Evolution of the oceanic lithosphere inferred from Po/So waves traveling in the Philippine Sea Plate

Abstract

Po/So waves are characterized by their high-frequency content and long-duration travel over great distances (up to 3000km) through the oceanic lithosphere. Po/So waves are developed by the multiple forward scattering of P and S waves due to small-scale stochastic random heterogeneities. To study the nature of these heterogeneities, Po/So waves are analyzed in the Philippine Sea Plate, which consists of three regions with different lithospheric ages. In the Philippine Sea Plate, Po/So waves propagate in the youngest region (15 Ma) and propagate more effectively in older regions. We investigate the mechanism of this propagation efficiency using numerical finite difference method simulations of 2-D seismic wave propagation. The results of this study demonstrate that the increase in propagation efficiency of Po/So waves depends on the age of the oceanic lithosphere, and this relationship can be qualitatively explained by thickening of the oceanic lithosphere including small-scale heterogeneities and a reduction in the intrinsic attenuation. These small-scale heterogeneities may form continuously in oceanic lithosphere from the time of its formation at a spreading ridge, via the solidification of melts distributed in the asthenosphere.

Toward the reconciliation of seismological and petrological perspectives on oceanic lithosphere heterogeneity

“…amplitude of stochastic variation 2%). The work of Shito et al [2013] demonstrates that the same model of heterogeneity is applicable in the old oceanic lithosphere of the NW Pacific and in the subducting Pacific plate beneath Japan A similar uniform distribution of heterogeneity (amplitude 2%) has been used by Shito et al [2015] in a study of Po and So propagation in the much younger lithosphere of the Philippine Sea plate. They note systematic thickening of the lithosphere with increasing age, and favor an even longer horizontal correlation length than for the northwest Pacific.…”

“…A similar uniform distribution of heterogeneity (amplitude 2%) has been used by Shito et al . [] in a study of Po and So propagation in the much younger lithosphere of the Philippine Sea plate. They note systematic thickening of the lithosphere with increasing age, and favor an even longer horizontal correlation length than for the northwest Pacific.…”

Active-source seismic survey on the northeastern Hawaiian Arch: insights into crustal structure and mantle reflectors

“…However, we detected mantle reflectors beneath the normal ocean basin, where crustal thickening due to intraplate volcanism does not occur. The other interpretation is that the mantle reflectors represent quasi-laminate heterogeneities in the lithospheric mantle; such heterogeneities have been identified previously by analyzing Po and So waves, which are characterized by their high-frequency (> 2.5 Hz) content and their propagation for great distances in the oceanic lithosphere, with quasi-laminate features with a horizontal correlation length of around 10 km and a vertical correlation length of 0.5 km and a velocity perturbation of a few percent (e.g., Shito et al 2015;Kennett and Furumura 2015). These heterogeneities are considered to have formed continuously in the oceanic lithosphere from the time of its formation at a mid-ocean ridge via the solidification of melts attached to the bottom of the lithosphere (Shito et al 2015).…”

“…The other interpretation is that the mantle reflectors represent quasi-laminate heterogeneities in the lithospheric mantle; such heterogeneities have been identified previously by analyzing Po and So waves, which are characterized by their high-frequency (> 2.5 Hz) content and their propagation for great distances in the oceanic lithosphere, with quasi-laminate features with a horizontal correlation length of around 10 km and a vertical correlation length of 0.5 km and a velocity perturbation of a few percent (e.g., Shito et al 2015;Kennett and Furumura 2015). These heterogeneities are considered to have formed continuously in the oceanic lithosphere from the time of its formation at a mid-ocean ridge via the solidification of melts attached to the bottom of the lithosphere (Shito et al 2015). In a previous active-source seismic survey conducted southeast of the Shatsky Rise, we observed such frozen melts at depths of 37-59 km (Ohira et al 2017b), but based on that study we were unable to determine whether such heterogeneities are ubiquitous in the lithosphere or restricted to specific regions.…”

Lateral variation of the uppermost oceanic plate in the outer-rise region of the Northwest Pacific Ocean inferred from Po-to-s converted waves

“…When P and S waves are excited by an earthquake occurring within the subducting slab, Po and So waves are developed from the multiple scatterings of the P and S waves owing to small-scale stochastic random heterogeneities in the oceanic mantle (Shito et al 2013). These waves propagate over distances up to 3000 km and often manifest long-duration, high-frequency (> 2 Hz) recordings in the seafloor observation (Kennett and Furumura, 2013;Shito et al 2013Shito et al , 2015. This study used Po waves observed in the northwestern part of the Pacific Plate ( Fig.…”