Tork Qashqai, Mehdi1 and Saygin, Erdinc1
1Deep Earth Imaging Future Science Platform, CSIRO, Perth, Western Australia
Teleseismic body-waves coda recorded on the radial and the vertical components of a seismogram have been used for decades to image the local structures below a seismic station through the inversion of P-to-S receiver functions (RFs). It has been shown that the observed response of the subsurface structure to a seismic wave, generated by a deep source, can be converted to a zero-offset reflection response by autocorrelating the recorded signals. Recently, the autocorrelation of the teleseismic P-wave coda or its inversion (Tork Qashqai et al., 2019) has emerged as a powerful tool to obtain complementary constraints on subsurface structures. Compared to the RFs, the autocorrelations of the teleseismic P-wave coda recorded on the radial and vertical components of a seismogram contain additional information. They include both P- and P-to-S converted phases, whereas the RFs mainly contain P-to-S phases as the P-waves are attenuated by the deconvolving the vertical component (P) from the radial (Sv) component. Therefore, one can account for the variability of both the Vp and Vs structures if the radial and vertical component autocorrelations are jointly inverted. Here, we present a new approach which can simultaneously estimate the crustal Vp, Vs, and Vp/Vs ratio structures below a seismic station by jointly inverting the vertical and radial component autocorrelations of the teleseismic P-wave coda. This has significant implications for characterizing the Vp/Vs ratio, which can be a good indication of the crustal composition. Our synthetic inversion tests showed substantial improvements in the estimation of the crustal properties (especially the Vp/Vs ratio) compared to the inversion of either the teleseismic RFs or the autocorrelation of the vertical component. The application of this method on passive seismic data recorded by a north-south oriented passive seismic experiment in central Australia (BILBY) provided the first comprehensive joint estimates of all crustal properties (Vp, Vs, and Vp/Vs ratio) for this experiment. We imaged crustal structures across the transition between the northern and southern Australian cratons which includes east-west trending geological domains of central Australia (the Gawler Craton, Eromanga and Officer Basins, Musgrave Province, Amadeus Basin, Arunta Block and the Georgina Basin). The comparison of the Moho estimates from the previous studies with our velocity and Moho models indicates that they might have imaged the top of a high-velocity lower crust in some regions. The overall trend of our Moho model follows the long-wavelength pattern of the Moho structure interpreted from the deep seismic reflection method along the GOMA seismic line that is parallel to the BILBY profile. It is also closer to the change of the reflectivity seen at the base of the crust in the GOMA migrated seismic section. Our approach is cost-effective and can be used in conjunction with the deep active seismic reflection profiling to obtain additional information, especially at depths where the deep seismic reflection method has penetration problems.
Tork Qashqai, M., Saygin, E., & Kennett, B. L. N. (2019). Crustal imaging with Bayesian inversion of teleseismic P wave coda autocorrelation. Journal of Geophysical Research: Solid Earth, 124, 5888-5906.
Mehdi is a research scientist at the CSIRO Deep Earth Imaging Future Science Platform and graduated from Macquarie University in 2016. His main research interests lie in the developing and applications of the passive seismic methods for imaging the subsurface structure from the near-surface to the base of lithosphere.