A time-dependent moment tensor inversion technique is applied to 20 seismic events recorded ahead of an advancing tunnel excavation at the Underground Research Laboratory (URL) in Canada. The data are recorded on a 16-sensor triaxial accelerometer array, and the events are chosen from a section of the tunnel such that the event-sensor paths are unaffected by the presence of the tunnel. Factorization of the moment tensor rate functions for each event and subsequent decomposition of the subevents into isotropic and deviatoric components indicate that the events are of three types. The first group shows implosional/shear failure source mechanisms. These are associated with the observed collapse of excavation holes drilled at the side of the tunnel and provide confidence that the method is producing realistic results. The second group of events occurs in the region ahead of the face and show tensile/shear failure mechanisms. These are interpreted in terms of crack initiation ahead of the face and may explain why the tunnel undergoes notching and failure in the floor and roof when the modeled stresses are below the measured rock strength. The third group of events shows predominant shear failure throughout their rupture history. These are interpreted in terms of failure on pre-existing crack surfaces. This study highlights the value of using source mechanism studies of associated seismicity to understand the processes that may be taking place in environments where the stress field is complex both spatially and temporally.