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Characterization of Hard Rock Site Effects on Ground Motions in Western Canada

Session: How Well Can We Assess Site Effects So Far? II

Type: Oral

Date: 4/20/2021

Presentation Time: 02:30 PM Pacific

Description: 

The seismic response of many important structures (e.g. concrete dams) is sensitive to the high frequency content of ground motions. Common ground-motion models (GMMs) are usually referenced to soft-rock site condition (V_S30 =760 m/s) (time averaged shear-wave velocity on the top 30 m). However, many of the critical structures in British Columbia (B.C.), are located on hard-rock site condition (i.e. V_S30 of 1500 m/s - 2800 m/s). Therefore, the seismic hazard results based on soft-rock site condition should be adjusted accordingly. Generic hard-rock adjustment factors (i.e. hard-rock to soft-rock amplification ratio) from the literature show large amount of variability at high frequencies with amplification or de-amplification of up to a factor of 2 to 3 being common. Specific to individual sites, BC Hydro (BCH) uses Vs-kappa corrections to account for these conditions.

Here we present recent work carried out at BCH to investigate another approach to quantifying the hard-rock adjustment factors, by estimating the non-ergodic site response for selected BCH facilities located on hard-rock sites. The scope of the work consists of four main tasks: 1) compilation of an empirical ground-motion database for rock seismograph sites in B.C., from which to determine regional source, attenuation and site terms; 2) development of regionally-calibrated hard-rock GMMs; 3) site-specific instrumental deployments at selected BCH facilities to enable direct computation of non-ergodic site terms relative to the developed GMMs; and 4) derivation of non-ergodic site response for selected BCH facilities using the site-specific ground motion records. Preliminary results have been obtained for the first three tasks. The outcomes of this work will enhance our understanding of the hard-rock site effects on ground motions, especially on the high-frequency response, and may ultimately reduce the uncertainty in seismic hazard analysis for selected BCH facilities through reduction of site-to-site variability.

Presenting Author: Behzad Hassani

Student Presenter: No

Authors