Evidence for extensile crack initiation in point source time-dependent moment tensor solutions

Abstract 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.

[1]  K. Koch,et al.  Moment tensor inversion of local earthquake data—I. Investigation of the method and its numerical stability with model calculations , 1991 .

[2]  Monitoring Progressive Failure Around a Tunnel In Massive Granite , 1995 .

[3]  Mechanisms of local earthquakes in 3-D inhomogeneous media determined by waveform inversion , 1995 .

[4]  A. McGarr,et al.  Moment tensors of ten witwatersrand mine tremors , 1992 .

[5]  A. McGarr,et al.  An implosive component in the seismic moment tensor of a mining‐Induced tremor , 1992 .

[6]  C. Martin.  Brittle Rock Strength Failure: Laboratory And In Situ , 1995 .

[7]  Robert B. Herrmann,et al.  A Student’s Guide to and Review of Moment Tensors , 1989 .

[8]  D. Giardini.  Moment tensor inversion from Mednet data (1) large worldwide earthquakes of 1990 , 1992 .

[9]  J. Šílený,et al.  Seismic events with non-shear component: I. Shallow earthquakes with a possible tensile source component , 1985 .

[10]  G. Panza,et al.  Inversion of seismograms to determine simultaneously the moment tensor components and source time function for a point source buried in a horizontally layered medium , 1991 .

[11]  C. Martin,et al.  The strength of massive Lac du Bonnet granite around underground openings , 1993 .

[12]  Giuliano F. Panza,et al.  Seismic moment tensor resolution by waveform inversion of a few local noisy records—I. Synthetic tests , 1996 .

[13]  G. Natale,et al.  Earthquake Focal Mechanisms from Inversion of First P And S Wave Motions , 1989 .

[14]  Giuliano F. Panza,et al.  Waveform inversion for point source moment tensor retrieval with variable hypocentral depth and structural model , 1992 .

[15]  Mark A. Riedesel,et al.  Display and assessment of seismic moment tensors , 1989 .

[16]  I. G. Wong,et al.  Implosional failure in mining-induced seismicity: a critical review : Proc 2nd International Symposium on Rockbursts and Seismicity in Mines, Minneapolis, 8–10 June 1988 P45–51. Publ Rotterdam: A A Balkema, 1990 , 1991 .

[17]  L. Johnson,et al.  Second-order moment tensors of microearthquakes at The Geysers geothermal field, California , 1988 .

[18]  M. Schäfer,et al.  Source parameters of seismic events at Heinrich Robert mine, Ruhr Basin, Federal Republic of Germany: Evidence for nondouble-couple events , 1990 .

[19]  R. P. Young,et al.  A controlled in‐situ investigation of the relationship between stress, velocity and induced seismicity , 1995 .

[20]  T. Lay,et al.  Rapid source mechanism determination of large (Mw≥5) earthquakes in the western United States , 1993 .

[21]  G. Cespuglio,et al.  Seismic moment tensor resolution by waveform inversion of a few local noisy records—II. Application to the Phlegraean Fields (Southern Italy) volcanic tremors , 1996 .

[22]  S. J. Gibowicz,et al.  Source parameters of seismic events at the Underground Research Laboratory in Manitoba, Canada: Scaling relations for events with moment magnitude smaller than −2 , 1991 .

[23]  NábÄ lek,et al.  Determination of earthquake source parameters from inversion of body waves , 1984 .

[24]  K. Satake.  Effects of station coverage on moment tensor inversion , 1985 .

[25]  R. Paul Young,et al.  Moment tensor inversion of induced microseisnmic events: Evidence of non-shear failures in the -4 < M < -2 moment magnitude range , 1992 .