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TitlePaleoseismic investigations on the Leech River Fault Zone, southern Vancouver Island
AuthorBednarski, J; Rogers, G
SourceCANQUA - CGRG Biennial Meeting, programme and abstracts volume; 2009 p. 37 Open Access logo Open Access
LinksOnline - En ligne
Alt SeriesEarth Sciences Sector, Contribution Series 20090161
MeetingCANQUA-CGRG Biennial Meeting; Burnaby; CA; May 3-8, 2009
Mediapaper; on-line; digital
ProvinceBritish Columbia
NTS92B/05; 92B/06; 92B/11; 92B/12; 92C/09; 92C/10
AreaVancouver Island; Victoria
Lat/Long WENS-125.0000 -123.0000 48.7500 48.2500
Subjectstectonics; structural geology; geophysics; structural features; structural interpretations; faults; Oligocene; Miocene; Eocene; earthquakes; earthquake risk; earthquake studies; Leech River Fault Zone; Devil's Mountain Fault; Cenozoic; Tertiary
ProgramReducing Risk from Natural Hazards
Released2009 01 01
AbstractVictoria, southern Vancouver Island, is the largest Canadian urban centre situated next to major geologic fault, the Leech River fault zone. Although the fault appears to have been inactive in recent history, the impact of a slip could be devastating. This is a report on recent investigations to assess the prehistoric activity of the Leech River fault zone. The Leech River fault zone records Late Eocene accretion of Leech River schist (Pacific Rim Terrane) and Eocene basalt (Crescent Terrane) against the continental edge of western North America. The fault forms a north-dipping thrust that places the Metchosin volcanic rocks under the Leech River schist. Movement along the fault occured during the Eocene, but some disruption of sediments of the Late Oligocene Sooke Formation sediments suggest that significant movement may have also taken place as late as the Miocene. On the west coast of Vancouver Island the Leech River fault zone is expressed as a steep narrow valley cutting through the Island Ranges. Trending southeast, it passes through the rapidly expanding communities of Colwood and Langford and then extends along the Victoria waterfront connecting with the Devil's Mountain Fault in the Strait of Juan de Fuca. Two damaging-level paleo-earthquakes have been identified on a splay of the Devil's Mountain Fault about 40 km east of Victoria, which occurred between 100 to 500 and 1100 to 2200 calendar years ago. Moreover, recent marine multibeam surveys have shown displaced Holocene sediments over the Devil's Mountain Fault just off the Victoria waterfront and in a splay fault through the southern Gulf Islands. In the immediate Victoria area, the Leech River Fault zone is buried beneath thick glaciofluvial sediments. These deposits are mostly cross-stratified gravels and sands deposited during deglaciation, about 13.6 ka BP, when the retreating glacier still occupied the land to the north. An ice-contact delta formed as meltwater flowed into a high relative sea level of at least 75 m. Successively lower meltwater channels were cut into the delta as relative sea level fell and the ice margins retreated northward. This resulted in a series of terraces and scarps on the delta surface. However some scarps could also have been produced by surface ruptures if the Leech River fault slipped during postglacial time. In 2007, a Lidar survey was flown along the length of the fault zone to detect surface lineaments which could be possible surface ruptures. Several identified lineaments were further investigated by ground resistivity and ground penetrating radar surveys, but the results are inconclusive. Human disturbance of the original landscape and heavy urban development in the area hinders more intensive investigations. Although no evidence of prehistoric earthquakes has yet been found and the current models of seismic hazard for the Victoria area do not need revision, more needs to be done to determine if the Leech River fault zone has been active in geologically recent times.

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