What's Happening to mb

Abstract
Introduction
Can we blame the prototype IDC?
Are we correcting a censoring effect?
Are we being fooled by aftershocks?

Conclusion

It is not all down to the pIDC

Annual changes in mb > 4.5 seismicity do not correlate with annual changes in pIDC reporting.

It's not all down to censoring

Changes in station magnitude skewness are too small to explain the large apparent shifts of mb.

It's not all down to aftershocks

There aren't enough aftershocks, and the apparent rate of mb < 4.5 independent events is decreasing.