Coda Q in the northern Cascadia subduction zone

Using 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 Q (sub C) =72f (super 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 Q (sub C) =73f (super 0.89) , and for in-slab events Q (sub C) can be expressed as Q (sub C) =69f (super 0.94) . In general, Q (sub 0) (Q (sub C) 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 approximately 7 and 1946 M approximately 7.3) on Vancouver Island, have the lowest Q (sub 0) values in our study area, suggesting a contrast in Q between the north and south of the island. Online Material: Figure showing principal tectonic units and station locations, and tables of average Q (sub 0) and alpha values with estimated uncertainties.