Direct measurement of stick‐slip and shear fracture energy of granite at elevated pressures

Abstract

The dynamics of frictional sliding in stick‐slip for a precut Westerly granite is studied at elevated pressures. Piston displacement measured outside the pressure vessel shows a small but significant delay (∼ 2 ms) during a stick‐slip. As a result, most part of the stress‐drop occurs with little piston displacement while most piston displacement occurs while the stress has already dropped to a residual level, giving a peculiar "L"‐shape in the stress‐displacement curve. On the other hand, displacement measured using an internal transducer mounted directly across the rock fracture shows that most slip on the rock fracture occurs concurrently with stress‐drop. Based on the directly measured stress‐displacement histories, we estimate a critical slip‐weakening displacement of 200 µm and a shear‐fracture energy of 300 J/m² at a normal stress of 85 MPa. This result shows that the discrepancy between the extrapolated values for the shear fracture energy [Okubo and Dieterich, 1984] and the estimated values given here and by Rice [1980] cannot be accounted for by different modes of measuring displacement.