Hey Davin,

Unfortuately, I never took that class. Sounds like you know more about this topic than I. That's a specialty even among mine engineers, who tend to make fun of mine geologists (which I'm not, I explore the surface, usually) who typically don't get to play with that stuff, just the chips from the holes and the fresh surface exposed.

But I've seen it used.

The great thing (from a mining point of view) about the type of explosive we are talking about here is that it is cheap and effective. In an open pit mine, 12" holes are drilled at regular spacing along a ledge to a depth of 30 or so feet. There might be a couple of dozen holes drilled for a single blast. (that's where the chips that a mine geologist's samples come from) Those holes are filled with the mixture along with a triggering explosive. Then, usually at the same time everday, a siren is sounded, all activity stops, and they let 'er rip. Truely amazing. Scary. After witnessing such a sight, I have no doubt that a rental truck full of this stuff could have done the horrific damage on that sad day in April.

I've been in tunnels during blasts, that too is scary, but is usually a different, more discriminating explosive. You don't want all that fuel oil in a tunnel. The sound echoing down the tunnel, the darkness (all power is turned off for a blast), the pressure drop as the shock wave comes rolling down the tunnel...I'll never forget it.

Now the part I know a little more about is this supposed double explosive signature on the seismic log. Seismic events create several different type of waves (shear, pressure, longitudinal) and these waves travel at different speeds through the earth. Knowing those speeds it is easy math to determine the distance from the epicenter of the event to the recording seismic station. If you've got three stations you can determine the exact location of the event. Of course you are right, Davin, that the shock wave would have bounced off discontinous layers ( like the mantle) and returned to the surface, but it would have been much later than the separation of the S and P waves that are often cited as a double explosion. (Note, I've not seen the seismic record of this event, and I am quite sure that Nick would be the one to go to for evaluation of that record.)

As an aside, it is interesting to note that a big quake will keep reverberating within the earth for some time, essentially ringing it like a bell.

Sorry I couldn't be of more technical help to the discussion.

jms