Wednesday, March 21, 2007

Earthly hum

Earth hums: why? The earth produces a sound which is detected by seismologists (earth-quake scholars), the source?

seismo hum story: click here

Which includes these bottom abcd pics, the max humming occurs seasonally at the bottlenecks (a=Chukchi/Bering sea flushes, b=Antarctic circumpolar current flush) yet the authors say nothing of that. Why?

My guess: the tidal grinder effect caused by the lunar-solar tidal differential and associated waves at the shallow sea coasts, (the chaordic (chaotic order) point of physical state equilibrium of solid, liquid and gas), where erosion & deposition continuously result in massive aggregate particulate displacement, which is the primary causative agent of tectonic plate migration (continental drift), splitting and then recombining supercontinents over geologic time.

Now, compare that explanation with the below abstract (correct interpretation IMO), and following summary paragraph, which (IMO incorrectly) states that movement of the plate derives from viscous traction at the base of the plate.

shelves hum story: click here

Letter Nature 445, 754-756 (15 February 2007) | doi:10.1038/nature05536; Received 2 November 2006; Accepted 12 December 2006

The Earth's 'hum' is driven by ocean waves over the continental shelves
Spahr C. Webb 1. Lamont Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA

Observations show that the seismic normal modes of the Earth at frequencies near 10 mHz are excited at a nearly constant level in the absence of large earthquakes1. This background level of excitation has been called the 'hum' of the Earth2, and is equivalent to the maximum excitation from a magnitude 5.75 earthquake3. Its origin is debated, with most studies attributing the forcing to atmospheric turbulence, analogous to the forcing of solar oscillations by solar turbulence2, 4, 5, 6, 7. Some reports also predicted that turbulence might excite the planetary modes of Mars to detectable levels4. Recent observations on Earth, however, suggest that the predominant excitation source lies under the oceans8, 9, 10. Here I show that turbulence is a very weak source, and instead it is interacting ocean waves over the shallow continental shelves that drive the hum of the Earth. Ocean waves couple into seismic waves through the quadratic nonlinearity of the surface boundary condition, which couples pairs of slowly propagating ocean waves of similar frequency to a high phase velocity component at approximately double the frequency. This is the process by which ocean waves generate the well known 'microseism peak' that dominates the seismic spectrum near 140 mHz (refs 11, 12), but at hum frequencies, the mechanism differs significantly in frequency and depth dependence. A calculation of the coupling between ocean waves and seismic modes reproduces the seismic spectrum observed. Measurements of the temporal correlation between ocean wave data and seismic data9, 10 have confirmed that ocean waves, rather than atmospheric turbulence, are driving the modes of the Earth.

plate traction summary: click here

Editor's Summary 22 March 2007
The power of plate tectonics

A compilation of seismic images, geochronology and plate-motion reconstruction has been used to compare the location and chronology of a hotspot track at the base and top of the North American lithosphere. The work reveals a conspicuous misalignment between the surface and deep parts of the tracks. Misalignment increases with age along the track, and is best explained by creep in the mantle lithosphere beneath North America. The sense of shear implied by these observations indicates that motion of the plate is driven by viscous traction at the base of the plate. This is of significance for understanding the driving forces of plate tectonics.
Letter: Seismic evidence for convection-driven motion of the North American plate

David W. Eaton and Andrew Frederiksen


DDeden said...

I forgot to mention, the Pacific is a big punchbowl-like basin, the sounds of the waves crashing along the coasts are reflected back towards the ocean center to form a central echo in the mid Pacific. The Atlantic has a mid-Atlantic mountain ridge, so the parabola effect is split into 2 basins with weaker concentration of sounds.

When the bottlenecked currents, ice sheets, tidal erosion and parabolic echo effects are combined with normal geotectonic activity along the Pacific rim, the sum shows up as displayed in the maps.
Also consider the accumulation of sonar and whale song carried long distances through seawater at various depths?

DDeden said...

I just learned that during summer, glaciers (in Greenland) move down slope and create seismic sound waves, so they add more to the Earthly hum.