Abstract

Dynamic shear ruptures (up to intersonic velocities) have been observed in brittle materials from micro to macro scales including earthquakes. Earthquake laboratory experiments have revealed two key features of extreme ruptures:

  1. intensive microcracking in the rupture tip and

  2. dramatic strength weakening of the rupture head. Although essential for understanding earthquakes, rock mechanics, tribology, and fractures, the relation between the intensive microcracking and strong interface weakening remains enigmatic despite continuous studies over decades. The fact that rupture propagation is always accompanied by intensive microcracking allows the hypothesis that the microstructure generated by this process plays an essential role in determining the relative displacement, speed and resistance of the rupture faces. This paper proposes further development of a recently identified shear rupture mechanism according to which the shearing rupture faces can create, under certain conditions, a fan-shaped microstructure in the rupture head, representing a mechanical system with extremely low shear resistance (up to an order of magnitude less than the frictional strength). The fan-structure represents also a self-sustaining mechanism of stress intensification. These two features provide driving power for extreme ruptures. The paper discusses the role of the fan-structure in creation of extreme ruptures propagating with the pulse-like mode.

1. Features of microcracking process in extreme ruptures

Extreme dynamics of shear ruptures (up to intersonic velocities) has been observed in brittle materials from micro to earthquake scales [1–9]. Extreme ruptures can propagate through intact materials and along pre-existing faults with frictional and coherent (bonded) interfaces. Intensive experimental studies of extreme ruptures, however, have been mainly focused on the rupture development in pre-existing faults. One of the reasons for that is the fact that in pre-existing faults the interface for shear rupture propagation is pre-determined, which facilitates the rupture process observation and registration, unlike extreme ruptures in intact materials. Natural and laboratory observations show that extreme ruptures can propagate as either self-healing slip pulses (pulse-like mode), where the fault relocks shortly behind the rupture front, or sliding cracks (crack-like mode), where a large section of the interface slides behind a fast-moving rupture front [1, 3, 5, 8, 9]. It is observed that key features of extreme ruptures of any mode are: the intensive microcracking process in the rupture tip and dramatic strength weakening of the fault in the rupture head [1–11]. The use of modern techniques allows researchers to go deeper into the details of the microcracking process in extreme ruptures. For example, laser techniques in experiments on transparent brittle polymethyl methacrylate (PMMA) specimens allowed visualizing the variation in contact area and interface slip associated with microcracking process [6, 7]; experiments on brittle photoelastic homalite specimens visualized distribution of stress field around the propagating rupture head and recorded features of slip and rupture velocities for the crack-like and pulse-like rupture modes [3–5, 8, 9]. Fig. 1 introduces some results obtained in these experiments which are very important for understanding of the rupture mechanism governing the extreme ruptures. A schematic representation of specimens used in experiments is shown in Fig. 1a. The frictional interface betweentwo blocks is pre-stressed by normal σn and shear τ stresses generated by forces Pn and Ps. In this paper we will consider extreme ruptures of the pulse-like mode because the understanding of this mode is more questionable. The rupture head propagating from right to left along the interface is shown in Fig. 1a as an ellipse where the green dot indicates the rupture tip and the red dot indicates the head back. The relative slip between the blocks takes place within the rupture head only. In front and behind the head the interface is locked.

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