EVIDENCE FOR EXTENSILE CRACK INITIATION IN POINT-SOURCE TIME-DEPENDENT MOMENT TENSOR SOLUTIONS

A time-dependent moment tensor inversion technique is applied to 20 se ismic events recorded ahead of an advancing tunnel excavation at the u nderground 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 una ffected by the presence of the tunnel. Factorization of the moment ten sor rare functions for each event and subsequent decomposition of the subevents into isotropic and deviatoric components indicate that the e vents are of three types. The first group shows implosional/shear fail ure source mechanisms. These are associated with the observed collapse of excavation holes drilled at the side of the tunnel and provide con fidence that the method is producing realistic results, The second gro up of events occurs in the region ahead of the face and show tensile/s hear failure mechanisms, These are interpreted in terms of crack initi ation ahead of the face and may explain why the tunnel undergoes notch ing and failure in the floor and roof when the modeled stresses are be low the measured rock strength. The third group of events shows predom inant shear failure throughout their rupture history. These are interp reted in terms of failure on pre-existing crack surfaces. This study h ighlights the value of using source mechanism studies of associated se ismicity to understand the processes that may be taking place in envir onments where the stress held is complex both spatially and temporally .