In seismology, a microseism is defined as a faint earth tremor caused by natural phenomena. Sometimes referred to as a "hum", it should not be confused with the anomalous acoustic phenomenon of the same name. 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. Characteristics of microseism are discussed by Bhatt. Because the ocean wave oscillations are statistically homogeneous over several hours, the microseism signal is a long-continuing oscillation of the ground. The most energetic seismic waves that make up the microseismic field are Rayleigh waves, but Love waves can make up a significant fraction of the wave field, and body waves are also easily detected with arrays. Because the conversion from the ocean waves to the seismic waves is very weak, the amplitude of ground motions associated to microseisms does not generally exceed 10 micrometers.
Detection and characteristics As noted early in the history of seismology, microseisms are very well detected and measured by means of a long-period seismograph, This signal can be recorded anywhere on Earth.
Dominant microseism signals from the oceans are linked to characteristic ocean swell periods, and thus occur between approximately 4 to 30 seconds. Microseismic noise usually displays two predominant peaks. 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. These microseisms have the same period as the water waves that generate them, and are usually called 'primary microseisms'. The stronger peak, for shorter periods, is also due to surface gravity waves in water, but arises from the interaction of waves with nearly equal frequencies but nearly opposite directions (the clapotis). These tremors have a period which is half of the water wave period and are usually called 'secondary microseisms'. A slight, but detectable, incessant excitation of the Earth's free oscillations, or normal modes, with periods in the range 30 to 1000 s, and is often referred to as the "Earth hum". For periods up to 300 s, the vertical displacement corresponds to Rayleigh waves generated like the primary microseisms, with the difference that it involves the interaction of infragravity waves with the ocean bottom topography. 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. 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, 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.
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 surface waves of different frequencies and directions generates wave groups. For waves propagating almost in the same direction, this gives the usual sets of waves that travel at the group speed, which is slower than phase speed of water waves (see animation). For typical ocean waves with a period around 10 seconds, this group speed is close to 10 m/s. 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.
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