Development of a Vs30 (NEHRP) map for the city of Ottawa, Ontario, Canada

Abstract

Four different seismic methods were used extensively to evaluate the shear wave velocity of soils and rock in the city of Ottawa, Canada, from which the travel-time weighted average shear wave velocity (Vs) from surface to 30 m in depth (Vs30) and the fundamental frequency (F0) were computed. Three main geological or geotechnical units were identified with distinct shear wave velocities: these consist of very loose thick post-glacial fine-grained sands, silts, and clays (Vs <150 m/s, thickness up to 110 m), firm glacial sediments (Vs~580 m/s, thickness ~3 m), and very firm bed-rock (Vs ~1750-3550 m/s). The seismic methods applied were downhole interval Vs measurements at 15 borehole sites, seismic refraction-reflection profile measurements for 686 sites, high-resolution shear wave reflection "landstreamer" profiling for 25 km in total, and horizontal-to-vertical spectral ratio (HVSR) of ambient seismic noise to evaluate the fun-damental frequency for ~400 sites. Most of these methods are able to distinguish the very high shear wave impedance of and depth to bedrock. Sparse earthquake recordings show that the soil amplification is large for weak motion when the soil behaves linearly.