Japan Geoscience Union Meeting 2016

Presentation information

International Session (Oral)

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

[S-IT09] Hard-Rock Drilling: Oceanic Lithosphere to Continental Crust Formation

Mon. May 23, 2016 10:45 AM - 12:15 PM Convention Hall A (2F)

Convener:*Yoshihiko Tamura(R & D Center for Ocean Drilling Science, Japan Agency for Maine-Earth Science and Technology), Tomoaki Morishita(School of Natural System, Colleage of Science and Technology, Kanazawa University), Susanne Straub(Lamont-Doherty Earth Observatory, Columbia Univ., USA), Takashi Sano(Department of Geology and Paleontology, National Museum of Nature and Science), Susumu Umino(Department of Earth Sciences, Kanazawa University), Eiichi TAKAZAWA(Department of Geology, Faculty of Science, Niigata University), Katsuyoshi Michibayashi(Institute of Geosciences, Shizuoka University), Chair:Eiichi TAKAZAWA(Department of Geology, Faculty of Science, Niigata University), Takashi Sano(Department of Geology and Paleontology, National Museum of Nature and Science)

10:45 AM - 11:00 AM

[SIT09-07] Seismological structure of oceanic lithosphere inferred from Po/So waves

★Invited papers

*Azusa Shito1, Daisuke Suetsugu2, Takashi Furumura3 (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)

Keywords:Po/So waves, oceanic lithosphere, small-scale heterogeneities

Oceanic lithosphere, comprising oceanic crust and upper mantle, plays an important role in plate tectonics. Knowledge of the detailed structure of oceanic lithosphere is key to understanding its origin.

The propagation of Po/So waves over large distances across oceanic lithosphere provides information about lithospheric structure. Po/So waves, characterized by a high frequency, large amplitude, and long duration, were identified as early as 1935 [Linehan, 1940]. Many previous studies have attempted to quantitatively explain the generation and propagation mode of Po/So phases. Although it is generally accepted that Po/So phases are guided waves traveling very efficiently throughout the oceanic lithosphere, the generation and propagation processes of Po/So phases remain unclear.

In the past two decades, there have been great technological advances in computer simulations of high-frequency seismic waves in heterogeneous structures and in broadband seismic observations on the seafloor. These gains motivated us to further investigate the processes relating to guided waves in oceanic lithosphere and the generation of Po/So waves, using broadband seismic data and the numerical Finite Difference Method (FDM) to simulate high-frequency seismic waves. In this presentation, we outline recent progress in the study of Po/So waves.

Shito et al. [2013] reported that Po/So waves are generated by multiple forward scattering of P- and S-waves due to small-scale heterogeneities in oceanic lithosphere. The laterally elongated heterogeneities are described by a von Karman distribution function with a correlation length of 10 km in the horizontal and 0.5 km in thickness, with a velocity perturbation of 2%.

Kennett and Furumura [2013; 2014] and Shito et al. [2015] found that the propagation efficiency of Po/So waves depends on the age of oceanic lithosphere, and that this relationship can be qualitatively explained by thickening of oceanic lithosphere that contains small-scale heterogeneities and a reduction in intrinsic attenuation. Recently, Kennett and Furumura [2015] proposed a new model that the amplitude of such heterogeneities increases with depth to the bottom of the lithosphere. These results suggest that small-scale heterogeneities may form continuously in oceanic lithosphere, from the time of its formation at a spreading ridge, via the solidification of melts in the asthenosphere.

The petrological and mineralogical processes that cause the small-scale heterogeneities remain poorly known. Future studies that combine seismological observations with petrological analyses will yield a greater understanding of the origin of oceanic lithosphere.