Abstract
SUMMARYWe develop and apply a method to constrain the space- and frequency-dependent location of ambient noise sources. This is based on ambient noise cross-correlation inversion using numerical wavefield simulations, which honour 3-D crustal and mantle structure, ocean loading and finite-frequency effects. In the frequency range from 3 to 20 mHz, our results constrain the global source distribution of the Earth’s hum, averaged over the Southern Hemisphere winter season of 9 yr. During Southern Hemisphere winter, the dominant sources are largely confined to the Southern Hemisphere, the most prominent exception being the Izu-Bonin-Mariana arc, which is the most active source region between 12 and 20 mHz. Generally, strong hum sources seem to be associated with either coastlines or bathymetric highs. In contrast, deep ocean basins are devoid of hum sources. While being based on the relatively small number of STS-1 broad-band stations that have been recording continuously from 2004 to 2013, our results demonstrate the practical feasibility of a frequency-dependent noise source inversion that accounts for the complexities of 3-D wave propagation. It may thereby improve full-waveform ambient noise inversions and our understanding of the physics of noise generation.
Highlights
SUMMARY We develop and apply a method to constrain the space- and frequency-dependent location of ambient noise sources
This is based on ambient noise cross-correlation inversion using numerical wavefield simulations, which honour 3-D crustal and mantle structure, ocean loading and finite-frequency effects
During Southern Hemisphere winter, the dominant sources are largely confined to the Southern Hemisphere, the most prominent exception being the Izu-Bonin-Mariana arc, which is the most active source region between 12 and 20 mHz
Summary
In the absence of earthquakes, seismic stations record a wealth of ambient signals, the sources of which have been studied extensively in order to understand their physical origin, and to improve interferometric imaging of the Earth’s interior (e.g. Aster et al 2008; Stutzmann et al 2009; Yao & van der Hilst 2009; Traer et al 2012; Reading et al 2014; Gal et al 2015; Juretzek & Hadziioannou 2017). Recent studies have underlined the importance of accounting for nearby sources and for lateral velocity variations in the localization of noise sources (Juretzek & Hadziioannou 2017; Ward Neale et al 2017; Gal et al 2018; Takagi et al 2018), thereby questioning the plane-wave approximation. In this context, we develop and apply an inversion technique that constrains the space- and frequency-dependent distribution of ambient noise source PSD, while accounting for 3-D crust and mantle structure, ocean loading, and finite-frequency sensitivity. Jointly inverting for the space- and frequency-dependence of noise sources and (2) to investigate the global-scale source distribution of ambient noise at long periods from 50 to 350 s, commonly referred to as the Earth’s hum (e.g. Nishida & Kobayashi 1999)
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