A new catalogue of toroidal-mode overtone splitting function measurements

Abstract

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 3D variations in velocity, density structure and boundary topography. They may also constrain anisotropy, especially when combining information from spheroidal modes, which are 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 the splitting function studies by Resovsky and Ritzwoller (1998) and Deuss et al. (2013), 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 MW ≥ 7.4 from the years 1983-2018. Our data include the Tohoku event of 2011 (9.1MW), the Okhotsk event of 2013 (8.3MW) and the Fiji Island event from 2018 (8.2MW). 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.

Observing seismic attenuation in the Earth’s mantle and inner core using normal modes

“…We use the earthquake catalogue published by Deuss et al (2013) and Koelemeijer et al (2013), and extended it to add 14 additional large earthquakes that occurred between 2012 and 2018 (Table 2.1). Horizontal data was also added to the catalogue (Schneider and Deuss, 2020). The extended catalogue now consists of 107 earthquakes with magnitude M w > 7.4 which occurred in the period from 1975 to 2018, providing us with an average increase of 24% in number of spectra per mode, compared to Deuss et al (2013).…”

“…They have Table 6.1: Modes for which elastic and anelastic splitting functions are measured in this thesis. Spheroidal modes were selected from the elastic splitting functions observations of Deuss et al (2013), and toroidal modes from Schneider and Deuss (2020). Modes marked with † depend on the starting model, modes marked with ‡ are beyond 4 mHz and cannot be measured starting from PREM, and modes marked with have too large dst uncertainties or change their cst values significantly when measured together with dst.…”

“…been selected given their low angular order l and large number of data available, which allows these mode pairs to be modelled by fewer splitting function coefficients than other higher l mode pairs. In addition to spheroidal modes, we also tried to measure anelastic splitting functions of some of the isolated toroidal modes reported in Schneider and Deuss (2020).…”

“…In Table 6.1, we list all modes from Deuss et al (2013) and Schneider and Deuss (2020) that satisfied the initial selection criteria. After making the measurements for all modes listed in Table 6.1, we found out that only the anelastic splitting functions of spheroidal modes with n = 0, 1, 2, 3 are well constrained (Table 6.2).…”

Toroidal mode observations and their interpretation in terms of anisotropy and density

“…In normal mode seismology research, it is common to use the normal mode spectra to measure splitting functions, which are depth averages of how a mode 'sees' the Earth by giving the local frequency perturbation a specific mode (e.g. Woodhouse and Giardini, 1985;Deuss et al, 2011;Schneider and Deuss, 2020). In order to make such measurements, we need to identify:…”