Microseism
In
Detection and characteristics
As noted early in the history of seismology,
Dominant microseism signals from the oceans are linked to characteristic ocean swell periods, and thus occur between approximately 4 to 30 seconds.
As a result, from the short period 'secondary microseisms' to the long period 'hum', this seismic noise contains information on the sea states. It can be used to estimate ocean wave properties and their variation, on time scales of individual events (a few hours to a few days) to their seasonal or multi-decadal evolution. Using these signals, however, requires a basic understanding of the microseisms generation processes.
Generation of primary microseisms
The details of the primary mechanism was first given by Klaus Hasselmann,[5] with a simple expression of the microseism source in the particular case of a constant sloping bottom. It turns out that this constant slope needs to be fairly large (around 5 percent or more) to explain the observed microseism amplitudes, and this is not realistic. Instead, small-scale bottom topographic features do not need to be so steep, and the generation of primary microseisms is more likely a particular case of a wave-wave interaction process in which one wave is fixed, the bottom. To visualize what happens, it is easier to study the propagation of waves over a sinusoidal bottom topography. This easily generalizes to bottom topography with oscillations around a mean depth.[14]
For realistic seafloor topography, that has a broad spatial spectrum, seismic waves are generated with all wavelengths and in all directions. Because the dynamic pressures of ocean waves fall off exponentially with depth, the primary microseism source mechanism is restricted to shallower regions of the world ocean (e.g., less than several hundred meters for 14 - 20 s wave energy).
Generation of secondary microseisms
The interaction of two trains of
In the case of opposite propagation direction the groups travel at a much larger speed, which is now 2π(f1 + f2)/(k1 − k2) with k1 and k2 the wave numbers of the interacting water waves.
For wave trains with a very small difference in frequency (and thus wavenumbers), this pattern of wave groups may have the same velocity as seismic waves, between 1500 and 3000 m/s, and will excite acoustic-seismic modes that radiate away.
As far as seismic and acoustic waves are concerned, the motion of ocean waves in deep water is, to the
Real ocean waves are composed of an infinite number of wave trains and there is always some energy propagating in the opposite direction. Also, because the seismic waves are much faster than the water waves, the source of seismic noise is isotropic: the same amount of energy is radiated in all directions. In practice, the source of seismic energy is strongest when there are a significant amount of wave energy traveling in opposite directions. This occurs when swell from one storm meets waves with the same period from another storm,[6] or close to the coast due coastal reflection.
Depending on the geological context, the noise recorded by a seismic station on land can be representative of the sea state close to the station (within a few hundred kilometers, for example in Central California), or a full ocean basin (for example in Hawaii).[7] In order to understand the noise properties, it is thus necessary to understand the propagation of the seismic waves.
Seasonal and secular microseism variations
Seasonality variation in microseisms offers valuable insights into the dynamics of the Earth's surface and subsurface processes. Globally observable microseisms are generated by ocean waves. Seasonal changes in oceanic and atmospheric conditions, such as wave height, storm activity, and wind patterns, contribute to the observed variations in microseism intensity and frequency content. For instance, during the northern and southern hemisphere winters, storm activity and wave energy are on average higher in the corresponding winter hemispheres and microseism signals become more pronounced. In contrast, during hemispherical summers, when oceanic and atmospheric conditions are relatively calmer, the microseism signal exhibits its lowest annual intensity. By studying the seasonality variation of microseisms, researchers can gain a better understanding of the underlying physical processes and their influence on the Earth's dynamic systems.[16] Because they are driven by ocean wave energy, microseism signals around the Earth also show large spatial scale variations that reflect average wave energy over large expanses of the global oceans.
Decadal scale studies have shown that microseism energy is growing as global storms, and their associated waves, increase in intensity
Body wave microseisms
Body wave microseisms are a type of seismic wave that propagates through the Earth's interior, distinct from surface waves. These microseisms are generated by various sources, including atmospheric pressure fluctuations, oceanic interactions, and anthropogenic activities. Unlike surface waves, which predominantly travel along the Earth's surface, body wave microseisms propagate through the deeper layers of the Earth. Seasonal variations in body-wave noise has been reported, consistent with differences in storm activity between the northern and southern hemisphere.[21]
See also
References
- ^ The American Heritage Dictionary of the English Language (Fourth ed.), Houghton Mifflin Company, 2000
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- ^ Ardhuin, Fabrice, Lucia Gualtieri, and Eleonore Stutzmann. "How ocean waves rock the Earth: two mechanisms explain seismic noise with periods 3 to 300 s." Geophys. Res. Lett. 42 (2015).
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- ^ Ruff, L.J. "Hurricane Season & Microseisms". MichSeis. Archived from the original on 2008-05-29. Retrieved 2008-08-26.
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- ^ Ardhuin, Fabrice. "Large scale forces under surface gravity waves at a wavy bottom: a mechanism for the generation of primary microseisms." Geophys. Res. Lett. 45 (2018), doi: 10.1029/2018GL078855.
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