Abstract
THE mechanism of deep-focus earthquakes has been a puzzle since their discovery almost 70 years ago1, 2, because brittle fracture and frictional sliding at depths in excess of 100–200 km would require unrealistic rock strengths3, 4. Rock strength does increase with pressure, but a few hundred MPa (equivalent to 10–20 km depth) suffices to inhibit most fracture, and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength. A range of mechanisms has been proposed for deep earthquakes, including plastic instabilities5–7, shear-induced melting8–11 and instabilities accompanying recrystallization12, 13 or polymorphic phase transformation14–23. Each of these proposed mechanisms has exhibited certain inherent weaknesses (see Kirby22 for review and discussion). Here we report experimental observations of high-pressure faulting of metastable Mg2GeO4 olivine as it undergoes incipient transformation to a spinel 24, 25 structure. We present a model in which this faulting arises from the generation, propagation and linking-up of spinel-filled anticracks26. When applied to the olivine → spinel transformation in the Earth's mantle, the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding.
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Department of Geology, University of California, Davis, California 95616, USA
- H. W. Green II
- & P. C. Burnley
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