The influence of viscosity of thin fluid films on the frictional interaction mechanism of rock blocks

Abstract

In the past few years, as a result of accomplishment of a number of international drilling projects in frac ture zones, new important results have been published about the structure of fractures of mega earthquakes and the properties of the geomaterials that make them up [1]. Especially interesting data were obtained as a result of sampling in the Nankai Trough [2, 3] and the Japan Trench including sampling in the fracture of the Tohuku mega earthquake fracture of March 11, 2011, with moment amplitude М w = 9.0 [4,5]. We focus attention on the fact that the frictional properties of the materials taken from various fractures are signifi cantly different [2][3][4]. This leads to the formation of different deformation regimes under the conditions of a close stressed state: slow earthquakes [2, 3], tsunami genic earthquakes of the Tohuku type [4], and "typi cal" events [2]. Some of the authors think that the dif ferent frictional behaviors may be related to the differ ent composition of clayey materials, which compose from 37 to 85% of the ground material sampled from the central parts of the fracture zones [2,4]. Such a high concentration of clay is characteristic of the upper part of the fracture located within the accretion prism and near it. As the depth increases, the concen tration of clay decreases and the content of quartz increases, which leads to the possibility of instability [3]. Nevertheless, the material at sufficiently shallow depths in the main part of the fracture invariably con tains minerals such as smectite, kaolinite, and others. Colloidal films formed by the clayey minerals have clearly manifested viscous properties and even being present in small quantities can have a strong influence on the frictional characteristics of the fracture.In this paper we present the results of laboratory experiments in which we found a new effect of the rad ical variation in the shear deformation of a crack after passing a specific viscosity limit of the thin fluid films that wet the particle surfaces of the filling material.In our experiments a granite block with a size of 8 × 8 × 3 cm was located on an immovable granite basement. The contact between the rough surfaces was filled with a layer of quartz sand 2.5 mm thick with a mean size of the grains d = 0.2-0.3 mm. The mean depth of the inhomogeneities of the rough surface was notably greater than the grain size; it was equal to 0.5-0.8 mm. A normally directed load was applied to a mobile block using a special device that excluded the appearance of shear stresses at its upper surface. The applied stress σ n ranged between 50 and 150 kPa. The shear load was applied to the block using a spring ele ment with rigidness K = 10-40 N/mm. The rate of the spring deformation was constant and equal to u s = 20 µm/s. The shear stress applied to the upper block was measured with a force sensor SMS/5kN with an accuracy to 1 N, while the relative displacement of the blocks was recorded using laser sensors ILD2220-10 (G) in the frequency band 0-5 kHz with an accuracy to 0.1 µm.I...

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