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TitleCrustal stress indicators for southwest British Columbia: What controls faulting in the crust?
AuthorBalfour, N J; Cassidy, J; Dosso, S
SourceSeismological Society of America Annual Meeting, Abstracts; Seismological Research Letters vol. 81, no. 2, 2010 p. 333
LinksOnline - En ligne
Alt SeriesEarth Sciences Sector, Contribution Series 20100026
MeetingSeismological Society of America Annual Meeting; Portland; US; April 21-23, 2010
Mediapaper; on-line; digital
ProvinceBritish Columbia
Subjectsstructural geology; structural features; faults; structural interpretations; structural analysis; Juan de Fuca plate
ProgramTargeted Hazard Assessments in Western Canada, Public Safety Geoscience
AbstractWe present new stress and anisotropy results, which are combined with seismicity, local tectonics, and GPS data (velocity and strain) to determine regional stresses that control faulting in the North American crust above the Cascadia subduction zone in SW British Columbia (BC). The region experiences crustal earthquakes with recorded magnitudes up to 7.3. These events have a shorter reoccurrence interval than megathrust earthquakes and pose a significant hazard to major population centers. The North American plate in this region includes a change in margin orientation from N-S in Washington State to NW-SE in BC and involves a complex region of deformation above a bend in the subducting Juan de Fuca plate. Over 1000 focal mechanisms have been calculated to identify the dominant style of faulting and inverted to estimate the 3 principal stress orientations and stress ratio. These results are then compared with results from recent anisotropy work where over 4000 station-event pairs have been analysed for shear-wave splitting. By comparing stress orientations with fast directions we investigate whether the source of anisotropy is stress or structure related and identify any variations in the stress field. Preliminary results indicate a change in fast direction with distance from the trench from margin-parallel on Vancouver Island to margin-normal on Mainland BC. This change is also reflected in GPS velocity vectors and may suggest there is some variation in the stress with distance from the margin.