Seismic-hazard assessment relies on accurate knowledge of path effects, where path effects describe the attenuation of earthquake ground-motion amplitude with distance, including geometric spreading and anelastic attenuation effects. At distances less than 100 km from the source, the attenuation of amplitudes is dominated by geometric spreading, and can be modeled as R−b. A modified version of the reverse two-station method (rtsm) of Chun et al. (1987) was used to determine the attenuation coefficient b for selected paths in the Charlevoix Seismic Zone (csz).
The modified rtsm gives values of the attenuation coefficient for the crustal paths between a pair of stations. Average values over many paths give an approximation of the regional attenuation. The rate of attenuation is high within the csz, the most seismically active region with eastern Canada. The average b value within the csz is 1.36 (standard deviation, ±0.65; standard error, ±0.01). This is much higher than the value of 1.0 that applies to direct-wave spreading in a homogeneous elastic whole space.