Introduction To Ambient Seismic Noise

Introduction to Ambient Seismic Noise

The seismic ambient field is an emerging topic in seismology these days along with rising in implementation of high density array of seismometer for example the AlpArray project and the USArray. Both aspect, the seismic ambient noise research and dense array ….

Prerequisite knowledge

Everything you should know about ambient noise

• Definition

  • The ambient field generated by these forces constitutes the seismic background radiation of the Earth, historically referred to as microseisms.

  • The absence of an easily identifiable, deterministic signal has led to the common classification of the ambient field as seismic ambient noise.

  • This natural noise is particularly strong in the microseism band (0.04–0.2 Hz), which is also of interest for imaging purposes at the lithospheric and global scales.

• Source Spectrum
  • Short periods (0.1-1 s) ambient noise power levels are dominated by human-generated (i.e., “cultural”) seismic energy radiated from the electrical grid, cars, trains, and machinery within a few kilometers of the recording station.
    • Cultural noise
    • Clacier calving
    • Wind
  • Intermediate periods (1–30 s) are dominated by microseisms, that can be many orders of magnitude higher in power than other parts of the seismic spectrum.
    • Microseism Broad and dominant peaks near the periods of 7 and 14 seconds.
    • Extra-Tropical Storms/Tropical Cyclones
    • Sea ice
  • Long-period (30–500 s) signals are generally caused by ocean infragravity waves generated by storm-forced, shoreward-directed winds, commonly referred to as “Hum.”
    • Hum
      Hum is a term referring to bell-like ringing associated with the fundamental long-period resonant spheroidal oscillations of Earth. Hum excitation is due to the portion of ocean swell that reaches coastlines and is transformed into much longer periods (> 50 s).
    • Seiche
      Long-period noise results from a Seiche wave that forms in the canal channel from the constructive interference of the wakes of passing ships and wind-forced waves reflecting off the nearby shorelines. These seiche waves crash onto the shore and cause movement of the soils, resulting in tilt of the seismic vault at a period of ∼ 100 s.
• To understand the seismic ambient noise levels experienced by a network of seismic stations, assessment can be made by describing the probability density function of power spectral density (PSDPDF) method.

  • The standard Peterson (1993) new low-noise model (NLNM) is a noise model constructed by removing earthquakes and other transient signals from the seismic records in order to identify the minimum noise levels representative of quiet periods at continental interior seismic stations distributed around the world.

  • The new high-noise model (NHNM) is also constructed using mostly island-based stations within the GSN (Peterson, 1993).

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the xperts

• Michael H. Ritzwoller (University of Colorado)
• Michel Campillo (ISTerre)
• Barbara A. Romanowicz (UC BErkeley)
• Nikolai Shapiro (ISTerre)
• Philippe Roux (ISTerre)
• Laurent Stehly (ISTerre)
• Paul Cupillard (University of Lorraine)
• ..

Some Papers

Introduction

1. Aki, K. 1957. Space and time spectra of stationary stochastic waves, with special reference to microtremors. Bull. Earthq. Res. Inst., Univ. Tokyo, 35, 415–457.

2. Sabra, K. G., Gerstoft, P., Roux, P., and Kuperman, W. A. 2005. Surface wave tomography from microseisms in Southern California. Geophys. Res. Lett., 32, doi:10.1029/2005GL023155.

3. Shapiro, N. M., Campillo, M., Stehly, L., and Ritzwoller, M. 2005. High resolution surface wave tomography from ambient seismic noise. Science, 307, 1615–1618.

Chapter 1 Visualization

1. Bensen, G., Ritzwoller, M., and Shapiro, N. M. 2008. Broadband ambient noise surface wave tomography across the United States. Journal of Geophysical Research: Solid Earth, 113(B05306).

2. Berger, J., Davis, P., and Ekström, G. 2004. Ambient earth noise: a survey of the global seismographic network. Journal of Geophysical Research: Solid Earth, 109(11307).

3. Brune, J. N. 1970. Tectonic stress and the spectra of seismic shear waves from earthquakes. Journal of Geophysical Research, 75(26), 4997–5009.

4. Stehly, L., Campillo, M., and Shapiro, N. 2006. A study of the seismic noise from its long-range correlation properties. Journal of Geophysical Research: Solid Earth, 111(B10), B10306.

Chapter 2 Beamforming and Polarization Analysis

Noise Sources

1. Frantti, G., Willis, D., and Wilson, J. T. 1962. The spectrum of seismic noise. Bulletin of the Seismological Society of America, 52(1), 113–121.