This thesis examines processes causing faulting in the North American crust in the
northern Cascadia subduction zone. A combination of seismological methods, including source mechanism determination, stress inversion and earthquake relocations
are used to determine where earthquakes occur and what forces influence faulting.
We also determine if forces that control faulting can be monitored using seismic
anisotropy. Investigating the processes that contribute to faulting in the crust is
important because these earthquakes pose significant hazard to the large population
centres in British Columbia and Washington State.
To determine where crustal earthquakes occur we apply double-difference earthquake
relocation techniques to events in the Fraser River Valley, British Columbia, and the
San Juan Islands, Washington. This technique is used to identify "hidden" active
structures using both catalogue and waveform cross-correlation data. Results have
significantly reduced uncertainty over routine catalogue locations and show lineations
in areas of clustered seismicity. In the Fraser River Valley these lineations or streaks
appear to be hidden structures that do not disrupt near-surface sediments; however,
in the San Juan Islands the identified lineation can be related to recently mapped
surface expressions of faults.
To determine forces that influence faulting we investigate the orientation and sources
of stress using Bayesian inversion results from focal mechanism data. More than 600
focal mechanisms from crustal earthquakes are calculated to identify the dominant
style of faulting and inverted to estimate the principal stress orientations and the
stress ratio. Results indicate the maximum horizontal compressive stress (SHmax)
orientation changes with distance from the subduction interface, from margin-normal
along the coast to margin-parallel further inland. We relate the margin-normal stress
direction to subduction-related strain rates due to the locked interface between the
North America and Juan de Fuca plates just west of Vancouver Island. Further
from the margin the plates are coupled less strongly and the margin-parallel SHmax
relates to the northward push of the Oregon Block. Active faults around the region
are generally thrust faults that strike east-west and might accommodate the margin-
parallel compression.
Finally, we consider whether crustal anisotropy can be used as a stress monitoring
tool in this region. We identify sources and variations of crustal anisotropy using
shear-wave splitting analysis on local crustal earthquakes. Results show spatial variations in fast directions, with margin-parallel fast directions at most stations and
margin-perpendicular fast directions at stations in the northeast of the region. To
use seismic anisotropy as a stress indicator requires identifying which stations are primarily in
uenced by stress. We determine the source of anisotropy at each station by
comparing fast directions from shear-wave splitting results to the SHmax orientation.
Most stations show agreement between these directions suggesting that anisotropy is
stress-related. These stations are further analysed for temporal variations and show
variation that could be associated with earthquakes (ML 3{5) and episodic tremor
and slip events.
The combination of earthquake relocations, source mechanisms, stress and anisotropy
is unique and provides a better understanding of faulting and stress in the crust of
northern Cascadia. / Graduate
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/3636 |
| Date | 19 October 2011 |
| Creators | Balfour, Natalie Joy |
| Contributors | Cassidy, John Francis, Dosso, Stanley Edward |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Rights | Available to the World Wide Web |
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