11:00 AM - 1:00 PM
[SSS03-P06] Olivine Fabrics in the Oceanic Lithosphere Constrained by the Pn Azimuthal Anisotropy
Keywords:seismology, anisotropy, lithosphere
Olivine fabrics in the oceanic lithosphere are assumed to be consequences of the stress and chemical conditions at the time of the plate formation (e.g., Karato et al. 2008), and it is important to constrain them from seismological observations. In the previous studies, surface wave analyses to constrain the radial and azimuthal anisotropy provided the primary clues (e.g., Karato et al. 2008, Russel 2021). In this study, we show that the detailed azimuthal dependence of Pn velocities could provide another important clues.
Pn azimuthal anisotropy was studied in various regions (e.g., Morris et al. 1969, Mark et al., 2019), however, azimuthal dependence of Pn velocities are often represented by using only cos(2φ) and sin(2φ) terms, where φ is the azimuth. However, as Backus (1965) explicitly showed, there should be contributions from cos(4φ) and sin(4φ) terms as well even if we assumed infinitesimally weak anisotropy. Theoretical computations of group velocities for pure olivine crystal clearly show that there exist significant discrepancies in azimuthal dependency among A, D, and E-type olivine which are assumed major fabrics in the oceanic lithosphere.
We conducted the NOMan Project (Takeo et al, 2018, Baba et al., 2017) between 2010 and 2014 and deployed the BBOBSs in the northwestern Pacific. The primary purpose was passive-source experiments, but we also conducted active-source experiments in the region of the northern subarray (Area-A) with 4 event sites and 8 stations. We analyze the data from the active-source experiments to reveal the 2φ and 4φ terms and test whether we could constrain the fabrics in this region. We use the 1-D model of Shinohara et al. (2008), which were obtained by exploration experiments in the adjacent region, as the reference model and assume that the velocity structures in this region are slight perturbations from their model. We determine the optimal amplitudes of 2φ and 4φ terms using least squares fitting. The effects of event and station corrections are also took into account. According to our preliminary analysis, the region shows significant amplitudes of 4φ terms, which is the unique features of the A-type olivine.
Pn azimuthal anisotropy was studied in various regions (e.g., Morris et al. 1969, Mark et al., 2019), however, azimuthal dependence of Pn velocities are often represented by using only cos(2φ) and sin(2φ) terms, where φ is the azimuth. However, as Backus (1965) explicitly showed, there should be contributions from cos(4φ) and sin(4φ) terms as well even if we assumed infinitesimally weak anisotropy. Theoretical computations of group velocities for pure olivine crystal clearly show that there exist significant discrepancies in azimuthal dependency among A, D, and E-type olivine which are assumed major fabrics in the oceanic lithosphere.
We conducted the NOMan Project (Takeo et al, 2018, Baba et al., 2017) between 2010 and 2014 and deployed the BBOBSs in the northwestern Pacific. The primary purpose was passive-source experiments, but we also conducted active-source experiments in the region of the northern subarray (Area-A) with 4 event sites and 8 stations. We analyze the data from the active-source experiments to reveal the 2φ and 4φ terms and test whether we could constrain the fabrics in this region. We use the 1-D model of Shinohara et al. (2008), which were obtained by exploration experiments in the adjacent region, as the reference model and assume that the velocity structures in this region are slight perturbations from their model. We determine the optimal amplitudes of 2φ and 4φ terms using least squares fitting. The effects of event and station corrections are also took into account. According to our preliminary analysis, the region shows significant amplitudes of 4φ terms, which is the unique features of the A-type olivine.