On the videos I watched I heard the very loud sound that accompanied the shockwave (almost simultaneous with the glass shattering) and another much quieter one about 5-10 sec. later - perhaps the echo you mentioned? However, that specific video was taken beside the school and close to the zinc factory, so the second boom may have been the sound of the wall/roof caving in.
A different video looked to be taken a little outside the city, and the sound/shockwave resembled the sound of rolling thunder more than a singular explosion.
What I found the most interesting was how long after the explosion it took for the shockwave/soundwave to travel. Don’t get me wrong, I know light travels faster than sound. I’m familiar with counting time between lightning and thunder to get approximated distance from the strike, but there was just so much time between the explosion and the shock-/sound-wave that it was surprising to me.
- sorry for any typos, or confusion. I’m posting from our nintendo wii via our tv as my comp’s down with a busted video card.