4 ms·

Never heard of this, so I had to do some digging. My money is on this being an ocean wave phenomenon. The Gulf of Guinea, specifically the Bight of Bonny, is u

Never heard of this, so I had to do some digging. My money is on this being an ocean wave phenomenon. The Gulf of Guinea, specifically the Bight of Bonny, is up to ~20,000ft deep. Average ocean waves have a period less than 15s and the average ocean depth of the Atlantic is ~11,000ft. Oddly similar ratios! So it's plausible this 26s wave period is caused by the increase in depth [0]. The abnormally large seismic amplitude can be similarly attributed, as pressure patterns conditionally form along the bottom of the ocean (in either direction) due to combined features of both the wave and the ocean bottom topography. [1] [0] https://en.wikipedia.org/wiki/Wind_wave#Physics_of_waves https://en.wikipedia.org/wiki/Wind_wave#Physics_of_waves [1] https://en.wikipedia.org/wiki/Microseism#Generation_of_secondary_microseisms https://en.wikipedia.org/wiki/Microseism#Generation_of_secon...

mucsun 6y ago
Never heard of this either, but I don't agree with your theory, as waves usually form only superficial on the ocean.
- hugey010 6y ago
  This is an unusual measurement which is unexplained by current models. This chart [0] helps show why and when waves are not superficial to only surface of the ocean. [0] https://en.wikipedia.org/wiki/Microseism#/media/File:Interference_of_water_waves_with_bottom_topography,_relevant_for_microseism_generation.gif https://en.wikipedia.org/wiki/Microseism#/media/File:Interfe...
pa7x1 6y ago
Wave-depth and period are related, waves of 15s or even 26 are much too shallow to be affected by the seabed at those depths. EDIT: Some back of the envelope physics to see how plausible your hypothesis is: For deep water waves: wave_speed = gravity * wave_period / 2pi wave_speed (26 sec period) = 40.56 m/s wave_speed = wavelength / period => wavelength = 1054 m = 3458 ft. The effect of the seabed (i.e. shallow water wave physics) is usually taken into account at depths of the order 1/20 the wavelength. That is, when the depth is smaller than 1/20 the wavelength we have to start taking into account the effect of the seabed which makes the dispersion relation of waves non-linear and brings entirely new physics. Of course that 1/20 is not a magic number just a rough figure when the effects of the seabed start to be noticeable. Unfortunately, we are still off by several orders of magnitude so I would think the effect you describe is unlikely.
- hugey010 6y ago
  Thanks, you're probably right that it's not only depth being a factor in this. I think we both over-simplified as these are logarithmic ratios, not linear. On to speculation: the interaction between the waves and the ocean floor may be hitting one, of many, of these magic numbers in this scenario. It's possible two periods of waves interact to cause this 26s interval, since waves are always grouped. Also, consider how the waves may have combinatorial feedback loops due to stokes drift (doppler effect) in deeper water. It's frustrating the latest research on microeism seems to be from the 50's. There's very few resources describing what happens to waves as they move from shallow to deeper water.
  - pa7x1 6y ago
    To be fair, that kind of back of the envelope analysis shows that with our current knowledge we cannot assign this phenomenon to your hypothesis. But maybe that's precisely why the phenomenon remains unexplained because we have to think a bit outside the box. Fluid dynamics is complicated (Navier-Stokes still has a 1 million prize attached to it), surface wave propagation is complicated but when you include the effects of the seabed it's even more complicated. A lot of non-linear behavior pops out which always brings very nasty math. I'm more on the side that this is not the explanation but I wouldn't bet too much on it.
jacquesm 6y ago
The same effects that cause a tone when blowing across a bottle rim might cause a vibration like this when an ocean current moves over a deep trench.
- iaw 6y ago
  It's called a Helmholtz Resonator fyi
  - jacquesm 6y ago
    Yes, but I think more people know what a bottle is than what a Helmholtz resonator is.
  - birdyrooster 6y ago
    TIL

mucsun 6y ago

Never heard of this either, but I don't agree with your theory, as waves usually form only superficial on the ocean.

hugey010 6y ago
This is an unusual measurement which is unexplained by current models. This chart [0] helps show why and when waves are not superficial to only surface of the ocean. [0] https://en.wikipedia.org/wiki/Microseism#/media/File:Interference_of_water_waves_with_bottom_topography,_relevant_for_microseism_generation.gif https://en.wikipedia.org/wiki/Microseism#/media/File:Interfe...

pa7x1 6y ago

Wave-depth and period are related, waves of 15s or even 26 are much too shallow to be affected by the seabed at those depths. EDIT: Some back of the envelope physics to see how plausible your hypothesis is: For deep water waves: wave_speed = gravity * wave_period / 2pi wave_speed (26 sec period) = 40.56 m/s wave_speed = wavelength / period => wavelength = 1054 m = 3458 ft. The effect of the seabed (i.e. shallow water wave physics) is usually taken into account at depths of the order 1/20 the wavelength. That is, when the depth is smaller than 1/20 the wavelength we have to start taking into account the effect of the seabed which makes the dispersion relation of waves non-linear and brings entirely new physics. Of course that 1/20 is not a magic number just a rough figure when the effects of the seabed start to be noticeable. Unfortunately, we are still off by several orders of magnitude so I would think the effect you describe is unlikely.

hugey010 6y ago
Thanks, you're probably right that it's not only depth being a factor in this. I think we both over-simplified as these are logarithmic ratios, not linear. On to speculation: the interaction between the waves and the ocean floor may be hitting one, of many, of these magic numbers in this scenario. It's possible two periods of waves interact to cause this 26s interval, since waves are always grouped. Also, consider how the waves may have combinatorial feedback loops due to stokes drift (doppler effect) in deeper water. It's frustrating the latest research on microeism seems to be from the 50's. There's very few resources describing what happens to waves as they move from shallow to deeper water.
- pa7x1 6y ago
  To be fair, that kind of back of the envelope analysis shows that with our current knowledge we cannot assign this phenomenon to your hypothesis. But maybe that's precisely why the phenomenon remains unexplained because we have to think a bit outside the box. Fluid dynamics is complicated (Navier-Stokes still has a 1 million prize attached to it), surface wave propagation is complicated but when you include the effects of the seabed it's even more complicated. A lot of non-linear behavior pops out which always brings very nasty math. I'm more on the side that this is not the explanation but I wouldn't bet too much on it.

jacquesm 6y ago

The same effects that cause a tone when blowing across a bottle rim might cause a vibration like this when an ocean current moves over a deep trench.

iaw 6y ago
It's called a Helmholtz Resonator fyi
- jacquesm 6y ago
  Yes, but I think more people know what a bottle is than what a Helmholtz resonator is.
- birdyrooster 6y ago
  TIL