SCEC Award Number 22143 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Evaluation of velocity models and azimuthal anisotropy in Southern Californai using Eikonal tomography with surface waves generated by full waveforms simulations
Investigator(s)
Name Organization
Nori Nakata Massachusetts Institute of Technology Fan-Chi Lin University of Utah
Other Participants
SCEC Priorities 4b, 1c, 1d SCEC Groups Seismology, CXM, GM
Report Due Date 03/15/2023 Date Report Submitted 05/10/2023
Project Abstract
 The research contributes to the development of techniques for the challenging problem of evaluating velocity models and azimuthal anisotropy patterns constructed from surface wave tomography. The obtained results imply that clear artificial azimuthal anisotropy is observed at the edge of low velocity boundaries (e.g., basins). A similar pattern is also seen in azimuthal anisotropy maps resolved from surface waves reconstructed using ambient noise (Qiu et al., 2019), suggesting the anisotropy obtained near sharp structural boundaries may be artifacts. The performed research indicates that azimuthal anisotropy obtained from surface wave tomography requires extra verifications before being used to correlate with existing structural properties (e.g., mantle flow, alignment of fractures). Moreover, we will compare phase velocities obtained from Eikonal tomography using synthetic waveforms with those computed using 1-D velocity profiles from the input model. This comparison, in complement to the checkerboard test, provide a better evaluation of the model resolution, i.e., regions with large mismatch are likely also poorly imaged in previous surface wave tomography studies (e.g., Berg et al., 2018; Qiu et al., 2019).
Intellectual Merit The research contributes to the development of techniques for the challenging problem of evaluating velocity models and azimuthal anisotropy patterns constructed from surface wave tomography. The obtained results imply that clear artificial azimuthal anisotropy is observed at the edge of low velocity boundaries (e.g., basins). The performed research indicates that azimuthal anisotropy obtained from surface wave tomography requires extra verifications before being used to correlate with existing structural properties (e.g., mantle flow, alignment of fractures).
Broader Impacts The results highlight the potential of using synthetic waveforms generated by full waveform simulations to better understand the limitations of isotropic phase velocity and azimuthal anisotropy maps obtained from surface wave tomography. The employed method to identify regions with significant artificial azimuthal anisotropy or large mismatch to phase dispersion curve predicted by the input model may help other topics in seismology (e.g., interpretation and merging velocity models in SC). The project supported an early career postdoc.
Exemplary Figure Figure 5. Isotropic phase velocities & azimuthal anisotropy for Rayleigh wave at 7 s are illustrated in (a) and (b), respectively. (a) Number of azimuth bins that yield robust phase velocity estimates at each grid cell. Stars and triangles are virtual sources and receivers analyzed in the Eikonal tomography. The grid cell highlighted by the green square is used to demonstrate the azimuthal anisotropy fitting in Figure 6. (b) The isotropic phase velocity map is shown as the background image with color indicating the velocity value, whereas the azimuthal anisotropy is shown as straight lines with the length and direction as amplitude and fast axis of the anisotropy, respectively. Similar azimuthal anisotropy patterns are observed near basins in (c) and (d) for results derived from real data obtained from Qiu et al. (2019).