What's Happening to mb
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.