The 1918 and 1957 Vancouver Island earthquakes

Abstract

Since 1918, six significant earthquakes (5.2 < M < 7.2) have occurred in the region of central Vancouver Island where the Juan de Fuca, Explorer, and America plates interact. In this study, two of the largest earthquakes are examined in detail: the 1918 ( MS ≃ 7) and the 1957 ( MS ≃ 6) events. The preferred location of the 1918 earthquake is on Vancouver Island at 49.44°N, 126.22°W, with a focal depth of 15 km. Magnitudes determined are MS = 6.9 ± 0.3 and mb = 7.2 ± 0.4. Analysis of surface waves suggests this is a predominantly strike-slip earthquake occurring along either a NNW- or an ENE-striking fault. The seismic moment is estimated as 7.40 × 1025 dyne-cm and the stress drop to be 122 bars. The 1957 earthquake has been relocated on the continental shelf west of Vancouver Island at 49.64°N, 127.00°W, with a focal depth of 30 km. Magnitudes determined are MS = 5.9 ± 0.2 and mb = 6.3 ± 0.3. Surface wave and P -nodal analyses indicate that this is a predominantly strike-slip earthquake; either dextral along an NNW-striking fault, or sinistral along a ENE-striking fault. The seismic moment is estimated as 8.14 × 1024 dyne-cm and the stress drop to be 36 bars. The 1918 earthquake appears to be a crustal intraplate event occurring in the lithosphere of the America plate, resulting from the complicated interaction of the Explorer, Juan de Fuca, and America plates. The preferred epicenter, depth, focal mechanism, and stress drop for the 1957 earthquake are consistent with the left-lateral motion between the Juan de Fuca and Explorer plates along the subducted extension of the Nootka fault zone. This earthquake is identical, within uncertainties, to events occurring in 1972 and 1986. We believe that these three earthquakes provide the best definition to date of both the position of the subducting portion of the Nootka fault zone west of Vancouver Island and the direction of relative motion along this fault.