Baseline assessment of seismic hazard in British Columbia's north coast

Abstract

A review of existing geohazard studies pertaining to British Columbia's north coast (BCNC), an area with a number of proposed infrastructure projects, revealed that there are many unknowns and scientific gaps with regards to natural geologic hazards in the BCNC. Of particular interest is the potential of earthquakes and slope failures (i.e. landslides) and the relationship between the two. In addition to the well-known 1974 and 1975 Kitimat Arm landslides (Murty, 1979; Murty and Brown, 1979; Conway et al., 2012), recent bathymetric mapping of the Douglas Channel revealed two large submarine landslides and a previously unmapped geomorphic feature that is consistent with active faulting (Conway et al., 2012). Whilst not conclusive, the proximity of the feature to the landslides suggests that movement on the identified fault as a potential landslide trigger. Historically, the BCNC has been seismically quiescent. As a consequence, seismic monitoring and research related to the BCNC has been minimal. While larger earthquakes are felt and recorded, the configuration of the Canadian National Seismograph Network prior to 2014 did not allow earthquakes less than approximately ML 2.1 to be consistently located. Long-term, continuous monitoring of microseismicity, combined with geodetic and paleoseismic techniques could be used to assess the possibility of large earthquakes on the recently mapped Douglas Channel "fault". Moreover, these studies could identify other potentially unmapped faults in the BCNC region and provide an indication of their potential to host large earthquakes. Modelling of earthquakes similar to the 1973 ML4.9 Terrace earthquake, a hypothetical rupture on the Douglas Channel fault, and a MW 8.0 Haida Gwaii thrust earthquake reveal that shaking intensities from such earthquakes could be sufficient to induce slope failures in the BCNC. Regional GPS studies have shown that west and south of the BCNC there is significant crustal deformation (Hippchen, 2011; Mazzotti, et al., 2003a&b). Presently, the hypotheses is that some of the neighbouring deformation and hence strain is being transferred to the BCNC. Augmenting the permanent GPS network will help quantify how much strain is being transferred from surrounding regions to the BCNC, which will subsequently improve seismic hazard knowledge in the region. This report compiles the state of knowledge of geohazards in the BCNC region prior to 2014. Knowledge gaps are identified and recommendations to minimize those gaps are made. Recent efforts to fill the knowledge gaps are discussed.