![]() ![]() Under normal conditions, seismic noise has very low amplitude and cannot be felt by humans, and was also too low to be recorded by most early seismometers at the end of 19th century. Since the dispersion curve (phase velocity or slowness as a function of frequency) is related to the variations of the shear-wave velocity with depth, it can be used as a non-invasive tool to determine subsurface seismic structure and an inverse problem. These waves are dispersive, meaning that their phase velocity varies with frequency (generally, it decreases with increasing frequency). Seismic noise includes a small number of body waves (P- and S-waves), but surface waves ( Love and Rayleigh waves) predominate since they are preferentially excited by surface source processes. High and low background noise models as a function of frequency have been evaluated globally. The amplitude of seismic noise vibrations is typically in the order of 0.1 to 10 μm/ s. Non-anthropogenic activity includes pulses at intervals between 26 and 28 seconds (0.036–0.038 Hz) centered on the Bight of Bonny in the Gulf of Guinea that are thought to be caused by reflected storm waves, focused by the African coast, acting on the relatively shallow sea-floor. Īnthropogenic noise detected during periods of low seismic activity includes "footquakes" from soccer fans stamping their feet in Cameroon. Ībove 1 Hz, wind and other atmospheric phenomena can also be a major source of ground vibrations. Īt high frequency (above 1 Hz), seismic noise is mainly produced by human activities such as road traffic and industrial work but there are also natural sources, including rivers. In particular the globally observed peak between 0.1 and 0.3 Hz is clearly associated with the interaction of water waves of nearly equal frequencies but probating in opposing directions. Research on the origin of seismic noise indicates that the low frequency part of the spectrum (below 1 Hz) is principally due to natural causes, chiefly ocean waves. However, seismic noise also has practical uses, including determining the low-strain and time-varying dynamic properties of civil-engineering structures, such as bridges, buildings, and dams seismic studies of subsurface structure at many scales, often using the methods of seismic interferometry Environmental monitoring, such as in fluvial seismology and estimating seismic microzonation maps to characterize local and regional ground response during earthquakes. Seismic noise is often a nuisance for activities that are sensitive to extraneous vibrations, including earthquake monitoring and research, precision milling, telescopes, gravitational wave detectors, and crystal growing. It is often called the ambient wavefield or ambient vibrations in those disciplines (however, the latter term may also refer to vibrations transmitted through by air, building, or supporting structures.) Seismic noise is relevant to any discipline that depends on seismology, including geology, oil exploration, hydrology, and earthquake engineering, and structural health monitoring. Primary sources of seismic waves include human activities (such as transportation or industrial activities), winds and other atmospheric phenomena, rivers, and ocean waves. Low frequency waves (below 1 Hz) are commonly called microseisms and high frequency waves (above 1 Hz) are called microtremors. Physically, seismic noise arises primarily due to surface or near surface sources and thus consists mostly of elastic surface waves. In geophysics, geology, civil engineering, and related disciplines, seismic noise is a generic name for a relatively persistent vibration of the ground, due to a multitude of causes, that is often a non-interpretable or unwanted component of signals recorded by seismometers.
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