CodaQin the Northern Cascadia Subduction Zone

Abstract

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 C =72 f 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 C =73 f 0.89, and for in‐slab events Q C can be expressed as Q C =69 f 0.94. In general, Q ( Q 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 ∼7 and 1946 M ∼7.3) on Vancouver Island, have the lowest Q 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 and alpha values with estimated uncertainties.