PSI - Issue 2_B

Alexey A Ostapchuk et al. / Procedia Structural Integrity 2 (2016) 2810–2817 Author name / Structural Integrity Procedia 00 (2016) 000–000

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Depending on properties of the filler the dynamic events of one and the same type may be realized as either regularly repeated events (regular regime), or random stochastic events (irregular regime). This work deals only with regularly repeated dynamic events. It is convenient to characterize the state of a rock fault by its shear stiffness:

s dW k d  

(1)

s

where  is the shear stress, W s is the relative displacement of discontinuity sides. The regime of repeated episodes of accelerated creep is a set of alternating slow accelerations and gradual decelerations. Relative alteration of shear force in this regime is less than 1 %, which complicates a more detailed description of the process. The most pronounced changes of fault parameters are observed in regularly repeated dynamic failures (stick slip). Unlike in simplified stick-slip models, in which there are no motion between periods of sliding, here the block moves all the time at a finite velocity. The velocities at the preparation stage and during the dynamic failure (V max ) may differ 10 6 times. A typical pattern of alteration of fault parameters is shown in Fig.3. The stage of dynamic failure preparation can be divided into three sections, each exhibiting certain peculiarities of fault state alteration.

Fig. 3. Variation of block velocity (a) and fault stiffness (b) during repeated dynamic failure.

Despite the increase of shear force the motion gradually decelerates at the section A. It is the change of fault parameters that produces this effect, but not the block inertia. The increase of shear stiffness by more than an order of magnitude is observed at this section. The stiffness reaches its maximal value of ~ 100-200 MPa/m several seconds after the dynamic event. The end of section A can be regarded as the end of the strengthening stage of the fault. Variations of shear stiffness are negligibly small at the section B of relative stability, when block moves at a constant low velocity. Then, the stage of weakening C starts, at which the stiffness decreases radically with displacement. Duration of this stage, on average, is about 2/3 of the recurrence time.