Investigating hum sources using cross-correlation asymmetry

Abstract:

The sources and excitation of the Earth’s hum are current subjects of debate. Here, we propose an alternative approach to map power spectral density of hum sources. We are currently processing IRIS continuous seismic data spanning the last 10 years to obtain a cross-correlation dataset. The amplitude-independent phase cross-correlation provides correlation stacks that converge relatively rapidly and without the application of additional preprocessing such as spectral whitening. We calculate monthly stacks of approx. daily broadband geometrically normalized ‘classic’ and phase cross-correlations. These can be recombined into ‘seasonal’ stacks of several months. Also, they can be restacked by month over several years, so as to investigate long-term average behavior during different months. Although these correlation functions retain no absolute amplitude information, observational evidence from our data shows that the relative amplitudes of the causal and acausal branch differ between certain station pairs, indicating a preferential direction of wave propagation. We intend to use this directional characteristic to investigate source distribution of continuous seismic motions. In a first step, we project the measurements of amplitude asymmetry back along the station-station path using very simplified sensitivity kernels. These kernels can be obtained numerically or by using analytical 2-D Green’s functions for homogeneous media. We expect to obtain global maps of source regions for different seasons. In a further step (not to be presented here), the observations will be inverted for the geographical distribution of source power spectral density. We target hum as first example application, while the method could also be applied to other types of continuous seismic motion. The goal of our study is to obtain a global hum source map, which might benefit our understanding of the excitation mechanisms of hum.