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When last week's magnitude 8.2 earthquake struck beneath Bolivia, days after a catastrophic shallower quake in Colombia, its effects were muted by the 640 kilometers of rock between the quake and the surface. But since then what may be the largest deep earthquake of the century has shaken up the scientific community, sending researchers scrambling to decode this signal from the planet's depths. And the first message they are taking from it is a sharp reminder that the descent of Pacific ocean crust into the mantle beneath South America isn't following the textbook pattern.
Deep earthquakes trace this process of subduction because they take place within the descending slab of tectonic plate, marking its slope to depths as great as 670 kilometers. The deepest ones are triggered, say recent theories (Science, 26 April 1991, p. 510), when the pressures of the deep Earth transform the rock from...