Shear Heating in Granular Layers

Abstract

Abstract-Heat-flow measurements imply that the San Andreas Fault operates at lower shear stresses than generally predicted from laboratory friction data. This suggests that a dramatic weakening effect or reduced heat production occur during dynamic slip. Numerical studies intimate that grain rolling or localization may cause weakening or reduced heating, however laboratory evidence for these effects are sparse. We directly measure frictional resistance (v), shear heating and microstructural evolution with accumulated strain in layers of quartz powder sheared at a range of effective stresses (| n =5-70 MPa) and sliding velocities (V= 0.01-10 mm/s). Tests conducted at | n ]25 MPa show strong evidence for shear localization due to intense grain fracture. In contrast, tests conducted at low effective stress (| n = 5 MPa) show no preferential fabric development and minimal grain fracture hence we conclude that non-destructive processes such as grain rolling/sliding, distributed throughout the layer, dominate deformation. Temperature measured close to the fault increases systematically with | n and V, consistent with a one-dimensional heat-flow solution for frictional heating in a finite width layer. Mechanical results indicate stable sliding (v 0.6) for all tests, irrespective of deformation regime, and show no evidence for reduced frictional resistance at rapid slip or high effective stresses. Our measurements verify that the heat production equation (q = v| n V) holds regardless of localization state or fracture regime. Thus, for quasistatic velocities (V 510 mm/s) and effective stresses relevant to earthquake rupture, neither grain rolling/sliding or shear localization appear to be a viable mechanism for the dramatic weakening or reduced heating required to explain the heat flow paradox.

Rheological transitions in high‐temperature volcanic fault zones

“…The fracturing of granular melt found in our experiments was not reported in the previous friction tests using quartz with subrounded to angular shape and spherical glass beads at room temperature and the normal stress <10 MPa [Mair and Marone, 2000;Anthony and Marone, 2005]. The fracturing found in this study may reflect the irregular shape of the starting material (Figure 1), compared with quartz and glass beads used in the previous studies [Mair and Marone, 2000;Anthony and Marone, 2005].…”

Moisture‐related weakening and strengthening of a fault activated at seismic slip rates

“…In our experiments, the slip rate and sliding displacement were remarkably higher and longer than the previous studies, and thus the temperature reached 40°C due to its frictional heating. In low slip rate and short displacement experiments, temperature increase due to frictional heating is reported to be less than 10°C for both bare rocks and granular materials [Lockner and Okubo, 1983;Mair and Marone, 2000]. The larger increase in temperature at higher slip rates would contribute to the moisture-drained weakening.…”

A thermal pressurization model for the spontaneous dynamic rupture propagation on a three‐dimensional fault: 1. Methodological approach

“…Accordingly to Cardwell et al [1978] and Fialko [2004], we assume that the shear strain rate is constant within 2w. Mair and Marone [2000] have shown with laboratory experiments that this hypothesis might be adequate. Therefore the shear strain rate becomes the ratio of the total slip velocity v over the thickness of the slipping zone 2w.…”