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Lithological Controls on Seismicity in Granitic Rocks

Applied Seismology Consultants Ltd.
10 Belmont
Shrewsbury, Shropshire, SY1 1TE
England
(D. S. C.)

University of Liverpool, Department of Earth Sciences
Jane Herdman Laboratories
Brownlow Street
Liverpool, L69 3BX
England
(R. P. Y.)

Seismicity induced from a tunnel excavation through two lithological units, granite and granodiorite, at the Canadian Underground Research Laboratory (URL) is analyzed in an attempt to understand observed lithological differences in the damage-zone development. The results from seismicity recorded by the 16 triaxial accelerometer array found the damage zone around granite excavations to have more events occurring ahead of the tunnel face and a shorter overall seismic response time than the granodiorite. Petrographic analysis of the rock samples show stress relief microcracking predominantly in the larger quartz crystals, suggesting that these are the weakest mineral grains. We propose that initial in situ cracking occurs in the large quartz crystals, significantly reducing the strength and resulting in rapid formation of the excavation damage zone. The smaller-grained, more homogenous granodiorite shows less stress relief microcracking, probably due to the stresses being distributed over a larger number of grain boundaries. From the seismic and petrographic evidence we propose that the crack initiation stress is lower in the granite than the granodiorite.

The events from the granite and granodiorite have a similar range in magnitude (M_w = –2.9 to –4.2) and source dimension (r₀ = 0.13–0.51 m). They have a mean P- to S-wave corner frequency ratio of 1.0, probably indicating relatively slow rupture velocities. About 25% of the events have an S- to P-wave energy ratio less than 10, agreeing with previous source mechanism studies, which find a number of events at this depth have significant isotropic components.