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▲▲Figure 2. Earthquakes used in the m N – M W conversion. Gray symbols represent the pre-1995 events, black symbols the post-1995ones, and white symbols the Byam Martin Channel events not used in the analysis. The corresponding diagonal lines show the best fitstraight line in a least squares sense to each group. Most dots represent individual earthquakes but some represent two to six earthquakesthat lie on the same point.TABLE 2Comparison of M W – m N Conversion RelationsPre-1995Post-1995RelationMean Residual S. D. Mean Residual S.D.Constant (this study) –0.004 0.16 –0.005 0.19Line (this study) 0.007 0.16 –0.005 0.19Atkinson (1993) –0.061 0.16 0.046 0.19Boore and Atkinson (1987) 0.149 0.22 0.124 0.21Johnston et al. (1994) 0.405 0.30 0.301 0.26Hasegawa-a(1983) * –0.192 0.31 –0.284 0.27Hasegawa-b (1983) * –0.391 0.24 –0.140 0.27Nuttli (1983) 0.216 0.29 0.147 0.25Sonley and Atkinson (2005) –0.088 0.17 0.048 0.20* a refers to relation for m N < 4.2 and b for m N 4.2–6.6.tistically significant. This date corresponds to known changesin the CNSN and operating system. Further investigation isunderway to better understand the underlying reason for thechange in the magnitude relation.It should be cautioned that these relations were establishedusing earthquakes of m N 2.5 or greater and may not be appropriatefor smaller magnitudes. While it is also possible thatthey may not be appropriate for the largest earthquakes, thisfactor should not be a major problem as it should always be possibleto determine M W directly for these events. Comparisonswith instrumentally determined moment magnitudes showthat these conversions are very reliable on average when a largedata set is considered but will not always give the correct M Wfor any individual earthquake. While it would be expected thatthe conversions should hold true for eastern North America ingeneral, it must be emphasized that they are based on magni-Seismological Research Letters Volume 82, Number 6 November/December 2011 989