NEAR-FIELD PULSE-TYPE MOTION OF SMALL EVENTS IN DEEP GOLD MINES: OBSERVATIONS, RESPONSE SPECTRA AND DRIFT SPECTRA

摘要

Strong pulse-type motions are observed in the near source region of large earthquakes; however, there is a lack of waveforms collected from small earthquakes at very close distances recorded underground in mines. The results presented in this paper are relevant for structures of with the height of a few meters, placed in an underground excavation. The strong ground motion sensor was installed in a deep gold mine at the surface of a seismic active stope. The strongest monitored horizontal ground motion was caused by an event of magnitude 2 at a distance of 90 m with PGA 103 m/s2 and PGV 0.365 m/s, and the final displacement was 6.2 mm. The drift response technique based on a continuous shear beam model is employed here to estimate the drift response subjected the near-field earthquakes. The main features of velocity response spectra and drift spectra are similar, which is an indication that the larger damage comes from the large velocity peaks. The strong ground motions with PGA larger than 10 g cause drifts of 50%, which are unobserved at the earth's surface. Drift larger than several percents could cause non elastic deformation. The damping constant of a structure has to be increased significantly to decrease a value of the drift. The weak underground motion has spectral characteristics similar to the strong ground motion observed on the earth's surface; the drift spectrum has a maximum value less than 1% in frequency range relevant to a high underground structure.