At the low frequency end, it is well-captured by seismograms, with far better sensitivity than one's ear, in fact, well up into the audible range for the case of the Parkfield instrumentation, which I think records at 250 sps. We bury the seismometers >100 m down because that avoids the high level of background noise present at the surface.

Are you saying that at your distance from Parkfield, you can hear sound waves that seismometers buried right next to the fault and the people around Parkfield cannot hear? What frequency band do you have in mind?