[S-IT15] 地球深部科学 - 核・マントルの相互作用と共進化

[SIT15-P04] Inner core differential rotation inferred from antipodal seismic observations

*坪井 誠司¹、Butler Rhett² (1.海洋研究開発機構、2.ハワイ大学)

キーワード：内核、地震対蹠点、差分回転

The Earth’s Inner Core may rotate at a different speed than Earth’s crust–mantle. We use observations for antipodal earthquake and station pairs to examine the arrival time shift of the whispering-gallery wave which propagates tangentially along the top of the Inner Core. Observations of these waves for earthquakes which occurred at the same location reveal that travel times vary by 0.2 sec over the past 20 years. We propose that these time shifts are caused by the interaction of a zero rigidity patch (lateral heterogeneity) at the Inner Core surface with the Inner Core differential rotation. The simplest interpretation infers a rotation rate of 0.05 degree/year, which is one order of magnitude smaller than those reported by previous studies. Our result may also give constraint to the provenance of a zero rigidity patch at the Inner Core surface. The Fresnel zone coverage of whispering-gallery PKIIKP+ antipodal measurements beneath the Inner Core boundary extend over ~60% of the boundary surface, in contrast to PKiKP–PKP paths which individually measure specular reflections (~ 1 Hz) from the top of the ICB over Fresnel zones each encompassing 0.002% of the Inner–Outer Core boundary.