HO HUM?

June 12th, 2011

Do places with “mystery hums” have greater seismic risk?

The click-hungry Daily Telegraph has a piece about yet another village afflicted by a “mystery hum” — a subject I’ve been meaning to write about for a while.

I’ve had several experiences with low-frequency ground noises, and I have never spent much time being mystified by them.  In my case the sources were obvious. For others, the sound sources may be harder to identify, but the fact that low-frequency sound energy can travel long distances through the ground, form standing waves, vibrate the china and so forth, is not mysterious at all.  I do wonder though, whether places that are more susceptible to “hum” energy are also more susceptible to the seismic energy generated by earthquakes.

The most recent of my own “hum” experiences was about a week ago.  Some idiot one street over left a pickup truck parked with a noisy beat blasting out from its superwoofers.  The ground, being denser than air, carries low-frequency sound energy a lot more efficiently.  I guess that everyone living within a quarter mile of that parked truck and its superwoofers could hear/feel its tom-tom throb, coming not so much through the air, but up from the floor.  At such a distance, it was reduced to a basic hmm-hmm-hmm, from which all the higher frequency components had been filtered out.

I have also noticed, in more than one place near the seashore, what might best be called a “marine diesel” hum.  I usually feel it in the early mornings, in bed, when all other sounds are minimized.  The culprit, whenever I’ve investigated, has been a ship that’s passing within a few miles of the shore.  The hum apparently comes from the ship’s large engine and propeller, and if the seawater thermoclines and shoreline geology are right, then I guess that this hum-like noise is transmitted efficiently from the sea to the seabed to the shore and up through the concrete foundations and floor slab of the house.  (Incidentally, navies around the world have long used these “VLF” frequencies in the electromagnetic spectrum for long-distance communication with submerged submarines.)

The areas such as Bristol (a major seaport) and Taos and Woodland, England, where the “big hums” have been reported, may simply have local geologies that very efficiently transmit engine noise from distant man-made sources.  Quirks of tectonic movement and water tables may allow – during a window of weeks to years – the development of resonance-like patterns that deliver unusually large amounts of sound energy to these hum-locations.  And I guess in the worst case scenario, the sound sources are natural oceanic/tidal or geologic sources, which means that you can’t shut them off.

In principle, one could set up mobile ground stations to measure such sounds systematically, and thereby triangulate on the source.  But ground-sound energy is pretty tricky, and I would imagine that plenty of reflection and diffraction goes on, especially by mountains and shorelines.

As for the earthquake question, geologists have long known that the local geology can make a big difference in earthquake intensity and damage.  One classic case was that of the Marina District in San Francisco, which sustained heavy damage in the 1989 Loma Prieta quake — damage that was blamed on the presence of sandy, easily “liquefacted” subsoil.  Perhaps (I speculate) there was also a deeper or surrounding geology that served to concentrate seismic energy in the liquefaction-prone zone.  Four years before that quake, there were reports of a hum in the same San Francisco neighborhoods that would later be heavily affected by the quake.  In the words of Science Frontiers Online, in late 1985: “[O]ver at Pacific Heights and the Marina District on San Francisco Bay a new hum is driving people crazy.”

Hmmm.

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