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UBC Theses and Dissertations

The 1918 and 1957 Vancouver Island earthquakes Cassidy, John Francis


The oceanic Juan de Fuca and Explorer plates are subducting beneath the continental America plate west of Vancouver Island. The Nootka fault zone, which separates these oceanic plates, experiences left-lateral shear due to the different rates of subduction for the Juan de Fuca (4 cm/yr) and the Explorer (<2 cm/yr) plates. Since 1918, six significant earthquakes (M= 5.3 - 7.2) have occurred in the region where the projection of this fault zone intersects central Vancouver Island. In this study two of the largest events are examined; the 1918 (Ms ≃ 7) and the 1957 (Ms ≃ 6) earthquakes. Prior to this research, no comprehensive studies of these events had been carried out. A total of 46 seismograms from 24 stations worldwide were obtained for the 1918 earthquake, and 138 seismograms from 46 stations were obtained for the 1957 earthquake. The preferred epicentre for the 1918 earthquake is 49.47°N, 126.24°W, with an estimated uncertainty of ±30 km. The preferred focal depth of 15 km indicates that this was a crustal earthquake. Magnitude estimates are Ms = 6.9 ± 0.3, mb, = 7.2 ± 0.4 and MI = 7.0, in agreement with previous studies. Surface wave analysis suggests this is a predominantly strike-slip earthquake occurring along either a NNW or an ENE striking fault. A seismic moment of 7.40x10²⁵ dyne-cm and a stress drop of 122 bars, indicative of an intraplate event, are estimated. The preferred epicentre for the 1957 earthquake is 49.65°N, 127.02°W with an uncertainty of ± 20 km. The estimated focal depth of 30 km suggests this event occurred in the subducting oceanic plate. Magnitude estimates are Ms = 5.9±0.2, rrif, = 6.3±0.3 and Mi = 5.7. Surface wave and P-nodal analyses indicate that this is a predominantly strike-slip earthquake; either dextral along a NNW striking fault, or sinistral along a ENE striking fault. The seismic moment is estimated to be 8.14x10²⁴ dyne-cm, and the stress drop to be 36 bars, which is indicative of an interplate event. The quality of these data does not allow for an unambiguous interpretation of these earthquakes in terms of seismotectonic models. However, the results of this study indicate that these earthquakes do not have normal or thrust mechanisms. The 1918 earthquake appears to be a crustal, intraplate event resulting indirectly from the complicated interaction of the Explorer, Juan de Fuca and America plates. The preferred epicentre, depth and stress drop for the 1957 earthquake are consistent with left-lateral motion between the Juan de Fuca and Explorer plates along the Nootka fault zone where it is being subducted beneath Vancouver Island. Uncertainties in the above parameters however, do not rule out the possibility of this being a crustal earthquake along a NW striking fault.

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