Introduction
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For earthquakes all around the world, the 1996 mb / freqeuncy distribution is offset from earlier years, and the 1997 distribution is differs even more. The offset seems far too large to be attributed to natural variations in seismicity; it is implausible that the typical number of mb 5.0 earthquakes was nearly 400 through 1995, but that there were fewer than 200 in 1997 just by chance. The alternative is that the 1996 and 1997 curves are offset not downward, but leftward. Looking at the offset this way suggests that recent mb values are biased. That is, earthquakes are being assigned smaller magnitudes than in the past. |
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ISC Bulletin Sesimicity
* If b=1 and actual seismicity is stationary, then to change the apparent seismicity rate from N to N' would require redefining mb by log10(N'/N). |
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NEIS mb excludes pIDC amplitudes since 1996 August, but still shows a similar, although smaller, trend.
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