Toward an Improved Understanding of the MN-Mw Time Dependence in Eastern Canada

An earlier study by Bent (2011) established that the M N – M relation for eastern Canada changed by 0.12 magnitude units in approximately 1995. Although several possible factors were identified that might have contributed to the change, the exact cause was not determined. This study picks up where the earlier one left off and tests the effects of several changes made in the 1990s that might have affected magnitude calculations. Changes made to the instrumentation and sampling rate can explain approximately 60% of the observed change and are likely related to magnitudes being calculated at higher frequencies than they were in the past. Changes to the network distribution may be contributing an additional ∼10% of the difference, but the difference is not statistically significant. Increased precision of amplitude and period measurements can be ruled out as a factor. We have not been able to determine whether some of the unaccounted‐for difference is due to a change in the software package used for routine magnitude and epicentral determinations. The seismograph network in Canada has undergone several significant changes or upgrades during the past century, and the results of this study raise the question of whether there may have been other magnitude relations in the past. Preliminary results comparing and M N over seven decades show a significant change in the relation over time, with several step‐like increases that correspond well with network upgrades; this raises the question of whether magnitudes, in general, have remained constant with time.