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Deep Quake Deepens Mystery

Most earthquakes are shallow. They are concentrated no deeper than 20-25 kilometers down. However, a few extremely deep quakes rumble at depths of about 600 kilometers. On June 8, 1994, what may be the largest deep earthquake of the century -- magnitude 8.2 -- exploded 640 kilometers beneath Bolivia. "Exploded" may or may not be the proper word. Geophysicists are really not certain what causes the very deep quakes, because at 640 kilometers rocks are so hot that they flow rather than snap under geological stresses. The more common, shallow earthquakes are generally created when rocks snap and fracture. Since the deep quakes seem to be concentrated in subducted slabs of terrestrial crust that plunge down deep into the earth's mantle, geophysicists suppose that the increasing heat and pressure applied to the descending slabs may cause "explosive" phase changes in minerals contained in the slabs. Phase changes often involve volume changes that, if sudden, might generate seismic waves. Too, water of hydration in minerals may be explosively turned into vapor. But this is all surmise at present.

The Bolivian quake also caused the whole earth to ring like a bell. Every 20 minutes or so, the entire planet expanded and contracted a minute but detectable amount.

Another surprise: the Bolivian earthquake was felt a far away as Seattle -- the first time that a quake in that part of South America has been actually felt in North America.

(Kerr, Richard A.; "Bolivian Quake Deepens a Mystery," Science, 264:1659, 1994. Also: Monastersky, R.; "Great Quake in Bolivia Rings Earth's Bell," Science News, 145:391, 1994.)

Deep-focus earthquakes are cataloged in EQQ1 in our catalog: Inner Earth: A Search for Anomalies. Details here.

From Science Frontiers #95, SEP-OCT 1994. 1994-2000 William R. Corliss

Science Frontiers Sourcebook Project Reviewed in:

Quotes

  • "A sourcebook of unexplained phenomena is therefore a valuable addition to a collection of scientific literature. William R. Corliss has provided this in the past with his source books of scientific anomalies in several subjects, and now he has provided it for astronomy. He has done an excellent job of collecting and editing a large amount of material, taken in part from scientific journals and in part from scientific reporting in the popular or semi-scientific press." -- "The Mysterious Universe: A Handbook of Astronomical Anomalies", reviwed by Thomas Gold, Cornell University, in Icarus, vol.41, 1980

  • "An interesting, systematic presentation of unusual weather [..] This book is recommended for a general audience" --"Corliss, William R., Tornados, Dark Days, Anomalous Precipitation, and Related Weather Phenomena, Sourcebook Project, 1983.", revieweed in Choice, September 1983
  • "..the science is necessarily somewhat speculative, but Corliss's symthesis is based on reputable sources." -- "Corliss, William R. (Compiler). Lightning, Auroras, Nocturnal Lights, and Related Luminous Phenomena" reviwed by Joseph M. Moran, Univ. of Wisconsin in Science Books and Films, Sep/Oct 1983

  • "Before opening the book, I set certain standards that a volume which treads into dangerous grounds grounds like this must meet. The author scrupulously met, or even exceeded those standards. Each phenomenon is exhaustively documented, with references to scientific journals [..] and extensive quotations" -- "Book Review: The moon and planets: a catalog of astronomical anomalies", The Sourcebook Project, 1985., Corliss, W. R., Journal of the Royal Astronomical Society of Canada, Vol. 81, no. 1 (1987), p. 24., 02/1987