Seismometers.
Good point. Although, on 9/11 after the second tower collapse, I checked public earthquake sites to see if it registered as a measuarble event but didn't see anything. I hadn't heard about the Murrah, are you referring to the bombing or the demolition?
Another thing to remember is that it wasn't necessarily an impact into the surface but an airburst from the description. An airburst would not couple as strongly to the surface resulting in possibly a seismically undetectable event.