Mining-induced fault-slip in deep underground mines is known to cause devastating damage to mine openings. It is thus crucial to be able to estimate its seismic source parameters, such as magnitude and seismically released energy. When slip-weakening laws are employed in fault-slip simulations, the characteristic slip-weakening distance, dc, is one of the most important input parameters. This study proposes a methodology to constrain dc of a large-scale fault in an underground mine, based on seismic efficiency η defined by ER (seismically radiated energy) / ΔW (potential energy change). First, a 3D numerical model that encompasses a fault running parallel to a steeply dipping orebody is constructed with the help of finite difference software FLAC3D. Mining blocks of the orebody are extracted in static conditions. Subsequently, dynamic analysis is performed to simulate fault-slip with a linear slip-weakening law employing Barton's shear strength model. Model parametric studies with respect to dc and mining depth are performed, and seismic efficiency is computed for each model. The results show that when dc ranges from 1.0 cm to 2.5 cm, the analyses yield reasonable seismic efficiencies. The estimated range of dc is reasonable considering the difference in size between rock shear tests, natural earthquakes, and mining-induced fault-slips. Consequently, this study provides a clue to estimating a reasonable range of dc especially for mining-induced fault-slip, thus significantly contributing to more accurate evaluation of the intensity of ground motions induced by mining-induced fault-slip.

1. Introduction

The stress re-distribution caused by mining activities at great depths can reactivate pre-existing faults in underground mines, leading to fault-slip with intense seismic waves. As a result, severe damage to mine openings is inflicted. Importantly, seismic waves arising from fault-slip predominantly contribute to the damage initiation. Although previous studies mainly focus on the "static" behaviour of faults in underground mines (Hofmann, et al., 2011), a recent study (Sainoki and Mitri, 2015) has addressed the problem by performing dynamic analysis. It was found that critical slip-weakening distance dc, over which the shear stress acting on a fault decreases during fault-slip, is an important parameter that affects the severity of mining-induced fault-slip, i.e., the intensity of seismic waves and the amount of seismically radiated energy.

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