An important component of seismic hazard analysis is the magnitude-recurrence relation, which provides the cumulative rate of occurrence of earthquakes within a seismic source zone as a function of magnitude. In the national seismic hazard maps developed for the 2005 National Building Code of Canada, the magnitude-recurrence relation for the Cascade Mountains zone of southwestern British Columbia (casr), the seismic zone in which Vancouver and Victoria are located, is based mainly on earthquakes measured on the local scale ML (Richter scale), but other scales including body-wave magnitude (mb), coda duration magnitude (Mc), and moment magnitude (M) have been used for some of the events. The mixture of magnitude types is a potential source of bias in the relations.
By using data from recently calculated regional moment tensor solutions, a local magnitude-moment magnitude relationship is developed for magnitudes of 2.9 < ML < 6.0 in the casr zone. This relation, along with relations from other sources for mb and Mc, and specific moment magnitude values where available, are used to develop a homogeneous moment magnitude catalog for use in the formulation of a new casr zone magnitude-recurrence relation. The use of a homogeneous moment magnitude scale produces a significant change in the shape of the magnitude-recurrence curve in the casr zone. Perhaps more importantly, the use of a homogeneous catalog reduces the uncertainty in the recurrence rates of moderate to large earthquakes in casr by nearly an order of magnitude. We conclude that the use of a homogeneous magnitude scale is critical to producing unbiased magnitude-recurrence relations.