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TitleSeismic attenuation in the Anahim Volcanic Belt and adjacent regions of British Columbia
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AuthorFarahbod, A M; Cassidy, J F
SourceGeological Survey of Canada, Open File 8030, 2016, 50 pages, https://doi.org/10.4095/298894
Year2016
PublisherNatural Resources Canada
Documentopen file
Lang.English
Mediaon-line; digital
File formatpdf
ProvinceBritish Columbia
NTS82L; 82M; 92I; 92J; 92K; 92L; 92M; 92N; 92O; 92P; 93A; 93B; 93C; 93D; 93E; 93F; 93G; 93H; 93I; 93J; 93K; 93L
Lat/Long WENS-128.0000 -118.0000 55.0000 50.0000
Subjectstectonics; geophysics; earthquakes; earthquake studies; earthquake risk; earthquake mechanisms; earthquake magnitudes; p waves; s waves; seismic waves; crustal studies; crustal movements; seismicity; tectonic environments; tectonic setting; Anahim Volcanic Belt; Nechako Basin
Illustrationslocation maps; plots; tables
ProgramWestern Canada Geohazards Project, Public Safety Geoscience
Released2016 08 23
AbstractIn this study we investigated coda-wave attenuation (Qc) in the interior of British Columbia, with a focus on the Anahim Volcanic Belt (AVB) and specifically the Nazko Cone region using the single scattering approximation on records from short period and broadband stations of the regional Canadian National Seismic Network (CNSN) and POLARIS Network. Our dataset is comprised of 380 earthquakes recorded between 1999 and 2012 with magnitudes ranging from 1.6 to 3.9, depths from 0 to 36 km and epicentral distances of 15 to 100 km. This gives a total of 423 high signal to noise (S/N) traces (S/N?5.0) useful for Qc calculation (with a range of ellipse parameter, a2, of 30 to 110) across the region. Coda windows were selected to start at tc = 2tS (two times the travel time of the direct S wave), and were filtered at center frequencies of 2, 4, 8, 12 and 16 Hz. Although the dataset is relatively small, with substantial uncertainties, a consistent pattern emerges. We find that in the interior of BC, the lowest Q0 values (e.g., Q0 of 39) are in the vicinity of Nazko Cone, near the 2007 earthquake swarm. This sequence of more than 1000 earthquakes occurred at depths of 25-31 km and was interpreted as a magma injection at the base of the crust. Further, we find that all stations within the AVB show lower Q0 values compared to other stations in the BC interior. Averaging all data in the AVB with a2 of 30-50 km (UBRB, MCMB1, SULB, THMB) yields a low average Q0 of 53, compared to an average Q0 of 67 at other stations in the BC interior, and an average Q0 of 70 for stations in the Cascadia subduction zone. Our results showing low Q0 throughout the AVB (and the lowest Q0 at Nazko Cone) provides additional support for the interpretation of magma injection into the lower crust during the Nazko earthquake sequence, fracturing of the crust, and high seismic attenuation.
Summary(Plain Language Summary, not published)
This study examines seismic attenuation in the vicinity of the Anahim Volcanic Belt of British Columbia. Seismic attenuation is a critical factor in the development of earthquake hazard maps. The Anahim Volcanic Belt is a zone of relatively young volcanic eruptions across BC, and the site of an earthquake swarm in 2007. We find higher attenuation across the volcanic belt, suggesting heavily fractured rock that helps to absorb high-frequency seismic energy. Our research helps to constrain the boundaries of the Anahim Volcanic Belt, and provides new details on the seismic attenuation, both of which will contribute to improved earthquake and volcanic hazard models for this region.
GEOSCAN ID298894