15:30 〜 16:30
[S21-P-05] Azimuthal anisotropy in the Northwest Pacific oceanic lithosphere inferred from Po/So waves
Po/So waves, which have a high frequency, large amplitude, and long duration, propagate for large distances across oceanic lithosphere. These waves are generated by multiple forward scattering of P- and S-waves due to small-scale heterogeneities in oceanic lithosphere and P-waves trapped in seawater. To study the origin of such small-scale heterogeneities, we analyzed the azimuthal anisotropy of Po/So waves propagating.
Shito [2016, JPGU Abstract] examined the azimuthal anisotropy of Po/So wave propagation using seismic waveform data recorded at Broad Band Ocean Bottom Seismometers (BBOBSs) in the Northwest Pacific from 2010 to 2014. They determined travel times of the Po/So waves using an auto-picking algorithm based on an AR model, and estimated the average velocities of the Po/So waves between sources and stations. The average velocities of the Po/So waves traveling in the Northwest Pacific show clear variations as a function of azimuth, as follows:
VPo = 8.25 + 0.20 cos2(x − 153),
VSo = 4.71 + 0.04 cos2(x − 159).
The magnitudes of the anisotropy for Po and So waves velocities are 2.4% and 0.8%, respectively, which are smaller than the results of previous studies for Pn and Sn waves [Shimamura, 1984; Shinohara et al., 2008]. The fast direction is parallel to the past spreading direction of oceanic crust as estimated from magnetic anomalies [Nakanishi et al., 1992], which is roughly consistent with the previous studies [Shimamura, 1984; Shinohara et al., 2008].
In this study, we conduct Finite Difference Method (FDM) simulation of seismic wave propagation. We compare calculated and reported azimuthal anisotropy of the Po/So waves propagation and discuss the mechanism, which should be relating to the generation and evolution of the oceanic lithosphere.
Shito [2016, JPGU Abstract] examined the azimuthal anisotropy of Po/So wave propagation using seismic waveform data recorded at Broad Band Ocean Bottom Seismometers (BBOBSs) in the Northwest Pacific from 2010 to 2014. They determined travel times of the Po/So waves using an auto-picking algorithm based on an AR model, and estimated the average velocities of the Po/So waves between sources and stations. The average velocities of the Po/So waves traveling in the Northwest Pacific show clear variations as a function of azimuth, as follows:
VPo = 8.25 + 0.20 cos2(x − 153),
VSo = 4.71 + 0.04 cos2(x − 159).
The magnitudes of the anisotropy for Po and So waves velocities are 2.4% and 0.8%, respectively, which are smaller than the results of previous studies for Pn and Sn waves [Shimamura, 1984; Shinohara et al., 2008]. The fast direction is parallel to the past spreading direction of oceanic crust as estimated from magnetic anomalies [Nakanishi et al., 1992], which is roughly consistent with the previous studies [Shimamura, 1984; Shinohara et al., 2008].
In this study, we conduct Finite Difference Method (FDM) simulation of seismic wave propagation. We compare calculated and reported azimuthal anisotropy of the Po/So waves propagation and discuss the mechanism, which should be relating to the generation and evolution of the oceanic lithosphere.