Origin of seismic repeaters in a deep mine: what do we learn from in-situ investigation coupling geology, geomechanics and geophysics?

Overview & Synopsis

Seismic repeaters (i.e. seismic events with highly similar waveforms) have been observed in the deep mine of Garpenberg, Sweden. In natural seismology, repeaters are commonly associated with the presence of aseismic slip (i.e. creep) on fault planes, loading seismogenic asperities. This raises questions regarding what these asperities and creeping zone consist of and how they behave from a mechanical point of view. A local monitoring network has been installed in situ, close to the source area of one family of repeaters at 1km depth, with the objective of identifying the origin of these repeaters. The network comprises boreholes, geophones and strain cells. We show that the presence of a fault plane with creeping portions and an asperity is compatible with field data. We also discuss the difficulty of clearly identifying the origin of the repeaters, as we couldn’t reach the seismic asperity itself, even with such a local network.