A new catalogue of toroidal-mode overtone splitting function measurements

Spectra of whole Earth oscillations or normal modes provide important constraints on Earth's large-scale structure. The most convenient way to include normal mode constraints in global tomographic models is by using splitting functions or structure coefficients, which describe how the frequency of a specific mode varies regionally. Splitting functions constrain 3-D variations in velocity, density structure and boundary topography. They may also constrain anisotropy, especially when combining information from spheroidal modes, which arc mainly sensitive to P - SV structure, with toroidal modes, mainly sensitive to SH structure. Spheroidal modes have been measured extensively, but toroidal modes have proven to be much more difficult and as a result only a limited number of toroidal modes have been measured so far. Here, we expand previous splitting function studies, by focusing specifically on toroidal-mode overtone observations. We present splitting function measurements for 19 self-coupled toroidal modes of which 13 modes have not been measured before. They are derived from radial and transverse horizontal component normal mode spectra up to 5 mHz for 91 events with M-W >= 7.4 from the years 1983-2018. Our data include the Tohoku event of 2011 (9.1M(W)), the Okhotsk event of 2013 (8.3M(W)) and the Fiji Island event from 2018 (8.2M(W)). Our measurements provide new constraints on upper- and lower-mantle shear wave velocity structure and in combination with existing spheroidal mode measurements can be used in future inversions for anisotropic mantle structure. Our new splitting function coefficient data set will be available online.

Automated summary

Spectra of whole Earth oscillations or normal modes provide important constraints on Earth's large-scale structure. The most convenient way to include normal mode constraints in global tomographic models is by using splitting functions or structure coefficients. This study expands on previous splitting function research, specifically focusing on toroidal-mode overtone observations to provide new constraints on mantle structure.