Japan Geoscience Union Meeting 2016

Presentation information

International Session (Poster)

Symbol S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Techtonophysics

[S-IT08] Structure and Dynamics of Suboceanic Mantle: Theories and Observations

Tue. May 24, 2016 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall HALL6)

Convener:*Jun Korenaga(Department of Geology and Geophysics, Yale University), Hitoshi Kawakatsu(Earthquake Research Institute, University of Tokyo), James Gaherty(Lamont Doherty Earth Observatory), Kiyoshi Baba(Earthquake Research Institute, University of Tokyo)

5:15 PM - 6:30 PM

[SIT08-P03] Anisotropy in the Northwest Pacific oceanic lithosphere inferred from Po/So waves

*Azusa Shito1, Daisuke Suetsugu2, Takashi Furumura3, Hajime Shiobara3, Takehi Isse3, Hiroko Sugioka4, Aki Ito2, Akiko Takeo3, Hisashi Utada3 (1.Institute of Seismology and Volcanology, Kyushu University, 2.Department of Deep Earth Structure and Dynamics Research, Japan Agency for Marine-Earth Science and Technology, 3.Earthquake Research Institute, University of Tokyo, 4.Department of Planetology, Graduate School of Science, Kobe University)

Keywords:Po/So waves, anisotropy, oceanic lithosphere

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 in the Northwest Pacific.

Seismological observations using Broad Band Ocean Bottom Seismometers (BBOBSs) were conducted in the Northwest Pacific from 2010 to 2014 as a part of the Normal Oceanic Mantle Project. During the experiments, high-quality Po/So waves were recorded from earthquakes in the subducting Pacific plate. We 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].

We investigate the mechanism of the azimuthal anisotropy of Po/So wave propagation, which should be relating to the generation and evolution of the oceanic lithosphere using a Finite Difference Method (FDM) simulation of seismic wave propagation. We compare observed and calculated Po/So waves, and discuss the mechanism of their azimuthal anisotropy.