[S-SS09] Seismic wave propagation: Theory and Application

[SSS09-P06] Seismic Source Characterization in Anisotropic Media: Impact of Lithospheric Anisotropy on Moment Tensors

*Hitoshi Kawakatsu^1,2, Akiko Takeo¹ (1.Earthquake Research Institute, The University of Tokyo, 2.Institute of Earth Sciences, Academia Sinica)

Keywords:seismic anisotropy, moment tensor, non-double couple component, surface wave tomography

Kawakatsu (2024, BSSA) recently demonstrated that the systematic source-type dependence of the non-double couple (NDC) components of the GCMT moment tensors for shallow sources—previously reported by Kawakatsu (1991, GRL) for the Harvard CMT catalog—can be attributed (except in the case of strike-slip events) to the presence of transverse isotropy with a vertical symmetry axis (VTI or radial anisotropy) in the lithosphere, where the anisotropy parameter ξ>1. It is important to recognize that this is an intrinsic property of the moment tensor in an anisotropic medium, as long as we remain within the Burridge-Knopoff-type framework, and not merely an effect of unmodeled path influences (Aki and Richards, 1980). Given that most published moment tensor solutions (e.g., GCMT, W-phase, USGS, F-net, etc.) adopt this framework, direct interpretations of MT solutions (such as slip directions, regional stress or strain) based on the assumption of an isotropic source region structure are likely to be biased (e.g., Vavrycic, 2005, GJI). To assess the significance of this effect, it is essential to investigate how realistic anisotropic structures may give rise to NDC components in moment tensors.

In this study, we aim to quantify this effect by utilizing surface wave anisotropy tomography models of the lithosphere (e.g., Priestley et al., 2024, EPSL). Our approach incorporates both radial and azimuthal anisotropy in the modeling of moment tensors, including NDC components, while assuming seismic sources as planar faulting, following the methodology of Kawakatsu (2024). Preliminary global modeling suggests that azimuthal anisotropy may enhance NDC modeling; however, the extent of this improvement remains uncertain and is highly dependent on the assumed fabric structure. We aim to clarify this issue by the time of the presentation. Additionally, we will discuss regional modeling results for areas exhibiting strong lithospheric anisotropy in tomography.