Study on Crustal Dynamics and Earthquake Mechanism Bassed on Experiments on Deformation and Fracture of Rocks under High Confining pressure and Temperature

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In experimental works on the deformation and fracture of rocks, we have concluded that the fracture mechanism changes at the confining pressure where the compressive strength equals the frictional strength, from the ordinary brittle fracture (designated as the low-pressure type) to the high-pressure type. We have also suggested the possibility that the high-pressure type occurs in the crust, considering the size effect on rock strength.The change of micromechanisms between them was discussed from the point view of structural geology. The relationship between the performance of them in the crust and faulting styles was discussed.The strength of granite at a confining pressure of 1500 MPa in the high-pressure type regime was observed to temperature of 600℃. The strength values at 200〜280℃ were found to be very low with a minimum at around 250℃. This behavior was explained as the switching the dominating factors from the activation of crack groups to the inhibition of them and the dominance of crystal plasticity with increasing temperature. The lithospheric strength inferred from these experimental results suggests the presence of a low-strength zone in depth of 8〜12km. This could provide an alternative explanation for the seismogenic zone in the crust : earthquakes could occur or nucleate easily where the strength is low.Development of cracks was observed using granite samples recovered from several stress levels of fault formation process during triaxial compression controlled with circumferential-strain rate. The fault was developed again by re-compression of the recovered sample on which surface strain gauges were distributed near the fault nucleus, and the change of strain field near the fault was measured in the fault developing process. New idea on the faulting process was proposed.