NEOTECTONIC INVESTIGATIONS ON WESTERN VANCOUVER ISLAND, BRITISH COLUMBIA (92F/4) By P.T. Bobrowsky and J.J. Clague, Geological Survey of Canada

Interest in the contemporary geodynamic setting of southwestern British Columbia stems from a concern that a large earthquake may impact the major urban centres of the province. Indeed, monitoring of seismic activity in western Canada indicates that numerous smaller magnitude (MS) earthquake could also occur in the eastern North Pacific Ocean near populated areas of British Columbia (Rogers, 1988). The dire consequences of a great earthquake, as perceived by the public through popular articles ($ Koppel, 1989). are warranted insofar as previous MRf events elsewhere (r.g. M9.5 in Chile, 1960, and MX.6 in Alaska, 1964) have caused considerable propeny damage and loss of human life. Our understanding of these rare but devastating events has improved considershly during the last few decades. For example, they are now known to occur along zones of interaction between converging lithospheric plates. Commonly referred to as “megathrust” earthquakes, they result from the sudden release of large amounts of strain stored between two locked lithospheric plates (Rogers, IYXX). In the case of western Canada, the Cascadia subduction zone west of Vancouver Island maks the interface between the eastward subducting Juan de Fuca plate and the over-riding North American plate. The Cascadia subduction zone shares many of the same attributes as other subduction zones bordering the Pacific Ocean that have experienced megathrust earthquakes in recorded history (Rogers, 19xX). Responding to concern that a megathrust event could effect southwestern British Columbia, the Geological Survey of Canada and the provincial Geological Survey Branch have undertaken integrated geological investigations as part of a neotectonics program. The primary purpose of this research is to locate and interpret the geologic evidence for past earthquakes, and if possible determine the frequency and magnitude of these events. Such information could prove invaluable in formulating and implementing proactive policy and actions for safer building codes, emergency response procedures, as well as general land-use planning. SETTING