Ars
Technica: The earthquakes that most people are familiar
with generally occur within 15 km of Earth's surface. But 90
years ago, evidence was found suggesting some earthquakes occur
at depths of several hundreds of kilometers. How they occur is
unclear. At those depths, the pressures and temperatures cause
rock to flow, so it doesn't experience the sorts of shocks that
cause normal earthquakes. Thorne Lay of the University of
California at Santa Cruz examined a recent quake more than 600
km below the Okhotsk plate in the Pacific Ocean. He found
that the quake had enough energy to cause 180-km-long fractures
in the rock near the epicenter. Under the pressures at that
depth, the fractures spread at nearly 14 000
kph. Alexandre Schubnel of Ecole Normale Supérieure
in France suggests that the fractures are similar to those that
accompany the transformation of the mineral
olivine into
the mineral
spinel, which
occurs under extreme pressures and temperatures. Schubnel has
mimicked the occurrence of such fractures on a laboratory scale
and believes that a similar transformation could be happening
to minerals in Earth's crust to cause the deep
earthquakes.
