Induced seismicity during the construction of the Gotthard Base Tunnel, Switzerland: hypocenter locations and source dimensions
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A series of 112 earthquakes was recorded between October 2005 and August 2007 during the excavation of the MFS Faido, the southernmost access point of the new Gotthard Base Tunnel. Earthquakes were recorded at a dense network of 11 stations, including 2 stations in the tunnel. Local magnitudes computed from Wood–Anderson-filtered horizontal component seismograms ranged from −1.0 to 2.4; the largest earthquake was strongly felt at the surface and caused considerable damage in the tunnel. Hypocenter locations obtained routinely using a regional 3-D P-wave velocity model and a constant Vp/Vs ratio 1.71 were about 2 km below the tunnel. The use of seismic velocities calibrated from a shot in the tunnel revealed that routinely obtained hypocenter locations were systematically biased to greater depth and are now relocated to be on the tunnel level. Relocation of the shot using these calibrated velocities yields a location accuracy of 25 m in longitude, 70 m in latitude, and 250 m in focal depth. Double-difference relative relocations of two clusters with highly similar waveforms showed a NW–SE striking trend that is consistent with the strike of mapped faults in the MFS Faido. Source dimensions computed using the quasidynamic model of Madariaga (Bull Seismo Soc Am 66(3):639–666, 1976) range from 50 to 170 m. Overlapping source dimensions for earthquakes within the two main clusters suggests that the same fault patch was ruptured repeatedly. The observed seismicity was likely caused by stress redistribution due to the excavation work in the MFS Faido.
KeywordsInduced seismicity Earthquake locations Source dimensions Gotthard Base Tunnel Stress redistribution
We wish to thank AlpTransit Gotthard AG for financial support of installation and maintenance of the AlpTransit network. We further wish to thank S. Metzger and B. Baitsch-Ghiradello for their help in the fieldwork and data analysis, and the staff of the Swiss Seismological Service for maintenance and operation of the AlpTransit network. Comments and suggestions from two anonymous reviewers significantly improved the manuscript. This work benefited from fruitful discussion within the AG Mikrobeben consisting of the following members: M. Rehbock-Sander, R. Stadelmann, H. Hagedorn, T. Eppler (all Amberg Engineering AG), D. Kressig, F. Pochop (all AlpTransit Gotthard AG), H.-J. Ziegler (Kellerhals + Haefeli AG), P. Zwicky (Basler & Hofmann).
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