Microseism
![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.](/Images/godic/202501/27/ANMO_20_years2805.jpg")
![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 L1 is shorter than the bottom wavelength L2, 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/L1 - 1/L2).](/Images/godic/202501/27/Interference_of_water_waves_with_bottom_topography,_relevant_for_microseism_generation2805.gif")
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 homogenous 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.