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. Physically, seismic noise arises primarily due to surface or near-surface sources and thus consists mostly of elastic surface waves. Low frequency waves (below 1 Hz) are commonly called microseisms and high frequency waves (above 1 Hz) are called microtremors. Primary sources of seismic waves include human activities (such as transportation or industrial activities), winds and other atmospheric phenomena, rivers, and ocean waves. Seismic noise is relevant to any discipline that depends on seismology, including geology, oil exploration, hydrology, and earthquake engineering, and structural health monitoring. It is often called the ambient wavefield or ambient vibrations in those disciplines (however, the latter term may also refer to vibrations transmitted through air, buildings, or supporting structures.) 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. 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.
Causes 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. 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 propagating in opposing directions. At 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. Above 1 Hz, wind and other atmospheric phenomena can also be a major source of ground vibrations. Anthropogenic noise detected during periods of low seismic activity includes "footquakes" from soccer fans stamping their feet in Cameroon. 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.
Physical characteristics The amplitude of seismic noise vibrations is typically in the order of 0.1 to 10 μm/s. High and low background noise models as a function of frequency have been evaluated globally. 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. These waves are dispersive, meaning that their phase velocity varies with frequency (generally, it decreases with increasing frequency). 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.
History 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. However, by the early 20th century, Japanese seismologist Fusakichi Omori could already record ambient vibrations in buildings, where the amplitudes are magnified. He determined building resonance frequencies and studied their evolution as a function of damage. Globally visible 30 s–5 s seismic noise was recognized early in the history of seismology as arising from the oceans, and a comprehensive theory of its generation was published by Longuet-Higgins in 1950. Rapid advances beginning around 2005 in seismic interferometry driven by theoretical, methodological, and data advances have resulted in a major renewed interest in the applications of seismic noise.
Civil engineering After the 1933 Long Beach earthquake in California, a large experiment campaign led by D. S. Carder in 1935 recorded and analyzed ambient vibrations in more than 200 buildings. These data were used in the design codes to estimate resonance frequencies of buildings but the interest of the method went down until the 1950s. Interest on ambient vibrations in structures grew further, especially in California and Japan, thanks to the work of earthquake engineers, including G. Housner, D. Hudson, K. Kanai, T. Tanaka, and others. In engineering, ambient vibrations were however supplanted – at least for some time – by forced vibration techniques that allow to increase the amplitudes and control the shaking source and their system identification methods. Even though M. Trifunac showed in 1972 that ambient and forced vibrations led to the same results, the interest in ambient vibration techniques only rose in the late 1990s. They have now become quite attractive, due to their relatively low cost and convenience, and to the recent improvements in recording equipment and computation methods. The results of their low-strain dynamic probing were shown to be close enough to the dynamic characteristics measured under strong shaking, at least as long as the buildings are not severely damaged.
… excerpt ends here. Continue reading the full article.
