Seismic Noise Observations from Multiple Arrays in the Southern Hemisphere: Challenges and Opportunities

Abstract:

Seismic noise holds a two-fold interest for global geophysics. It is a source of energy that may be used to form 3D tomographic images of the Earth. It is also an observable that provides a continuous, and in some cases, multi-decadal record of ocean storm activity. Potential insights include the understanding of global changes in patterns of storm location and severity, and inputs to models of carbon dioxide entrainment into the oceans. While any continuous seismic record has the potential to contribute knowledge on ambient seismic noise (microseisms), seismic arrays provide information that includes inference of the seismic slowness and backazimuth of incoming signals. Given that noise observations have no onset time, array techniques are very important in contributing signal properties that allow backprojection and hence location determination for storm sources.

Seismic arrays located in Australia and southeast Asia enable deep ocean noise sources from much of the Southern Ocean and western Pacific to be investigated. We analyze seismic records including those from the Warramunga (WRA), Alice Springs (ASAR) and Pilbara (PSAR) arrays together with WAVEWATCH III oceanographic reanalysis models. We use the IAS Capon technique to identify incoming signals with the aim of detecting multiple noise sources impinging on the array at any one time. The combination of multi-decadal records from WRA, and multi-array perspectives for recent years provides an opportunity to observe seismic noise across a range of frequencies and across many years of seasonal variation.

We find some noise sources are consistent between different arrays, but others seem to be much better observed by a particular single array. We summarize these insights with a view to optimizing decadal plans for future installations of multiple arrays. Further, we address the challenge of using new arrays together with existing arrays with contrasting station configurations. Multiple array methods afford the opportunity of better understanding complex, extended ocean sources of seismic energy.