No. 95: Sep-Oct 1994
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.