![]() This noise is not instrumental or local but is ubiquitous irrespective of location. Even in seismically quiet periods, seismometers record persistent random fluctuations 1) known as “ambient noise”. Seismometers were developed to record ground motions caused by seismic events such as earthquakes and volcanic eruptions. For a comprehensive understanding, we must consider the solid Earth, the ocean, and the atmosphere as a coupled system. ![]() Recent developments in source location based on body-wave microseisms enable us to estimate forcing quantitatively. Excitation of secondary microseisms can be attributed to non-linear forcing by standing ocean swell at the sea surface in both pelagic and coastal regions. Excitation of primary microseisms can be attributed to topographic coupling between ocean swell and seismic waves on subtle undulation of continental shelves. Below 5 mHz, atmospheric disturbances may also contribute to this excitation. Topographic coupling between seismic waves and ocean infragravity waves at the abyssal floor can explain the observed shear traction sources. Below 20 mHz, pressure fluctuations of ocean infragravity waves reach the abyssal floor. This can be categorized as seismic hum (1–20 mHz), primary microseisms (0.02–0.1 Hz), and secondary microseisms (0.1–1 Hz). The ambient seismic wave field, also known as ambient noise, is excited by oceanic gravity waves primarily. ![]()
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