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Interesting that that low, deep tones were the first the author heard. This is due to sound waves traveling through the ground arriving first. The higher frequencies are attenuated much more. I've always noticed this occurring with on-ground explosions as well.

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Interesting. At what point does it stop being sounds waves traveling through the ground and start becoming straight-up seismic vibrations caused by the (effective) explosions happening on the pad? Probably a bit of a spectrum, as the ground is itself acting like a giant woofer of sorts. I wonder where such a launch registers on the Richter scale...

The sound waves in the ground actually are seismic waves, there's no real distinction. The difference is that air essentially just has longitudinal pressure waves (like P-waves), whereas the ground has transversely oriented shear (S) waves and a number of surface waves.

You can hear this on a lot of those EOD cache disposal videos on YouTube.

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