Though it wasn't quite friction they were picking up. It is 10 microns across. (About the same as one red blood cell.) Moving at 0.1% of the speed of light with a ton of mass behind it. It doesn't pause to interact long enough for friction (ie the force caused by stuff trying to stick to it) to do much. It just shoves aside whatever it runs into. We are detecting the shoved aside stuff.

But still, when something that small is picked up on seismic stations through the thickness of the Earth, there is a lot of energy being released.

Cheers,
Ben