Coda Q in southwestern British Columbia, Canada

Coda decay rates of 122 locally recorded earthquakes are used to infer frequency-dependent coda Q values at 21 short-period stations in southwestern British Columbia, Canada. Using the single-scattering model, coda Q values (Q_c) are derived for a range of sampling volumes corresponding to maximum lapse times of 36-59 sec and maximum sampling depths of 70-110 km. Maps of Q₀ (Q_c inferred at 1 Hz) for different sampling volumes show a consistent trend of increasing Q₀ to the northeast away from the subduction zone and into the Coast Belt batholith, and decreasing Q₀ toward the southeastern-most stations near the Vedder fault at the southern terminus of the Coast Belt. Q₀ increases gradually with sampling volume at all stations. The dependence of Q_c on frequencies between 2 and 16 Hz for different stations varies but on average ranges from 280 at 2 Hz to 850 at 16 Hz for sampling volumes that extend to depths of ∼90 km. An average of all data for the same sampling volume size gives a relationship of Q_c = 110 f^0.72 for frequencies in the 2- to 16-Hz range, which suggests significant differences in lithospheric coda attenuation properties in comparison with northwestern Washington State where Q_c = 63 f^0.97. Changes in coda Q correlate with the volume of brittle and fractured continental and subducting oceanic lithosphere sampled.