“Pre-Earthquake” Micro-Structural Effects Induced by Shear Stress on α-Quartz in Laboratory Experiments

Abstract

This paper presents the results of measurements performed on -quartz subjected to shear stress in dry conditions, to understand the relationship between the shear intensity and the resulting physical and chemical e ects. If a shear stress of intensity higher than 100 MPa is applied continuously to alpha quartz crystals, they will tend to lose their crystallinity, progressively reduce their friction coe cient (Cof) and change into a low-order material, apparently amorphous under X-ray di raction, but with a structure di erent from silica glass. Raman and Pair Distribution Function analyses suggested a structure like cristobalite, a silica polymorph well-known for its auxetic behavior, i.e., having a negative Poisson ratio. This elastic parameter pre-eminently controls the friction coe cient of the material and, if it is negative, the Cof lowering. As a result, the increase in low crystallinity cristobalite is su cientto explain the lowering of the quartz friction coe cient up to values able to contribute, in principle, to the triggering processes of active faults. This allows hypothesizing a slip induction mechanism that does not include the need to have the interposition of layers of hydrated silica, as invoked by many authors, to justify the low friction coe cients that are achieved in shear stress tests on rocks abundant in quartz.