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Sounds that shake the Earth: Do you know what causes seismic noise?
In geophysics, geology, and related fields such as civil engineering, seismic noise refers to relatively long-lasting ground vibrations caused by a variety of reasons. Such vibrations are often considered to be non-interpretable or unwanted components of the signal recording. The physical origin of seismic noise comes primarily from sources at or near the surface and consists almost entirely of elastic surface waves.
Low-frequency waves (below 1 Hz) are often called microseisms, while high-frequency waves (above 1 Hz) are called picoseisms.
The main sources of seismic waves include human activities (such as traffic or industrial activity), wind and other atmospheric phenomena, rivers and ocean waves. Seismic noise is relevant to all scientific fields that rely on seismology, including geology, oil exploration, hydrology, as well as earthquake engineering and structural health monitoring.
This noise is often a disruption in activities that are sensitive to external vibrations, such as earthquake monitoring and research, precision milling, telescopes, gravitational wave detection and crystal growth. However, seismic noise also has practical uses, such as determining the low-strain and time-varying dynamic properties of civil engineering structures (such as bridges, buildings, and dams); conducting seismic studies of underground structures of various sizes, often using seismic interferometry. and environmental monitoring, such as in river seismology.
Sources of seismic noise
Studies on the origins of seismic noise have shown that the low-frequency part of the spectrum (below 1 Hz) comes mainly from natural causes, mainly the impact of ocean waves. In particular, the globally observed peak between 0.1 and 0.3 Hz is clearly related to the interaction of water waves of the same frequency. In the high-frequency part (above 1 Hz), seismic noise is mainly caused by human activities such as road traffic and industrial work; but natural sources such as rivers can also contribute to this effect.
Above 1 Hz, wind and other atmospheric phenomena can also be major sources of ground vibrations.
For example, in Cameroon, "foot tremors" caused by football fans stomping their feet are among the non-anthropogenic activities observed during periods of low seismic activity. Around Bonny Bay in the Gulf of Guinea, pulses appear every 26 to 28 seconds, believed to be reflections from the rapids' undercurrents, a sign of the power of the waves.
Physical characteristics of seismic noise
The amplitude of seismic noise vibrations is typically between 0.1 and 10 μm/s. The globally evaluated background noise model shows frequency-dependent characteristics. Seismic noise includes a small amount of body waves (P and S waves), but surface waves (Love and Rayleigh waves) are the main components because they are preferentially excited by ground-source processes. These waves are scattered, meaning that their phase velocity varies with frequency (generally speaking, decreases with increasing frequency).
Because the dispersion curve (frequency function of phase velocity or laziness etc.) relates the variation of shear wave velocity with depth, it can be used as a non-intrusive tool to determine the seismic structure of the subsurface.
History of Seismic Noise
Under normal conditions, seismic noise has very low amplitude and is not perceptible to humans, nor could it be recorded on most early seismographs of the late 19th century. However, by the early 20th century, Japanese seismologist Ōmori Fumiyoshi was able to record ambient vibrations in buildings and determine the building's resonant frequency. The global 30-second to 5-second seismic noise was recognized as being caused by the ocean early in the history of seismology, and a comprehensive theory was published by Longuet-Higgins in 1950.
Current Applications of Seismic Noise
With the advancement of science and technology, especially with the rise of seismic interferometry since the 1990s, the application of seismic noise has continued to expand. For example, using analysis of ambient vibrations and random seismic wavefields, scientists can characterize subsurface structures using power spectra, H/V peak analysis, dispersion curves, and autocorrelation functions. The single-station method and the array method have undoubtedly brought new perspectives to seismology.
Finally, seismic noise is also considered a proxy indicator of economic development, reflecting changes in human activities.
With the impact of the COVID-19 epidemic, the reduction in human activities has significantly reduced seismic noise, becoming a unique window for observing the external environment. In the future, as our understanding of seismic noise deepens, will we be able to uncover more mysteries of natural phenomena?
