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TitleCoda Q in the northern Cascadia subduction zone
AuthorFarahbod, A M; Calvert, A J; Cassidy, J FORCID logo; Brillon, C
SourceBulletin of the Seismological Society of America vol. 106, no. 5, 2016 p. 1939-1947,
Alt SeriesEarth Sciences Sector, Contribution Series 20160082
PublisherSeismological Society of America (SSA)
Mediapaper; on-line; digital
File formatpdf
ProvinceBritish Columbia; Western offshore region
NTS92B; 92C; 92E; 92F; 92G; 92J; 92K; 92L; 102I
AreaCascadia Subduction Zone
Lat/Long WENS-130.0000 -121.5000 51.0000 47.0000
Subjectstectonics; geophysics; seismographs; array seismology; earthquakes; earthquake magnitudes
Illustrationslocation maps; graphs; plots; tables
ProgramPublic Safety Geoscience Western Canada Geohazards Project
Released2016 08 23
AbstractUsing seismograms recorded at 66 Canadian seismic stations, coda Q was estimated from earthquakes in southwestern British Columbia and northern Washington State, employing the single backscattering approximation. A total of 580 earthquakes with magnitudes ranging from 1.2 to 6.4, depths from 0 to 67 km, and epicentral distances of 5 - 110 km were selected to obtain 3022 high signal-to-noise
ratio traces for analysis. An average of all the data yields a relationship for coda Q of QC \'01 72f 0:91. There is little variation of this coda Q relationship when using either crustal or in-slab sources, which represent uniform sampling of the crust and upper mantle. Crustal earthquakes result in a relationship of QC \'01 73f 0:89, and for in-slab events QC can be expressed as QC \'01 69f 0:94. In general, Q0 (QC at 1 Hz) increases from the west coast of Vancouver Island to the east-southeast within the Coast belt. Stations on west-central Vancouver Island closest to the landward projection of the Nootka fault zone, and the location of the only two known large crustal earthquakes
(1918 M 7 and 1946 M 7:3) on Vancouver Island, have the lowest Q0 values in our study area, suggesting a contrast in Q between the north and south of the island.
Summary(Plain Language Summary, not published)
This study examines seismic attenuation (how ground shaking varies with distance from an earthquake) across the Cascadia subduction zone of southwest British Columbia. Seismic attenuation is a critical factor in the development of earthquake hazard maps. We find the highest attenuation along the seismically active west coast of Vancouver Island (and in the vicinity of the largest historical (M 7-7.3) earthquakes) and the lowest attenuation in the more seismically quiescent Coast Belt north of Vancouver. Our research helps to improve our understanding of the Cascadia subduction Zone and provides new details on the seismic attenuation - both of which will contribute to improved earthquake hazard models for this region.

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