In-situ search for the origin of seismic repeaters in a deep mine, using geological and geomechanical measurements

Abstract

Repeaters are repetitive seismic events with highly similar waveforms. They have been studied for decades by seismologists in natural environments and are commonly associated with the occurrence of aseismic slip (i.e. creep), loading seismogenic asperities on fault planes. Recently, long lasting repeater sequence (days to years) have been also observed in the active Garpenberg underground mine, Sweden. The presence of these phenomena may indicate that aseismic deformation induced by mining plays a role in triggering mine seismicity and thus has to be considered in seismic hazard assessment. To better understand the origin of these repeaters, we target one of the permanent repeater source located in the (∼25 m thick) horizontal sill pillar at 1 km depth using drilling cores and installation of strain cells in boreholes. By this in-situ investigation, we aim at observing structures and mechanisms responsible for the occurrence of repeaters, combining geological, geomechanical and geophysical observation. A clear co-seismic strain (∼6μɛ) during the occurrence of one repeater event enabled us to constrain the location of that source asperity, outside the area reached by the drillings. We show that the presence of an aseismic fault plane with at least one seismogenic asperity is consistent with field observations and that the drillings have probably crossed the aseismic portion of the fault plane. Finally, we discuss limitations of our installation, even if local and unique, to clearly identify the source mechanism behind the observed seismicity.