A local magnitude () relation for the western Canada sedimentary basin (WCSB) is developed using a rich ground‐motion dataset compiled from local and regional networks in the area. The assessment of amplitude decay with distance suggests that joining direct waves with postcritical reflections from Moho discontinuity modifies the attenuation pattern in the 100–200 km distance range. The distance correction is parameterized using a trilinear function that accounts for the observed attenuation attributes. Regression of ground‐motion amplitudes results in the following distance correction model () for earthquakes in WCSB:
in which is the hypocentral distance (km). The standard relations fail to capture the rates and shape of amplitude attenuation in the region, resulting in overestimated magnitudes by 0.3–0.6 units. The overestimation is larger for local networks due to the increased discrepancy between standard relations and actual attenuation properties at close distances. The derived relationship results in unbiased magnitude estimates in WCSB over a wide distance range (2–600 km), which ensures consistent magnitude estimates from local and regional networks.