Abstract
Seismic ambient noise sources have received increased attention
recently, creating new possibilities to study the Earth’s subsurface and
the atmosphere-ocean-solid Earth coupling. In efforts to locate such
noise sources using nonlinear finite-frequency inversions,
methodological developments such as pre-computed wavefields and
spatially variable grids were necessary. These make inversions feasible
for the secondary microseismic sources in a frequency range up to 0.2 Hz
on a daily basis. By obtaining a starting model for the inversion using
Matched Field Processing (MFP) we are able to further steer the
inversion towards acceptable global noise source models and improve the
final result. Analysis of one year of daily inversions shows the
seasonal variations of the secondary microseisms and their dependence on
the atmosphere-ocean-solid Earth coupling due to storm-induced ocean
waves. We present a web framework, SANS (Seismic Ambient Noise Sources,
sans.ethz.ch), where daily regional- to global-scale seismic ambient
noise source maps are made available to the public. This eases the
implementation of time-variable noise source distributions into
full-waveform ambient noise tomography and time-dependent subsurface
monitoring methods. Additionally, it encourages other studies to verify
if changes in the seismic data are due to changes in the subsurface
velocity or spatio-temporal variations of noise sources.