A shear press has been used to determine mechanical behavior of serpentinized dunite and forsterite at normal pressures to 50 kb, temperatures to 900°C, and at strain rates from 10−1/sec to 10−1/sec. Less than 5% by weight of serpentine reduces shear strength by at least 30% as the temperature is raised from 300°C to 500°C due to reduction of effective normal stress within the compressed wafers. The reduction of effective stress and subsequent weakening is thought to be due to generation of pore pressure resulting from dehydration of associated hydrous minerals. Mechanical behavior of upper mantle mineral analogs suggests that earthquakes are not produced by ordinary faulting, but may be associated with deformation of “weak” hydrous phases.

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