On March 20, 2012 at 14:16:45 market activity exploded in 3 waves.
The NBBO spread for thousands of stocks, immediately disintegrated.
You can find charts of SPY during this event here.
The chart below shows the number of stocks
(green) with increasing quote spreads and the number of stocks (red) with decreasing quote spreads during each second between 14:15:20 and 14:16:05 Eastern.
