Abstract

The December 2015 moment magnitude (M) 4.7 Vancouver Island earthquake is one of the largest recorded inslab earthquakes within Georgia Strait, British Columbia, Canada. Ground motions from all seismic stations within 500 km from the epicenter are examined. Peak ground acceleration reached a maximum of 4.65%g in Greater Victoria. We compare the observed horizontal‐component 5% damped pseudospectral acceleration (PSA) values with the inslab ground‐motion prediction equation (GMPE) suite used in the 2015 National Building Code of Canada (NBCC). PSA values from this event are significantly lower than the expected 2015 NBCC values for an M 4.7 inslab event, highlighting poor GMPE scaling at this low magnitude level. We develop an event‐specific GMPE to constrain the source and attenuation parameters in comparison with seismological models. Potential site effects are examined at strong‐motion stations within 85 km of the epicenter. High amplification at 4–6 Hz is observed on both thick sediment sites and on the northern edge of the Fraser delta, as observed in previous earthquakes. Gross generalizations of observed variable earthquake shaking are captured by current regional seismic microzonation maps; however, local discrepancies are present. This earthquake generated the first borehole array recordings obtained at depth in British Columbia. Recorded motions increase toward the surface and are of similar or higher amplitude than at nearby surface stations. Amplification between top and bottom sensors is a consistent factor of 7–8 in all three arrays over a similar 40–45‐m depth interval of Fraser delta sediments. Cross‐correlation analysis determines shear‐wave velocity estimates less than 350  m/s consistent with these delta sediments.

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