Shear Wave Velocity Measurements For Earthquake Hazards Studies, Fraser River Delta, British Columbia

Shear wave velocity-depth information in thick unconsolidated overburden is one of the fundamental<br>requirements for estimation of seismic site responses such as liquefaction potential and ground motion<br>amplification in an earthquake-prone area such as the Fraser River delta. The delta area (approximately 600 km*)<br>was surveyed in a reconnaissancem anner using direct shearw ave (S-wave) measurement methods such as surface<br>refraction, seismic cone penetrometer, and downhole logging. In addition, S-wave velocities of the entire<br>overburden thickness have been estimated from conventional exploration seismic reflection velocity analyses.<br>In all, S-wave velocitydepths have been obtained for over 400 sites where unconsolidated overburden thicknesses<br>range from ~5 m near the delta edges to > 1000 m in the south central region. Results indicate:<br>1) large S-wave velocity-depth gradients within the unconsolidated overburden which can give rise to<br>amplification effects for simple elastic models; confiiation of such amplifications are given from observations<br>of local earthquakes;<br>2) large S-wave velocity contrasts within the overburden sequence associated with the Holocene-Pleistocene<br>boundary; such contrasts can give rise to considerable lateral variation in site period response to earthquake<br>shaking;<br>3) widespread occurrence of low S-wave velocities in the near surface materials (mainly fine sands), suggesting<br>that the seismic liquefaction potential may be relatively high