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Contribution to SSA Annual Meeting 2022 in Bellevue (Washington, USA)
A large-scale application of multizonal transdimensional Bayesian inversion for developing a 3D geophysical model in Basel, Switzerland
Afifa Imtiaz (1), Francesco Panzera (1), Miroslav Hallo (1), Horst Dresmann (2), Brian Steiner (2), Donat Fäh (1)
(1) Swiss Seismological Service (SED), ETHZ, Switzerland (afifa.imtiaz@sed.ethz.ch)
(2) Applied and Environmental Geology (AUG), University of Basel, Switzerland
Local shear-wave velocity (Vs) structure and sedimentary thickness are considered to be the controlling parameters in seismic amplification. Using dispersion characteristics of surface waves from ambient vibration array data has proven to be an effective tool for imaging subsurface Vs profiles. However, the conventional optimization inversion techniques are limited in their ability to account for the inherent non-uniqueness of this inverse problem and related uncertainty of the profiles. Therefore, we apply a novel Bayesian inversion approach based on a Multizonal Transdimensional Inversion (MTI), with the aim of developing a 3D Vs model for Basel, Switzerland. It is based on a joint inversion of multimodal Rayleigh- and Love-wave dispersion curves (DCs) along with Rayleigh-wave ellipticity. The key advantages of the MTI are that the number of layers is determined self-adaptively from the measured data, and model uncertainties can be assessed quantitatively. Moreover, the solution of the Bayesian inversion is the posterior Probability Density Function that results from prior expectations and observed data supplemented by an expected distribution of data errors. Hence, the model uncertainty can be duly propagated from DCs to Vs profiles.
We apply MTI to retrieve 1D Vs profiles from 32 passive arrays located within about 130 sq. km area by using single-zone transdimensional model space with homogeneous prior assumptions. It delivered a major improvement in our results because such joint inversions of surface wave dispersion and ellipticity curves could be performed only for a few sites in Basel in the past. Mulitizonal inversion is performed by drawing additional constraints on depths of intermediate layers from a rigorous 3D geological model. The results are promising in better resolving the interfaces corresponding to major velocity contrasts and the rate of Vs change with depth, especially in the complex sedimentary structure of the Rhine Graben formation.