Ambient noise seismic imaging

Article By:

Ritzwoller,Michael H. Department of Physics, University of Colorado, Boulder, Colorado.

Last reviewed:2009

DOI:https://doi.org/10.1036/1097-8542.YB090130

Traditional seismic imaging of large-scale structures within the Earth's interior is based on observations of surface displacements following earthquakes or human-caused explosions. Explosive sources are used less often because of their expense and environmental impact. These methods measure body and surface wave travel times as well as whole waveforms typically following earthquakes. Such measurements are unraveled (inverted) to reveal the isotropic and anisotropic variation of compressional (Vp) and shear (Vs) wave speeds in the Earth's crust, mantle, and core, which are then interpreted in terms of temperature, composition, and fluid content. The ability of earthquake-based methods to resolve structural features within the Earth degrades during the propagation of the wave over long (teleseismic) distances. For seismic surface waves, such as Rayleigh waves (vertically polarized waves) and Love waves (horizontally polarized waves transverse to the direction of motion), teleseismic transmission results in the loss of the high frequencies needed to infer information about the Earth's crust and uppermost mantle. A recent innovation in seismic imaging based on using long time sequences of ambient seismic noise moves beyond some of the limitations imposed on earthquake-based methods to reveal higher-resolution information about the crust and uppermost mantle. This method is called ambient noise tomography (ANT), and it has been applied predominantly to seismic surface waves. With the application of ANT to data from ambitious new deployments of seismic arrays, such as the EarthScope USArray in the United States, improved seismic models of the Earth's crust and uppermost mantle are rapidly emerging at unprecedented resolution.

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