No. 112: Jul-Aug 1997
Earthquakes are concentrated along the boundaries between tectonic plates where oceanic plates dive under continental plates. Stresses naturally accumulate during such slow-motion collisions. The result: plenty of quakes. Mechanically, this model is very appealing, but there are puzzling exceptions. There are sections along plate boundaries obviously in collision where no earthquakes at all occur to relieve stresses. Quakes are felt on either side of these segments, but all is serene inside. These segments are termed "seismic gaps." They may stretch for hundreds of kilometers.
Theory insists that all seismic gaps must eventually be filled in. After all, the rocks can take only so much stress. Theory may be wrong because at least ten seismic gaps seem to be permanent. Something unexplained is transpiring beneath the surface that allows oceanic plates to slide quietly down under the continents and deep into the mantle.
One such permanent seismic gap is especially embarrassing to geophysicists. It stands out prominently on earthquake maps of the very active Peruvian coast. When the immense quake of 1974 shook this coastline, this gap was unperturbed. Neither did the many aftershocks violate this charmed region. Not believing in subterranean magic, some geophysicists confidently (and very loudly) predicted this reluctant gap would soon yield. After 23 years it is still there!
(Penvenne, Laura Jean; "When It's Better to Build on the Fault," New Scien tist, p. 14, January 11, 1997.)
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