The Pacific Northwest Seismic Network began to notice geophysical anomalies in the south end of the city of Seattle on Sunday afternoons not long after the NFL Seahawks opened their new stadium in 2002. These were recorded by spectrograms and seismograms that rated frequency content and envelope function and the fascinating sort of things that seismologists like to discuss at cocktail parties, but the bottom line was that the ground was shaking.
It isn't enough to open the Cascadia fault, or threaten to dump the city into Puget Sound, but on many occasions the roar of nearly 70,000 fans and the attendant shaking of the stadium itself is more than enough to unnerve visiting football teams.