Authors: Malovichko, D; Cuello, D; Rojas, E

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Malovichko, D, Cuello, D & Rojas, E 2018, 'Analysis of damaging seismic event on 24 December 2011 in the Pilar Norte sector of El Teniente mine', in Y Potvin & J Jakubec (eds), Caving 2018: Proceedings of the Fourth International Symposium on Block and Sublevel Caving, Australian Centre for Geomechanics, Perth, pp. 637-650,

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Numerous tunnels of the extraction and undercut levels of the Pilar Norte sector experienced violent damage on 24 December 2011. A seismic event with a moment magnitude of 2.4 was recorded by the mine-wide seismic system at the same time. It is difficult to explain the observed damage with peak particle velocities/peak ground velocities calculated for the basic source parameters of this event (location and magnitude). Seismic data recorded in Pilar Norte in 2011 indicate that failure in the sources is driven by sub-vertical compressional stress. The majority of seismic events located around the undercut level have crush-type source mechanisms (significant implosive component, pancake-shaped deviatoric parts with sub-vertical P-axis) evidencing vertical convergence of the excavations. There are no clear indications of active planar geological structures in the seismic data. The large seismic event that occurred on 24 December 2011 during the blasting sequence and its waveforms are complex. The source mechanism estimated from the low frequency part of the waveforms is of a crushtype. It was hypothesised that the source of this event represents a cascading damage of tunnels. The hypothesis was tested by means of comparing the modelled waveforms of spatially distributed episodes of tunnel damage with the recorded waveforms. A reasonable match was obtained for the scenario including an initial 20 mm convergence of the extraction and undercut drives around the southern Pilar Norte undercut front at a rate of 0.6–0.8 m/s and subsequent spread of smaller (3–6 mm) convergence along the eastern Pilar Norte undercut front and eastern Sub 6 cave abutment at a rate of 0.2–0.4 m/s. There was also significant shearing (ride) deformation along the southern and eastern Pilar Norte cave front. The hypothesis of cascading damage of tunnels also agrees with the underground observations (more than 90% of damage was attributed to the side walls of tunnels) and predictions of sub-vertical orientation of maximum principal stress around the tunnels of the extraction level according to a numerical stress model.

Keywords: seismic source mechanism, mechanism of damage, cascading damage of tunnels

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