USGS Earthquake Notification Service changed their standards for "notifying" of earthquakes, Aftershock Exclusion
Just received this email from the USGS:
After the magnitude 9.0 Tohoku earthquake on March 11, 2011, a change was made in ENS to enable the 'aftershock exclusion' feature by default. This filters out notifications for all but the largest aftershocks, eliminating the large deluge of aftershock notifications after a large mainshock.
If you want to check your settings or turn 'aftershock exclusion' off, (thereby potentially receiving a large number of aftershock notifications) you can log in to your ENS account and do this from the 'Account Preferences' tab on the main ENS webpage.