Microseism

The term is most commonly used to refer to the dominant background seismic and electromagnetic noise signals on Earth, which are caused by water waves in the oceans and lakes.

As noted early in the history of seismology,[10] microseisms are very well detected and measured by means of a long-period seismograph, This signal can be recorded anywhere on Earth.

The weaker is for the larger periods, typically close to 16 s, and can be explained by the effect of surface gravity waves in shallow water.

[13] The dominant sources of this vertical hum component are likely located along the shelf break, the transition region between continental shelves and the abyssal plains.

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.

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.

The generation of secondary-microseism Love waves involves mode conversion by non-planar bathymetry and, internally, through seismic wavespeed heterogeneity within the Earth.

[15] Seasonality variation in microseisms offers valuable insights into the dynamics of the Earth's surface and subsurface processes.

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.

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.

Seasonal variations in body-wave noise has been reported, consistent with differences in storm activity between the northern and southern hemisphere.

[21] As evidenced by the seismic recordings, body wave microseisms including P-, SV-, and SH-wave types, can be evident at a broad range of periods.

Power spectral density probability density function (color scale at right) for 20 years of continuous vertical component seismic velocity data recorded at Albuquerque, New Mexico by the ANMO station of the IRIS Consortium / USGS Global Seismographic Network. The high and low bounds are representative noise limits for seismographs deployed worldwide. The solid and dashed lines indicate the median and mode of the probability density function, respectively.
Interference of ocean waves with a fixed bottom topography. Here waves with period 12 s interact with bottom undulations of 205 m wavelength and 20 m amplitude in a mean water depth of 100 m. These conditions give rise to a pressure pattern on the bottom that travels much faster than the ocean waves, and in the direction of the waves if their wavelength L 1 is shorter than the bottom wavelength L 2 , or in the opposite direction if their wavelength is longer, which is the case here. The motion is exactly periodic in time, with the period of the ocean waves. The large wavelength in the bottom pressure is 1/(1/ L 1 − 1/ L 2 ).
Wave groups generated by waves with same directions. The blue curve is the sum of the red and black. In the animation, watch the crests with the red and black dots. These crests move with the phase speed of linear water waves , and the groups of large waves propagate slower ( Animation )
Wave groups generated by waves with opposing directions. The blue curve is the sum of the red and black. In the animation, watch the crests with the red and black dots. These crests move with the phase speed of linear water waves , but the groups propagate much faster ( Animation )