient. Regionality in the structure of the lithos phere is suggested. The depth to the upper boundary and the thickness of the 8.6 km/ sec layer are seemingly different between the two experimental sites.