As the F15 would have been flying on a north south bearing and as supersonic booms always trail the aircraft I think we can assume that the F15s flew past down town Seattle supersonically and most likely at altitudes > 20,000 ft. (The aircraft weren't really observed making the pass overhead)

Although F15s are very capable I have not really heard of a F15 exceeding Mach 1.5 and very rarely exceeding Mach 1.3 operationally. With external tanks and weapons supersonic flight would most likely have occurred at altitude i.e greater than 20,000 feet. The time differences going supersonic over a range of 150-200 miles at altitude to a low level subsonic flight plan would have been pretty negligible, after taking into account climb to height times and greater acceleration and deceleration times to eventually match the very slow level level target, i.e drop back down to a couple of thousand feet and 120 mph, which the F15s might have had to escort (the F15s would have had great difficulty maintaining the same speed as the float plane).



Again this make the supersonic flight over Seattle even more of a mystery. Even the F15s airborne RADAR probably wouldn't have been picked up the float plane until the F15 was within about 35-40 miles radius, by which time the float plane had apparently already landed or would have left the exclusion zone to make his landing approach.

Anyway getting back to the point of the thread, I would have personally been a little more concerned than other folks have stated hearing the sonic boom knowing that the PONUS was in town. Supersonic overland jet fighters usually mean a serious immediate war threat i.e. something like a Tu140 barrelling along somewhere nearby.

http://news.bbc.co.uk/1/hi/scotland/highlands_and_islands/8585432.stm