Earthquake site response studies using microtremor measurements in southwestern British Columbia

Abstract

In the linear range, the fundamental period and lower-bound estimate of site amplification can be easily determined from a single three-component seismic instrument recording of microtremor (ambient noise), by the horizontal-to-vertical (h/v) spectral ratio method. Across southwestern British Columbia, this technique has been utilized for (1) validation with available earthquake records, (2) seismic hazard assessment projects, (3) soil-structure interaction studies, and (4) comparison with shear-wave velocity measurement techniques. Comparison of microtremor and earthquake response at sites across SW British Columbia, show similar peak periods, and in Victoria, remarkably similar amplitudes. This validates the use of noise recordings for estimation of the fundamental period of a site to linear earthquake motion across SW British Columbia, as well as the estimated level of amplification in greater Victoria. Seismic hazard assessments at 18 greater Victoria schools, across the Fraser River delta, and along a 10-km stretch of the Sea-to-Sky highway that links Vancouver to Whistler (sites of the 2010 Winter Olympic Games) included the use of microtremor measurements for rapid and economical mapping of relative subsurface physical property variations. To investigate possible soil-structure interaction at five sites that host an earthquake recording instrument, noise measurements were conducted both inside, and outside of the structure. The resulting fundamental period compared with available indoor, weak-motion earthquake recordings are similar, suggesting little soil-structure interaction. Furthermore, examination of microtremor recordings made along a transect from the foundation of a 14-storey building to over 150 m away showed no deviation in site response. Comparison of microtremor response with shear-wave velocity measurement techniques are presented in 9CCEE Paper 1173.