Years ago we used analog tuners to set middle C at 262 Hz. That was a professional standard throughout the music industry. In the posts below with John Vidale I mentioned the Guralp CMG-3T seismometers used in Mexico. They can pick a range of frequencies, but I believe they look at 1Hz or 2 Hz for deep earth movement. That would be a bass string made of basalt. Your ear is hit by it, but the human brain does not process it. Maybe. Can animals hear frequency that low? Maybe I shouldn't open that can of worms.

glen